Add Type.GetNullableUnderlyingType() virtual API

[AaronRobinsonMSFT]

Closes#125388
Fixes#124216

Breaking change documentation: dotnet/docs#53407

Summary

Adds a new public virtual Type.GetNullableUnderlyingType() method so that Type subclasses (e.g. MetadataLoadContext's RoType) can correctly identify Nullable types. Nullable.GetUnderlyingType() now forwards to this virtual.

This follows the same pattern as Enum.GetUnderlyingType() forwarding to Type.GetEnumUnderlyingType().

Contract

• Type.GetNullableUnderlyingType() returns a non-null result for both:
  • a constructed Nullable<T> (returns T), and
  • the generic type definition typeof(Nullable<>) (returns the generic type parameter T).
• Nullable.GetUnderlyingType(Type) continues to return null for the generic type definition for COMPAT.
• The base Type.GetNullableUnderlyingType() throws NotSupportedException. Custom Type subclasses outside the BCL must override it; this is a behavioral breaking change documented in [Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType docs#53407.

Changes

Public API

• Type.cs: New public virtual Type? GetNullableUnderlyingType() that throws NotSupportedException(SR.NotSupported_SubclassOverride) (matches IsByRefLike pattern per @MichalStrehovsky's feedback). XML doc documents that the open generic Nullable<> is treated as nullable and yields the generic type parameter.
• System.Runtime.cs / System.Reflection.Emit.cs (ref assemblies): New API + TypeDelegator/TypeBuilder/EnumBuilder/GenericTypeParameterBuilder overrides.

Nullable.GetUnderlyingType rewire

• Nullable.cs: Now delegates to the new virtual after preserving the IsGenericTypeDefinition COMPAT short-circuit.

Runtime overrides (all three runtimes)

• RuntimeType.CoreCLR.cs / RuntimeType.Mono.cs: Override that handles both constructed and open-generic cases. Open Nullable<> returns GetGenericArguments()[0] since the native fast-path can't yield a MethodTable for the formal type parameter T.
• RuntimeType.NativeAot.cs: Same handling for constructed Nullable<X> via the EEType fast-path.
• RuntimeTypeInfo.cs (NativeAOT): Added public virtual returning null (per @jkotas's feedback).
• NativeFormatRuntimeNamedTypeInfo.cs (NativeAOT): Sealed override that returns the generic parameter only when the type is typeof(Nullable<>).
• RuntimeConstructedGenericTypeInfo.cs (NativeAOT): override for constructed generics.

Reflection subclasses

• TypeDelegator.cs: Override forwarding to typeImpl.GetNullableUnderlyingType().
• SignatureType.cs / SignatureConstructedGenericType.cs / SignatureModifiedType.cs: Overrides that delegate through the generic definition.
• ModifiedType.cs: Override delegating through the unmodified type.

Reflection.Emit

• TypeBuilder.cs / EnumBuilder.cs / GenericTypeParameterBuilder.cs / TypeBuilderInstantiation.cs: Overrides returning null (or appropriate result for instantiations).
• SymbolType.cs: Override returning null so Nullable.GetUnderlyingType doesn't throw on MakeArrayType/MakePointerType/MakeByRefType results from TypeBuilder.

MetadataLoadContext

• RoType.cs: Override using CoreType.NullableT identity comparison; uses GetGenericArguments()[0] so the open Nullable<> returns the MLC-projected generic parameter rather than indexing empty GenericTypeArguments.
• RoModifiedType.cs: Override delegating through the unmodified type (required because RoModifiedType.GetGenericTypeDefinition() throws).

Tests

• NullableTests.cs: Coverage for RuntimeType (constructed + open-generic) and TypeDelegator.
• SignatureTypes.cs: Coverage for SignatureConstructedGenericType and SignatureModifiedType.
• ModifiedTypeTests.cs: New NullableModifiedTypeHolder (uses volatile delegate*<int?> to obtain a ModifiedType wrapping Nullable<int>) and tests for modified Nullable / non-Nullable.
• TypeBuilderGetNullableUnderlyingType.cs (new): Coverage for TypeBuilder, EnumBuilder, GenericTypeParameterBuilder, TypeBuilderInstantiation, and SymbolType (Array / multi-dim Array / Pointer / ByRef).
• TypeTests.Nullable.cs (MetadataLoadContext): Coverage for RoType (constructed + open-generic).

Note

This PR description was updated with assistance from GitHub Copilot.

[Copilot]

Pull request overview

This PR introduces a new public virtual Type.GetNullableUnderlyingType() API so non-RuntimeTypeType providers (notably MetadataLoadContext’s RoType) can correctly identify closed Nullable<T> types, and updates Nullable.GetUnderlyingType(Type) to delegate to this new virtual.

Changes:

• Add Type.GetNullableUnderlyingType() and implement/override it for RuntimeType (CoreCLR/Mono), NativeAOT RuntimeType, TypeDelegator, and MetadataLoadContext’s RoType.
• Change Nullable.GetUnderlyingType(Type) to forward to Type.GetNullableUnderlyingType().
• Add System.Runtime tests covering both RuntimeType and MetadataLoadContext behavior, plus a test project reference to System.Reflection.MetadataLoadContext.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds the new public virtual GetNullableUnderlyingType() method.
src/libraries/System.Private.CoreLib/src/System/RuntimeType.cs	Overrides GetNullableUnderlyingType() for runtime types.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	Adds NativeAOT override of GetNullableUnderlyingType().
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual through TypeDelegator.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Implements nullable detection for MLC RoType.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Updates Nullable.GetUnderlyingType to delegate to the new virtual.
src/libraries/System.Runtime/ref/System.Runtime.cs	Updates the ref assembly surface area for the new API and TypeDelegator override.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Switches Mono’s internal nullable check to use GetNullableUnderlyingType().
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds tests for Type.GetNullableUnderlyingType() and MLC scenarios.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System.Runtime.Tests.csproj	Adds test-time project reference to System.Reflection.MetadataLoadContext.

Copilot's findings

• Files reviewed: 10/10 changed files
• Comments generated: 3

[Copilot]

Pull request overview

Adds a new virtual Type.GetNullableUnderlyingType() so non-RuntimeType implementations (notably MetadataLoadContext’s RoType) can correctly detect Nullable<T>, and updates Nullable.GetUnderlyingType(Type) to forward to the virtual.

Changes:

• Introduces Type.GetNullableUnderlyingType() and wires Nullable.GetUnderlyingType() to call it.
• Implements overrides for CoreCLR, Mono, NativeAOT RuntimeType, and key wrapper types (TypeDelegator, modified/signature types).
• Adds tests for RuntimeType and MetadataLoadContext behavior.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds new virtual GetNullableUnderlyingType() API.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Forwards Nullable.GetUnderlyingType to Type.GetNullableUnderlyingType().
src/coreclr/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs	Implements CoreCLR RuntimeType override using MethodTable fast-path + fallback.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Implements Mono RuntimeType override; updates IsNullableOfT to use it.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	Implements NativeAOT RuntimeType override.
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual to the underlying Type.
src/libraries/System.Private.CoreLib/src/System/Reflection/ModifiedType.cs	Forwards the new virtual to the unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureModifiedType.cs	Forwards the new virtual to the unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureConstructedGenericType.cs	Explicitly returns null for the new virtual.
src/libraries/System.Private.CoreLib/src/System/Reflection/Emit/TypeBuilderInstantiation.cs	Explicitly returns null for the new virtual.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Implements MLC RoType override via CoreType.NullableT identity.
src/libraries/System.Runtime/ref/System.Runtime.cs	Adds the new API to the ref assembly and TypeDelegator override surface.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds tests for Type.GetNullableUnderlyingType() on runtime types.
src/libraries/System.Reflection.MetadataLoadContext/tests/src/Tests/Type/TypeTests.Nullable.cs	Adds MLC-specific tests for nullable detection, including open-generic case.
src/libraries/System.Reflection.MetadataLoadContext/tests/System.Reflection.MetadataLoadContext.Tests.csproj	Includes the new MLC test file in compilation.

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs:99

• The new GetNullableUnderlyingType test cases don't cover the open generic definition typeof(Nullable<>). Given the API contract is "closed generic Nullable only", add a case asserting typeof(Nullable<>).GetNullableUnderlyingType() returns null to prevent regressions (and to catch the current behavior in the runtime overrides).

 [Theory]
[InlineData(typeof(int?), typeof(int))]
[InlineData(typeof(int), null)]
[InlineData(typeof(G<int>), null)]
public static void GetNullableUnderlyingType_RuntimeType(Type type, Type? expected)
{
Assert.Equal(expected, type.GetNullableUnderlyingType());
}

• Files reviewed: 15/15 changed files
• Comments generated: 4

[Copilot]

Pull request overview

Adds a new public virtual Type.GetNullableUnderlyingType() API so non-RuntimeType implementations (notably MetadataLoadContext’s RoType) can correctly recognize closed Nullable<T> and provide the underlying T. Nullable.GetUnderlyingType(Type) is updated to delegate to this virtual, mirroring the existing Enum.GetUnderlyingType() → Type.GetEnumUnderlyingType() pattern.

Changes:

• Introduce Type.GetNullableUnderlyingType() (virtual) and wire Nullable.GetUnderlyingType(Type) to call it for constructed generic types.
• Implement/forward the virtual across CoreCLR, Mono, NativeAOT, MetadataLoadContext (RoType), and common wrapper types (TypeDelegator, modified/signature types, TypeBuilderInstantiation).
• Add tests covering both RuntimeType and MetadataLoadContext behavior.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds the new public virtual GetNullableUnderlyingType() API and docs.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Updates Nullable.GetUnderlyingType to delegate to Type.GetNullableUnderlyingType().
src/coreclr/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs	CoreCLR override that recognizes Nullable<T> via method table fast-path and fallback.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Mono override implementation + internal IsNullableOfT updated to use the new virtual.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	NativeAOT override implementation.
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual to the delegated typeImpl.
src/libraries/System.Private.CoreLib/src/System/Reflection/ModifiedType.cs	Forwards the new virtual to the underlying unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureModifiedType.cs	Forwards the new virtual to the underlying unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureConstructedGenericType.cs	Sealed override returning null to preserve existing signature-type semantics.
src/libraries/System.Private.CoreLib/src/System/Reflection/Emit/TypeBuilderInstantiation.cs	Adds override to surface underlying T for constructed Nullable<T> in Reflection.Emit instantiations.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Adds RoType override (guarded) using core-type identity comparison for Nullable<T>.
src/libraries/System.Runtime/ref/System.Runtime.cs	Updates ref surface area for Type and TypeDelegator.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds direct Type.GetNullableUnderlyingType() runtime tests.
src/libraries/System.Reflection.MetadataLoadContext/tests/src/Tests/Type/TypeTests.Nullable.cs	Adds MLC tests validating both Nullable.GetUnderlyingType and the direct virtual call.
src/libraries/System.Reflection.MetadataLoadContext/tests/System.Reflection.MetadataLoadContext.Tests.csproj	Includes the new MLC test file in the test project.

Copilot's findings

• Files reviewed: 15/15 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 22/22 changed files
• Comments generated: 1

[dotnet-policy-service]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.

[Copilot]

Copilot's findings

• Files reviewed: 22/22 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 28/28 changed files
• Comments generated: 2

[AaronRobinsonMSFT]

Note

This comment was generated with assistance from GitHub Copilot.

Filed the breaking-change documentation issue: dotnet/docs#53407.

Remaining checklist item from the policy bot above:

• Email a link to the docs issue to the .NET Breaking Change Notification DL.

[AaronRobinsonMSFT]

Closes#125388
Fixes#124216

Breaking change documentation: dotnet/docs#53407

Summary

Adds a new public virtual Type.GetNullableUnderlyingType() method so that Type subclasses (e.g. MetadataLoadContext's RoType) can correctly identify Nullable types. Nullable.GetUnderlyingType() now forwards to this virtual.

This follows the same pattern as Enum.GetUnderlyingType() forwarding to Type.GetEnumUnderlyingType().

Contract

• Type.GetNullableUnderlyingType() returns a non-null result for both:
  • a constructed Nullable<T> (returns T), and
  • the generic type definition typeof(Nullable<>) (returns the generic type parameter T).
• Nullable.GetUnderlyingType(Type) continues to return null for the generic type definition for COMPAT.
• The base Type.GetNullableUnderlyingType() throws NotSupportedException. Custom Type subclasses outside the BCL must override it; this is a behavioral breaking change documented in [Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType docs#53407.

Changes

Public API

• Type.cs: New public virtual Type? GetNullableUnderlyingType() that throws NotSupportedException(SR.NotSupported_SubclassOverride) (matches IsByRefLike pattern per @MichalStrehovsky's feedback). XML doc documents that the open generic Nullable<> is treated as nullable and yields the generic type parameter.
• System.Runtime.cs / System.Reflection.Emit.cs (ref assemblies): New API + TypeDelegator/TypeBuilder/EnumBuilder/GenericTypeParameterBuilder overrides.

Nullable.GetUnderlyingType rewire

• Nullable.cs: Now delegates to the new virtual after preserving the IsGenericTypeDefinition COMPAT short-circuit.

Runtime overrides (all three runtimes)

• RuntimeType.CoreCLR.cs / RuntimeType.Mono.cs: Override that handles both constructed and open-generic cases. Open Nullable<> returns GetGenericArguments()[0] since the native fast-path can't yield a MethodTable for the formal type parameter T.
• RuntimeType.NativeAot.cs: Same handling for constructed Nullable<X> via the EEType fast-path.
• RuntimeTypeInfo.cs (NativeAOT): Added public virtual returning null (per @jkotas's feedback).
• NativeFormatRuntimeNamedTypeInfo.cs (NativeAOT): Sealed override that returns the generic parameter only when the type is typeof(Nullable<>).
• RuntimeConstructedGenericTypeInfo.cs (NativeAOT): override for constructed generics.

Reflection subclasses

• TypeDelegator.cs: Override forwarding to typeImpl.GetNullableUnderlyingType().
• SignatureType.cs / SignatureConstructedGenericType.cs / SignatureModifiedType.cs: Overrides that delegate through the generic definition.
• ModifiedType.cs: Override delegating through the unmodified type.

Reflection.Emit

• TypeBuilder.cs / EnumBuilder.cs / GenericTypeParameterBuilder.cs / TypeBuilderInstantiation.cs: Overrides returning null (or appropriate result for instantiations).
• SymbolType.cs: Override returning null so Nullable.GetUnderlyingType doesn't throw on MakeArrayType/MakePointerType/MakeByRefType results from TypeBuilder.

MetadataLoadContext

• RoType.cs: Override using CoreType.NullableT identity comparison; uses GetGenericArguments()[0] so the open Nullable<> returns the MLC-projected generic parameter rather than indexing empty GenericTypeArguments.
• RoModifiedType.cs: Override delegating through the unmodified type (required because RoModifiedType.GetGenericTypeDefinition() throws).

Tests

• NullableTests.cs: Coverage for RuntimeType (constructed + open-generic) and TypeDelegator.
• SignatureTypes.cs: Coverage for SignatureConstructedGenericType and SignatureModifiedType.
• ModifiedTypeTests.cs: New NullableModifiedTypeHolder (uses volatile delegate*<int?> to obtain a ModifiedType wrapping Nullable<int>) and tests for modified Nullable / non-Nullable.
• TypeBuilderGetNullableUnderlyingType.cs (new): Coverage for TypeBuilder, EnumBuilder, GenericTypeParameterBuilder, TypeBuilderInstantiation, and SymbolType (Array / multi-dim Array / Pointer / ByRef).
• TypeTests.Nullable.cs (MetadataLoadContext): Coverage for RoType (constructed + open-generic).

Note

This PR description was updated with assistance from GitHub Copilot.

[Copilot]

Pull request overview

This PR introduces a new public virtual Type.GetNullableUnderlyingType() API so non-RuntimeTypeType providers (notably MetadataLoadContext’s RoType) can correctly identify closed Nullable<T> types, and updates Nullable.GetUnderlyingType(Type) to delegate to this new virtual.

Changes:

• Add Type.GetNullableUnderlyingType() and implement/override it for RuntimeType (CoreCLR/Mono), NativeAOT RuntimeType, TypeDelegator, and MetadataLoadContext’s RoType.
• Change Nullable.GetUnderlyingType(Type) to forward to Type.GetNullableUnderlyingType().
• Add System.Runtime tests covering both RuntimeType and MetadataLoadContext behavior, plus a test project reference to System.Reflection.MetadataLoadContext.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds the new public virtual GetNullableUnderlyingType() method.
src/libraries/System.Private.CoreLib/src/System/RuntimeType.cs	Overrides GetNullableUnderlyingType() for runtime types.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	Adds NativeAOT override of GetNullableUnderlyingType().
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual through TypeDelegator.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Implements nullable detection for MLC RoType.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Updates Nullable.GetUnderlyingType to delegate to the new virtual.
src/libraries/System.Runtime/ref/System.Runtime.cs	Updates the ref assembly surface area for the new API and TypeDelegator override.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Switches Mono’s internal nullable check to use GetNullableUnderlyingType().
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds tests for Type.GetNullableUnderlyingType() and MLC scenarios.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System.Runtime.Tests.csproj	Adds test-time project reference to System.Reflection.MetadataLoadContext.

Copilot's findings

• Files reviewed: 10/10 changed files
• Comments generated: 3

[Copilot]

Pull request overview

Adds a new virtual Type.GetNullableUnderlyingType() so non-RuntimeType implementations (notably MetadataLoadContext’s RoType) can correctly detect Nullable<T>, and updates Nullable.GetUnderlyingType(Type) to forward to the virtual.

Changes:

• Introduces Type.GetNullableUnderlyingType() and wires Nullable.GetUnderlyingType() to call it.
• Implements overrides for CoreCLR, Mono, NativeAOT RuntimeType, and key wrapper types (TypeDelegator, modified/signature types).
• Adds tests for RuntimeType and MetadataLoadContext behavior.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds new virtual GetNullableUnderlyingType() API.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Forwards Nullable.GetUnderlyingType to Type.GetNullableUnderlyingType().
src/coreclr/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs	Implements CoreCLR RuntimeType override using MethodTable fast-path + fallback.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Implements Mono RuntimeType override; updates IsNullableOfT to use it.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	Implements NativeAOT RuntimeType override.
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual to the underlying Type.
src/libraries/System.Private.CoreLib/src/System/Reflection/ModifiedType.cs	Forwards the new virtual to the unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureModifiedType.cs	Forwards the new virtual to the unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureConstructedGenericType.cs	Explicitly returns null for the new virtual.
src/libraries/System.Private.CoreLib/src/System/Reflection/Emit/TypeBuilderInstantiation.cs	Explicitly returns null for the new virtual.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Implements MLC RoType override via CoreType.NullableT identity.
src/libraries/System.Runtime/ref/System.Runtime.cs	Adds the new API to the ref assembly and TypeDelegator override surface.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds tests for Type.GetNullableUnderlyingType() on runtime types.
src/libraries/System.Reflection.MetadataLoadContext/tests/src/Tests/Type/TypeTests.Nullable.cs	Adds MLC-specific tests for nullable detection, including open-generic case.
src/libraries/System.Reflection.MetadataLoadContext/tests/System.Reflection.MetadataLoadContext.Tests.csproj	Includes the new MLC test file in compilation.

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs:99

• The new GetNullableUnderlyingType test cases don't cover the open generic definition typeof(Nullable<>). Given the API contract is "closed generic Nullable only", add a case asserting typeof(Nullable<>).GetNullableUnderlyingType() returns null to prevent regressions (and to catch the current behavior in the runtime overrides).

 [Theory]
[InlineData(typeof(int?), typeof(int))]
[InlineData(typeof(int), null)]
[InlineData(typeof(G<int>), null)]
public static void GetNullableUnderlyingType_RuntimeType(Type type, Type? expected)
{
Assert.Equal(expected, type.GetNullableUnderlyingType());
}

• Files reviewed: 15/15 changed files
• Comments generated: 4

[Copilot]

Pull request overview

Adds a new public virtual Type.GetNullableUnderlyingType() API so non-RuntimeType implementations (notably MetadataLoadContext’s RoType) can correctly recognize closed Nullable<T> and provide the underlying T. Nullable.GetUnderlyingType(Type) is updated to delegate to this virtual, mirroring the existing Enum.GetUnderlyingType() → Type.GetEnumUnderlyingType() pattern.

Changes:

• Introduce Type.GetNullableUnderlyingType() (virtual) and wire Nullable.GetUnderlyingType(Type) to call it for constructed generic types.
• Implement/forward the virtual across CoreCLR, Mono, NativeAOT, MetadataLoadContext (RoType), and common wrapper types (TypeDelegator, modified/signature types, TypeBuilderInstantiation).
• Add tests covering both RuntimeType and MetadataLoadContext behavior.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds the new public virtual GetNullableUnderlyingType() API and docs.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Updates Nullable.GetUnderlyingType to delegate to Type.GetNullableUnderlyingType().
src/coreclr/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs	CoreCLR override that recognizes Nullable<T> via method table fast-path and fallback.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Mono override implementation + internal IsNullableOfT updated to use the new virtual.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	NativeAOT override implementation.
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual to the delegated typeImpl.
src/libraries/System.Private.CoreLib/src/System/Reflection/ModifiedType.cs	Forwards the new virtual to the underlying unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureModifiedType.cs	Forwards the new virtual to the underlying unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureConstructedGenericType.cs	Sealed override returning null to preserve existing signature-type semantics.
src/libraries/System.Private.CoreLib/src/System/Reflection/Emit/TypeBuilderInstantiation.cs	Adds override to surface underlying T for constructed Nullable<T> in Reflection.Emit instantiations.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Adds RoType override (guarded) using core-type identity comparison for Nullable<T>.
src/libraries/System.Runtime/ref/System.Runtime.cs	Updates ref surface area for Type and TypeDelegator.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds direct Type.GetNullableUnderlyingType() runtime tests.
src/libraries/System.Reflection.MetadataLoadContext/tests/src/Tests/Type/TypeTests.Nullable.cs	Adds MLC tests validating both Nullable.GetUnderlyingType and the direct virtual call.
src/libraries/System.Reflection.MetadataLoadContext/tests/System.Reflection.MetadataLoadContext.Tests.csproj	Includes the new MLC test file in the test project.

Copilot's findings

• Files reviewed: 15/15 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 22/22 changed files
• Comments generated: 1

[dotnet-policy-service]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.

[Copilot]

Copilot's findings

• Files reviewed: 22/22 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 28/28 changed files
• Comments generated: 2

[AaronRobinsonMSFT]

Note

This comment was generated with assistance from GitHub Copilot.

Filed the breaking-change documentation issue: dotnet/docs#53407.

Remaining checklist item from the policy bot above:

• Email a link to the docs issue to the .NET Breaking Change Notification DL.

[AaronRobinsonMSFT]

Closes#125388
Fixes#124216

Breaking change documentation: dotnet/docs#53407

Summary

Adds a new public virtual Type.GetNullableUnderlyingType() method so that Type subclasses (e.g. MetadataLoadContext's RoType) can correctly identify Nullable types. Nullable.GetUnderlyingType() now forwards to this virtual.

This follows the same pattern as Enum.GetUnderlyingType() forwarding to Type.GetEnumUnderlyingType().

Contract

• Type.GetNullableUnderlyingType() returns a non-null result for both:
  • a constructed Nullable<T> (returns T), and
  • the generic type definition typeof(Nullable<>) (returns the generic type parameter T).
• Nullable.GetUnderlyingType(Type) continues to return null for the generic type definition for COMPAT.
• The base Type.GetNullableUnderlyingType() throws NotSupportedException. Custom Type subclasses outside the BCL must override it; this is a behavioral breaking change documented in [Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType docs#53407.

Changes

Public API

• Type.cs: New public virtual Type? GetNullableUnderlyingType() that throws NotSupportedException(SR.NotSupported_SubclassOverride) (matches IsByRefLike pattern per @MichalStrehovsky's feedback). XML doc documents that the open generic Nullable<> is treated as nullable and yields the generic type parameter.
• System.Runtime.cs / System.Reflection.Emit.cs (ref assemblies): New API + TypeDelegator/TypeBuilder/EnumBuilder/GenericTypeParameterBuilder overrides.

Nullable.GetUnderlyingType rewire

• Nullable.cs: Now delegates to the new virtual after preserving the IsGenericTypeDefinition COMPAT short-circuit.

Runtime overrides (all three runtimes)

• RuntimeType.CoreCLR.cs / RuntimeType.Mono.cs: Override that handles both constructed and open-generic cases. Open Nullable<> returns GetGenericArguments()[0] since the native fast-path can't yield a MethodTable for the formal type parameter T.
• RuntimeType.NativeAot.cs: Same handling for constructed Nullable<X> via the EEType fast-path.
• RuntimeTypeInfo.cs (NativeAOT): Added public virtual returning null (per @jkotas's feedback).
• NativeFormatRuntimeNamedTypeInfo.cs (NativeAOT): Sealed override that returns the generic parameter only when the type is typeof(Nullable<>).
• RuntimeConstructedGenericTypeInfo.cs (NativeAOT): override for constructed generics.

Reflection subclasses

• TypeDelegator.cs: Override forwarding to typeImpl.GetNullableUnderlyingType().
• SignatureType.cs / SignatureConstructedGenericType.cs / SignatureModifiedType.cs: Overrides that delegate through the generic definition.
• ModifiedType.cs: Override delegating through the unmodified type.

Reflection.Emit

• TypeBuilder.cs / EnumBuilder.cs / GenericTypeParameterBuilder.cs / TypeBuilderInstantiation.cs: Overrides returning null (or appropriate result for instantiations).
• SymbolType.cs: Override returning null so Nullable.GetUnderlyingType doesn't throw on MakeArrayType/MakePointerType/MakeByRefType results from TypeBuilder.

MetadataLoadContext

• RoType.cs: Override using CoreType.NullableT identity comparison; uses GetGenericArguments()[0] so the open Nullable<> returns the MLC-projected generic parameter rather than indexing empty GenericTypeArguments.
• RoModifiedType.cs: Override delegating through the unmodified type (required because RoModifiedType.GetGenericTypeDefinition() throws).

Tests

• NullableTests.cs: Coverage for RuntimeType (constructed + open-generic) and TypeDelegator.
• SignatureTypes.cs: Coverage for SignatureConstructedGenericType and SignatureModifiedType.
• ModifiedTypeTests.cs: New NullableModifiedTypeHolder (uses volatile delegate*<int?> to obtain a ModifiedType wrapping Nullable<int>) and tests for modified Nullable / non-Nullable.
• TypeBuilderGetNullableUnderlyingType.cs (new): Coverage for TypeBuilder, EnumBuilder, GenericTypeParameterBuilder, TypeBuilderInstantiation, and SymbolType (Array / multi-dim Array / Pointer / ByRef).
• TypeTests.Nullable.cs (MetadataLoadContext): Coverage for RoType (constructed + open-generic).

Note

This PR description was updated with assistance from GitHub Copilot.

[Copilot]

Pull request overview

This PR introduces a new public virtual Type.GetNullableUnderlyingType() API so non-RuntimeTypeType providers (notably MetadataLoadContext’s RoType) can correctly identify closed Nullable<T> types, and updates Nullable.GetUnderlyingType(Type) to delegate to this new virtual.

Changes:

• Add Type.GetNullableUnderlyingType() and implement/override it for RuntimeType (CoreCLR/Mono), NativeAOT RuntimeType, TypeDelegator, and MetadataLoadContext’s RoType.
• Change Nullable.GetUnderlyingType(Type) to forward to Type.GetNullableUnderlyingType().
• Add System.Runtime tests covering both RuntimeType and MetadataLoadContext behavior, plus a test project reference to System.Reflection.MetadataLoadContext.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds the new public virtual GetNullableUnderlyingType() method.
src/libraries/System.Private.CoreLib/src/System/RuntimeType.cs	Overrides GetNullableUnderlyingType() for runtime types.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	Adds NativeAOT override of GetNullableUnderlyingType().
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual through TypeDelegator.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Implements nullable detection for MLC RoType.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Updates Nullable.GetUnderlyingType to delegate to the new virtual.
src/libraries/System.Runtime/ref/System.Runtime.cs	Updates the ref assembly surface area for the new API and TypeDelegator override.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Switches Mono’s internal nullable check to use GetNullableUnderlyingType().
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds tests for Type.GetNullableUnderlyingType() and MLC scenarios.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System.Runtime.Tests.csproj	Adds test-time project reference to System.Reflection.MetadataLoadContext.

Copilot's findings

• Files reviewed: 10/10 changed files
• Comments generated: 3

[Copilot]

Pull request overview

Adds a new virtual Type.GetNullableUnderlyingType() so non-RuntimeType implementations (notably MetadataLoadContext’s RoType) can correctly detect Nullable<T>, and updates Nullable.GetUnderlyingType(Type) to forward to the virtual.

Changes:

• Introduces Type.GetNullableUnderlyingType() and wires Nullable.GetUnderlyingType() to call it.
• Implements overrides for CoreCLR, Mono, NativeAOT RuntimeType, and key wrapper types (TypeDelegator, modified/signature types).
• Adds tests for RuntimeType and MetadataLoadContext behavior.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds new virtual GetNullableUnderlyingType() API.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Forwards Nullable.GetUnderlyingType to Type.GetNullableUnderlyingType().
src/coreclr/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs	Implements CoreCLR RuntimeType override using MethodTable fast-path + fallback.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Implements Mono RuntimeType override; updates IsNullableOfT to use it.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	Implements NativeAOT RuntimeType override.
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual to the underlying Type.
src/libraries/System.Private.CoreLib/src/System/Reflection/ModifiedType.cs	Forwards the new virtual to the unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureModifiedType.cs	Forwards the new virtual to the unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureConstructedGenericType.cs	Explicitly returns null for the new virtual.
src/libraries/System.Private.CoreLib/src/System/Reflection/Emit/TypeBuilderInstantiation.cs	Explicitly returns null for the new virtual.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Implements MLC RoType override via CoreType.NullableT identity.
src/libraries/System.Runtime/ref/System.Runtime.cs	Adds the new API to the ref assembly and TypeDelegator override surface.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds tests for Type.GetNullableUnderlyingType() on runtime types.
src/libraries/System.Reflection.MetadataLoadContext/tests/src/Tests/Type/TypeTests.Nullable.cs	Adds MLC-specific tests for nullable detection, including open-generic case.
src/libraries/System.Reflection.MetadataLoadContext/tests/System.Reflection.MetadataLoadContext.Tests.csproj	Includes the new MLC test file in compilation.

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs:99

• The new GetNullableUnderlyingType test cases don't cover the open generic definition typeof(Nullable<>). Given the API contract is "closed generic Nullable only", add a case asserting typeof(Nullable<>).GetNullableUnderlyingType() returns null to prevent regressions (and to catch the current behavior in the runtime overrides).

 [Theory]
[InlineData(typeof(int?), typeof(int))]
[InlineData(typeof(int), null)]
[InlineData(typeof(G<int>), null)]
public static void GetNullableUnderlyingType_RuntimeType(Type type, Type? expected)
{
Assert.Equal(expected, type.GetNullableUnderlyingType());
}

• Files reviewed: 15/15 changed files
• Comments generated: 4

[Copilot]

Pull request overview

Adds a new public virtual Type.GetNullableUnderlyingType() API so non-RuntimeType implementations (notably MetadataLoadContext’s RoType) can correctly recognize closed Nullable<T> and provide the underlying T. Nullable.GetUnderlyingType(Type) is updated to delegate to this virtual, mirroring the existing Enum.GetUnderlyingType() → Type.GetEnumUnderlyingType() pattern.

Changes:

• Introduce Type.GetNullableUnderlyingType() (virtual) and wire Nullable.GetUnderlyingType(Type) to call it for constructed generic types.
• Implement/forward the virtual across CoreCLR, Mono, NativeAOT, MetadataLoadContext (RoType), and common wrapper types (TypeDelegator, modified/signature types, TypeBuilderInstantiation).
• Add tests covering both RuntimeType and MetadataLoadContext behavior.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds the new public virtual GetNullableUnderlyingType() API and docs.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Updates Nullable.GetUnderlyingType to delegate to Type.GetNullableUnderlyingType().
src/coreclr/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs	CoreCLR override that recognizes Nullable<T> via method table fast-path and fallback.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Mono override implementation + internal IsNullableOfT updated to use the new virtual.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	NativeAOT override implementation.
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual to the delegated typeImpl.
src/libraries/System.Private.CoreLib/src/System/Reflection/ModifiedType.cs	Forwards the new virtual to the underlying unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureModifiedType.cs	Forwards the new virtual to the underlying unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureConstructedGenericType.cs	Sealed override returning null to preserve existing signature-type semantics.
src/libraries/System.Private.CoreLib/src/System/Reflection/Emit/TypeBuilderInstantiation.cs	Adds override to surface underlying T for constructed Nullable<T> in Reflection.Emit instantiations.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Adds RoType override (guarded) using core-type identity comparison for Nullable<T>.
src/libraries/System.Runtime/ref/System.Runtime.cs	Updates ref surface area for Type and TypeDelegator.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds direct Type.GetNullableUnderlyingType() runtime tests.
src/libraries/System.Reflection.MetadataLoadContext/tests/src/Tests/Type/TypeTests.Nullable.cs	Adds MLC tests validating both Nullable.GetUnderlyingType and the direct virtual call.
src/libraries/System.Reflection.MetadataLoadContext/tests/System.Reflection.MetadataLoadContext.Tests.csproj	Includes the new MLC test file in the test project.

Copilot's findings

• Files reviewed: 15/15 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 22/22 changed files
• Comments generated: 1

[dotnet-policy-service]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.

[Copilot]

Copilot's findings

• Files reviewed: 22/22 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 28/28 changed files
• Comments generated: 2

[AaronRobinsonMSFT]

Note

This comment was generated with assistance from GitHub Copilot.

Filed the breaking-change documentation issue: dotnet/docs#53407.

Remaining checklist item from the policy bot above:

• Email a link to the docs issue to the .NET Breaking Change Notification DL.

[AaronRobinsonMSFT]

Closes#125388
Fixes#124216

Breaking change documentation: dotnet/docs#53407

Summary

Adds a new public virtual Type.GetNullableUnderlyingType() method so that Type subclasses (e.g. MetadataLoadContext's RoType) can correctly identify Nullable types. Nullable.GetUnderlyingType() now forwards to this virtual.

This follows the same pattern as Enum.GetUnderlyingType() forwarding to Type.GetEnumUnderlyingType().

Contract

• Type.GetNullableUnderlyingType() returns a non-null result for both:
  • a constructed Nullable<T> (returns T), and
  • the generic type definition typeof(Nullable<>) (returns the generic type parameter T).
• Nullable.GetUnderlyingType(Type) continues to return null for the generic type definition for COMPAT.
• The base Type.GetNullableUnderlyingType() throws NotSupportedException. Custom Type subclasses outside the BCL must override it; this is a behavioral breaking change documented in [Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType docs#53407.

Changes

Public API

• Type.cs: New public virtual Type? GetNullableUnderlyingType() that throws NotSupportedException(SR.NotSupported_SubclassOverride) (matches IsByRefLike pattern per @MichalStrehovsky's feedback). XML doc documents that the open generic Nullable<> is treated as nullable and yields the generic type parameter.
• System.Runtime.cs / System.Reflection.Emit.cs (ref assemblies): New API + TypeDelegator/TypeBuilder/EnumBuilder/GenericTypeParameterBuilder overrides.

Nullable.GetUnderlyingType rewire

• Nullable.cs: Now delegates to the new virtual after preserving the IsGenericTypeDefinition COMPAT short-circuit.

Runtime overrides (all three runtimes)

• RuntimeType.CoreCLR.cs / RuntimeType.Mono.cs: Override that handles both constructed and open-generic cases. Open Nullable<> returns GetGenericArguments()[0] since the native fast-path can't yield a MethodTable for the formal type parameter T.
• RuntimeType.NativeAot.cs: Same handling for constructed Nullable<X> via the EEType fast-path.
• RuntimeTypeInfo.cs (NativeAOT): Added public virtual returning null (per @jkotas's feedback).
• NativeFormatRuntimeNamedTypeInfo.cs (NativeAOT): Sealed override that returns the generic parameter only when the type is typeof(Nullable<>).
• RuntimeConstructedGenericTypeInfo.cs (NativeAOT): override for constructed generics.

Reflection subclasses

• TypeDelegator.cs: Override forwarding to typeImpl.GetNullableUnderlyingType().
• SignatureType.cs / SignatureConstructedGenericType.cs / SignatureModifiedType.cs: Overrides that delegate through the generic definition.
• ModifiedType.cs: Override delegating through the unmodified type.

Reflection.Emit

• TypeBuilder.cs / EnumBuilder.cs / GenericTypeParameterBuilder.cs / TypeBuilderInstantiation.cs: Overrides returning null (or appropriate result for instantiations).
• SymbolType.cs: Override returning null so Nullable.GetUnderlyingType doesn't throw on MakeArrayType/MakePointerType/MakeByRefType results from TypeBuilder.

MetadataLoadContext

• RoType.cs: Override using CoreType.NullableT identity comparison; uses GetGenericArguments()[0] so the open Nullable<> returns the MLC-projected generic parameter rather than indexing empty GenericTypeArguments.
• RoModifiedType.cs: Override delegating through the unmodified type (required because RoModifiedType.GetGenericTypeDefinition() throws).

Tests

• NullableTests.cs: Coverage for RuntimeType (constructed + open-generic) and TypeDelegator.
• SignatureTypes.cs: Coverage for SignatureConstructedGenericType and SignatureModifiedType.
• ModifiedTypeTests.cs: New NullableModifiedTypeHolder (uses volatile delegate*<int?> to obtain a ModifiedType wrapping Nullable<int>) and tests for modified Nullable / non-Nullable.
• TypeBuilderGetNullableUnderlyingType.cs (new): Coverage for TypeBuilder, EnumBuilder, GenericTypeParameterBuilder, TypeBuilderInstantiation, and SymbolType (Array / multi-dim Array / Pointer / ByRef).
• TypeTests.Nullable.cs (MetadataLoadContext): Coverage for RoType (constructed + open-generic).

Note

This PR description was updated with assistance from GitHub Copilot.

[Copilot]

Pull request overview

This PR introduces a new public virtual Type.GetNullableUnderlyingType() API so non-RuntimeTypeType providers (notably MetadataLoadContext’s RoType) can correctly identify closed Nullable<T> types, and updates Nullable.GetUnderlyingType(Type) to delegate to this new virtual.

Changes:

• Add Type.GetNullableUnderlyingType() and implement/override it for RuntimeType (CoreCLR/Mono), NativeAOT RuntimeType, TypeDelegator, and MetadataLoadContext’s RoType.
• Change Nullable.GetUnderlyingType(Type) to forward to Type.GetNullableUnderlyingType().
• Add System.Runtime tests covering both RuntimeType and MetadataLoadContext behavior, plus a test project reference to System.Reflection.MetadataLoadContext.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds the new public virtual GetNullableUnderlyingType() method.
src/libraries/System.Private.CoreLib/src/System/RuntimeType.cs	Overrides GetNullableUnderlyingType() for runtime types.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	Adds NativeAOT override of GetNullableUnderlyingType().
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual through TypeDelegator.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Implements nullable detection for MLC RoType.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Updates Nullable.GetUnderlyingType to delegate to the new virtual.
src/libraries/System.Runtime/ref/System.Runtime.cs	Updates the ref assembly surface area for the new API and TypeDelegator override.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Switches Mono’s internal nullable check to use GetNullableUnderlyingType().
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds tests for Type.GetNullableUnderlyingType() and MLC scenarios.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System.Runtime.Tests.csproj	Adds test-time project reference to System.Reflection.MetadataLoadContext.

Copilot's findings

• Files reviewed: 10/10 changed files
• Comments generated: 3

[Copilot]

Pull request overview

Adds a new virtual Type.GetNullableUnderlyingType() so non-RuntimeType implementations (notably MetadataLoadContext’s RoType) can correctly detect Nullable<T>, and updates Nullable.GetUnderlyingType(Type) to forward to the virtual.

Changes:

• Introduces Type.GetNullableUnderlyingType() and wires Nullable.GetUnderlyingType() to call it.
• Implements overrides for CoreCLR, Mono, NativeAOT RuntimeType, and key wrapper types (TypeDelegator, modified/signature types).
• Adds tests for RuntimeType and MetadataLoadContext behavior.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds new virtual GetNullableUnderlyingType() API.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Forwards Nullable.GetUnderlyingType to Type.GetNullableUnderlyingType().
src/coreclr/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs	Implements CoreCLR RuntimeType override using MethodTable fast-path + fallback.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Implements Mono RuntimeType override; updates IsNullableOfT to use it.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	Implements NativeAOT RuntimeType override.
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual to the underlying Type.
src/libraries/System.Private.CoreLib/src/System/Reflection/ModifiedType.cs	Forwards the new virtual to the unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureModifiedType.cs	Forwards the new virtual to the unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureConstructedGenericType.cs	Explicitly returns null for the new virtual.
src/libraries/System.Private.CoreLib/src/System/Reflection/Emit/TypeBuilderInstantiation.cs	Explicitly returns null for the new virtual.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Implements MLC RoType override via CoreType.NullableT identity.
src/libraries/System.Runtime/ref/System.Runtime.cs	Adds the new API to the ref assembly and TypeDelegator override surface.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds tests for Type.GetNullableUnderlyingType() on runtime types.
src/libraries/System.Reflection.MetadataLoadContext/tests/src/Tests/Type/TypeTests.Nullable.cs	Adds MLC-specific tests for nullable detection, including open-generic case.
src/libraries/System.Reflection.MetadataLoadContext/tests/System.Reflection.MetadataLoadContext.Tests.csproj	Includes the new MLC test file in compilation.

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs:99

• The new GetNullableUnderlyingType test cases don't cover the open generic definition typeof(Nullable<>). Given the API contract is "closed generic Nullable only", add a case asserting typeof(Nullable<>).GetNullableUnderlyingType() returns null to prevent regressions (and to catch the current behavior in the runtime overrides).

 [Theory]
[InlineData(typeof(int?), typeof(int))]
[InlineData(typeof(int), null)]
[InlineData(typeof(G<int>), null)]
public static void GetNullableUnderlyingType_RuntimeType(Type type, Type? expected)
{
Assert.Equal(expected, type.GetNullableUnderlyingType());
}

• Files reviewed: 15/15 changed files
• Comments generated: 4

[Copilot]

Pull request overview

Adds a new public virtual Type.GetNullableUnderlyingType() API so non-RuntimeType implementations (notably MetadataLoadContext’s RoType) can correctly recognize closed Nullable<T> and provide the underlying T. Nullable.GetUnderlyingType(Type) is updated to delegate to this virtual, mirroring the existing Enum.GetUnderlyingType() → Type.GetEnumUnderlyingType() pattern.

Changes:

• Introduce Type.GetNullableUnderlyingType() (virtual) and wire Nullable.GetUnderlyingType(Type) to call it for constructed generic types.
• Implement/forward the virtual across CoreCLR, Mono, NativeAOT, MetadataLoadContext (RoType), and common wrapper types (TypeDelegator, modified/signature types, TypeBuilderInstantiation).
• Add tests covering both RuntimeType and MetadataLoadContext behavior.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds the new public virtual GetNullableUnderlyingType() API and docs.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Updates Nullable.GetUnderlyingType to delegate to Type.GetNullableUnderlyingType().
src/coreclr/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs	CoreCLR override that recognizes Nullable<T> via method table fast-path and fallback.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Mono override implementation + internal IsNullableOfT updated to use the new virtual.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	NativeAOT override implementation.
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual to the delegated typeImpl.
src/libraries/System.Private.CoreLib/src/System/Reflection/ModifiedType.cs	Forwards the new virtual to the underlying unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureModifiedType.cs	Forwards the new virtual to the underlying unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureConstructedGenericType.cs	Sealed override returning null to preserve existing signature-type semantics.
src/libraries/System.Private.CoreLib/src/System/Reflection/Emit/TypeBuilderInstantiation.cs	Adds override to surface underlying T for constructed Nullable<T> in Reflection.Emit instantiations.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Adds RoType override (guarded) using core-type identity comparison for Nullable<T>.
src/libraries/System.Runtime/ref/System.Runtime.cs	Updates ref surface area for Type and TypeDelegator.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds direct Type.GetNullableUnderlyingType() runtime tests.
src/libraries/System.Reflection.MetadataLoadContext/tests/src/Tests/Type/TypeTests.Nullable.cs	Adds MLC tests validating both Nullable.GetUnderlyingType and the direct virtual call.
src/libraries/System.Reflection.MetadataLoadContext/tests/System.Reflection.MetadataLoadContext.Tests.csproj	Includes the new MLC test file in the test project.

Copilot's findings

• Files reviewed: 15/15 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 22/22 changed files
• Comments generated: 1

[dotnet-policy-service]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.

[Copilot]

Copilot's findings

• Files reviewed: 22/22 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 28/28 changed files
• Comments generated: 2

[AaronRobinsonMSFT]

Note

This comment was generated with assistance from GitHub Copilot.

Filed the breaking-change documentation issue: dotnet/docs#53407.

Remaining checklist item from the policy bot above:

• Email a link to the docs issue to the .NET Breaking Change Notification DL.

[AaronRobinsonMSFT]

Closes#125388
Fixes#124216

Breaking change documentation: dotnet/docs#53407

Summary

Adds a new public virtual Type.GetNullableUnderlyingType() method so that Type subclasses (e.g. MetadataLoadContext's RoType) can correctly identify Nullable types. Nullable.GetUnderlyingType() now forwards to this virtual.

This follows the same pattern as Enum.GetUnderlyingType() forwarding to Type.GetEnumUnderlyingType().

Contract

• Type.GetNullableUnderlyingType() returns a non-null result for both:
  • a constructed Nullable<T> (returns T), and
  • the generic type definition typeof(Nullable<>) (returns the generic type parameter T).
• Nullable.GetUnderlyingType(Type) continues to return null for the generic type definition for COMPAT.
• The base Type.GetNullableUnderlyingType() throws NotSupportedException. Custom Type subclasses outside the BCL must override it; this is a behavioral breaking change documented in [Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType docs#53407.

Changes

Public API

• Type.cs: New public virtual Type? GetNullableUnderlyingType() that throws NotSupportedException(SR.NotSupported_SubclassOverride) (matches IsByRefLike pattern per @MichalStrehovsky's feedback). XML doc documents that the open generic Nullable<> is treated as nullable and yields the generic type parameter.
• System.Runtime.cs / System.Reflection.Emit.cs (ref assemblies): New API + TypeDelegator/TypeBuilder/EnumBuilder/GenericTypeParameterBuilder overrides.

Nullable.GetUnderlyingType rewire

• Nullable.cs: Now delegates to the new virtual after preserving the IsGenericTypeDefinition COMPAT short-circuit.

Runtime overrides (all three runtimes)

• RuntimeType.CoreCLR.cs / RuntimeType.Mono.cs: Override that handles both constructed and open-generic cases. Open Nullable<> returns GetGenericArguments()[0] since the native fast-path can't yield a MethodTable for the formal type parameter T.
• RuntimeType.NativeAot.cs: Same handling for constructed Nullable<X> via the EEType fast-path.
• RuntimeTypeInfo.cs (NativeAOT): Added public virtual returning null (per @jkotas's feedback).
• NativeFormatRuntimeNamedTypeInfo.cs (NativeAOT): Sealed override that returns the generic parameter only when the type is typeof(Nullable<>).
• RuntimeConstructedGenericTypeInfo.cs (NativeAOT): override for constructed generics.

Reflection subclasses

• TypeDelegator.cs: Override forwarding to typeImpl.GetNullableUnderlyingType().
• SignatureType.cs / SignatureConstructedGenericType.cs / SignatureModifiedType.cs: Overrides that delegate through the generic definition.
• ModifiedType.cs: Override delegating through the unmodified type.

Reflection.Emit

• TypeBuilder.cs / EnumBuilder.cs / GenericTypeParameterBuilder.cs / TypeBuilderInstantiation.cs: Overrides returning null (or appropriate result for instantiations).
• SymbolType.cs: Override returning null so Nullable.GetUnderlyingType doesn't throw on MakeArrayType/MakePointerType/MakeByRefType results from TypeBuilder.

MetadataLoadContext

• RoType.cs: Override using CoreType.NullableT identity comparison; uses GetGenericArguments()[0] so the open Nullable<> returns the MLC-projected generic parameter rather than indexing empty GenericTypeArguments.
• RoModifiedType.cs: Override delegating through the unmodified type (required because RoModifiedType.GetGenericTypeDefinition() throws).

Tests

• NullableTests.cs: Coverage for RuntimeType (constructed + open-generic) and TypeDelegator.
• SignatureTypes.cs: Coverage for SignatureConstructedGenericType and SignatureModifiedType.
• ModifiedTypeTests.cs: New NullableModifiedTypeHolder (uses volatile delegate*<int?> to obtain a ModifiedType wrapping Nullable<int>) and tests for modified Nullable / non-Nullable.
• TypeBuilderGetNullableUnderlyingType.cs (new): Coverage for TypeBuilder, EnumBuilder, GenericTypeParameterBuilder, TypeBuilderInstantiation, and SymbolType (Array / multi-dim Array / Pointer / ByRef).
• TypeTests.Nullable.cs (MetadataLoadContext): Coverage for RoType (constructed + open-generic).

Note

This PR description was updated with assistance from GitHub Copilot.

[Copilot]

Pull request overview

This PR introduces a new public virtual Type.GetNullableUnderlyingType() API so non-RuntimeTypeType providers (notably MetadataLoadContext’s RoType) can correctly identify closed Nullable<T> types, and updates Nullable.GetUnderlyingType(Type) to delegate to this new virtual.

Changes:

• Add Type.GetNullableUnderlyingType() and implement/override it for RuntimeType (CoreCLR/Mono), NativeAOT RuntimeType, TypeDelegator, and MetadataLoadContext’s RoType.
• Change Nullable.GetUnderlyingType(Type) to forward to Type.GetNullableUnderlyingType().
• Add System.Runtime tests covering both RuntimeType and MetadataLoadContext behavior, plus a test project reference to System.Reflection.MetadataLoadContext.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds the new public virtual GetNullableUnderlyingType() method.
src/libraries/System.Private.CoreLib/src/System/RuntimeType.cs	Overrides GetNullableUnderlyingType() for runtime types.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	Adds NativeAOT override of GetNullableUnderlyingType().
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual through TypeDelegator.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Implements nullable detection for MLC RoType.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Updates Nullable.GetUnderlyingType to delegate to the new virtual.
src/libraries/System.Runtime/ref/System.Runtime.cs	Updates the ref assembly surface area for the new API and TypeDelegator override.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Switches Mono’s internal nullable check to use GetNullableUnderlyingType().
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds tests for Type.GetNullableUnderlyingType() and MLC scenarios.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System.Runtime.Tests.csproj	Adds test-time project reference to System.Reflection.MetadataLoadContext.

Copilot's findings

• Files reviewed: 10/10 changed files
• Comments generated: 3

[Copilot]

Pull request overview

Adds a new virtual Type.GetNullableUnderlyingType() so non-RuntimeType implementations (notably MetadataLoadContext’s RoType) can correctly detect Nullable<T>, and updates Nullable.GetUnderlyingType(Type) to forward to the virtual.

Changes:

• Introduces Type.GetNullableUnderlyingType() and wires Nullable.GetUnderlyingType() to call it.
• Implements overrides for CoreCLR, Mono, NativeAOT RuntimeType, and key wrapper types (TypeDelegator, modified/signature types).
• Adds tests for RuntimeType and MetadataLoadContext behavior.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds new virtual GetNullableUnderlyingType() API.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Forwards Nullable.GetUnderlyingType to Type.GetNullableUnderlyingType().
src/coreclr/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs	Implements CoreCLR RuntimeType override using MethodTable fast-path + fallback.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Implements Mono RuntimeType override; updates IsNullableOfT to use it.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	Implements NativeAOT RuntimeType override.
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual to the underlying Type.
src/libraries/System.Private.CoreLib/src/System/Reflection/ModifiedType.cs	Forwards the new virtual to the unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureModifiedType.cs	Forwards the new virtual to the unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureConstructedGenericType.cs	Explicitly returns null for the new virtual.
src/libraries/System.Private.CoreLib/src/System/Reflection/Emit/TypeBuilderInstantiation.cs	Explicitly returns null for the new virtual.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Implements MLC RoType override via CoreType.NullableT identity.
src/libraries/System.Runtime/ref/System.Runtime.cs	Adds the new API to the ref assembly and TypeDelegator override surface.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds tests for Type.GetNullableUnderlyingType() on runtime types.
src/libraries/System.Reflection.MetadataLoadContext/tests/src/Tests/Type/TypeTests.Nullable.cs	Adds MLC-specific tests for nullable detection, including open-generic case.
src/libraries/System.Reflection.MetadataLoadContext/tests/System.Reflection.MetadataLoadContext.Tests.csproj	Includes the new MLC test file in compilation.

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs:99

• The new GetNullableUnderlyingType test cases don't cover the open generic definition typeof(Nullable<>). Given the API contract is "closed generic Nullable only", add a case asserting typeof(Nullable<>).GetNullableUnderlyingType() returns null to prevent regressions (and to catch the current behavior in the runtime overrides).

 [Theory]
[InlineData(typeof(int?), typeof(int))]
[InlineData(typeof(int), null)]
[InlineData(typeof(G<int>), null)]
public static void GetNullableUnderlyingType_RuntimeType(Type type, Type? expected)
{
Assert.Equal(expected, type.GetNullableUnderlyingType());
}

• Files reviewed: 15/15 changed files
• Comments generated: 4

[Copilot]

Pull request overview

Adds a new public virtual Type.GetNullableUnderlyingType() API so non-RuntimeType implementations (notably MetadataLoadContext’s RoType) can correctly recognize closed Nullable<T> and provide the underlying T. Nullable.GetUnderlyingType(Type) is updated to delegate to this virtual, mirroring the existing Enum.GetUnderlyingType() → Type.GetEnumUnderlyingType() pattern.

Changes:

• Introduce Type.GetNullableUnderlyingType() (virtual) and wire Nullable.GetUnderlyingType(Type) to call it for constructed generic types.
• Implement/forward the virtual across CoreCLR, Mono, NativeAOT, MetadataLoadContext (RoType), and common wrapper types (TypeDelegator, modified/signature types, TypeBuilderInstantiation).
• Add tests covering both RuntimeType and MetadataLoadContext behavior.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds the new public virtual GetNullableUnderlyingType() API and docs.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Updates Nullable.GetUnderlyingType to delegate to Type.GetNullableUnderlyingType().
src/coreclr/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs	CoreCLR override that recognizes Nullable<T> via method table fast-path and fallback.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Mono override implementation + internal IsNullableOfT updated to use the new virtual.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	NativeAOT override implementation.
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual to the delegated typeImpl.
src/libraries/System.Private.CoreLib/src/System/Reflection/ModifiedType.cs	Forwards the new virtual to the underlying unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureModifiedType.cs	Forwards the new virtual to the underlying unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureConstructedGenericType.cs	Sealed override returning null to preserve existing signature-type semantics.
src/libraries/System.Private.CoreLib/src/System/Reflection/Emit/TypeBuilderInstantiation.cs	Adds override to surface underlying T for constructed Nullable<T> in Reflection.Emit instantiations.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Adds RoType override (guarded) using core-type identity comparison for Nullable<T>.
src/libraries/System.Runtime/ref/System.Runtime.cs	Updates ref surface area for Type and TypeDelegator.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds direct Type.GetNullableUnderlyingType() runtime tests.
src/libraries/System.Reflection.MetadataLoadContext/tests/src/Tests/Type/TypeTests.Nullable.cs	Adds MLC tests validating both Nullable.GetUnderlyingType and the direct virtual call.
src/libraries/System.Reflection.MetadataLoadContext/tests/System.Reflection.MetadataLoadContext.Tests.csproj	Includes the new MLC test file in the test project.

Copilot's findings

• Files reviewed: 15/15 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 22/22 changed files
• Comments generated: 1

[dotnet-policy-service]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.

[Copilot]

Copilot's findings

• Files reviewed: 22/22 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 28/28 changed files
• Comments generated: 2

[AaronRobinsonMSFT]

Note

This comment was generated with assistance from GitHub Copilot.

Filed the breaking-change documentation issue: dotnet/docs#53407.

Remaining checklist item from the policy bot above:

• Email a link to the docs issue to the .NET Breaking Change Notification DL.

[AaronRobinsonMSFT]

Closes#125388
Fixes#124216

Breaking change documentation: dotnet/docs#53407

Summary

Adds a new public virtual Type.GetNullableUnderlyingType() method so that Type subclasses (e.g. MetadataLoadContext's RoType) can correctly identify Nullable types. Nullable.GetUnderlyingType() now forwards to this virtual.

This follows the same pattern as Enum.GetUnderlyingType() forwarding to Type.GetEnumUnderlyingType().

Contract

• Type.GetNullableUnderlyingType() returns a non-null result for both:
  • a constructed Nullable<T> (returns T), and
  • the generic type definition typeof(Nullable<>) (returns the generic type parameter T).
• Nullable.GetUnderlyingType(Type) continues to return null for the generic type definition for COMPAT.
• The base Type.GetNullableUnderlyingType() throws NotSupportedException. Custom Type subclasses outside the BCL must override it; this is a behavioral breaking change documented in [Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType docs#53407.

Changes

Public API

• Type.cs: New public virtual Type? GetNullableUnderlyingType() that throws NotSupportedException(SR.NotSupported_SubclassOverride) (matches IsByRefLike pattern per @MichalStrehovsky's feedback). XML doc documents that the open generic Nullable<> is treated as nullable and yields the generic type parameter.
• System.Runtime.cs / System.Reflection.Emit.cs (ref assemblies): New API + TypeDelegator/TypeBuilder/EnumBuilder/GenericTypeParameterBuilder overrides.

Nullable.GetUnderlyingType rewire

• Nullable.cs: Now delegates to the new virtual after preserving the IsGenericTypeDefinition COMPAT short-circuit.

Runtime overrides (all three runtimes)

• RuntimeType.CoreCLR.cs / RuntimeType.Mono.cs: Override that handles both constructed and open-generic cases. Open Nullable<> returns GetGenericArguments()[0] since the native fast-path can't yield a MethodTable for the formal type parameter T.
• RuntimeType.NativeAot.cs: Same handling for constructed Nullable<X> via the EEType fast-path.
• RuntimeTypeInfo.cs (NativeAOT): Added public virtual returning null (per @jkotas's feedback).
• NativeFormatRuntimeNamedTypeInfo.cs (NativeAOT): Sealed override that returns the generic parameter only when the type is typeof(Nullable<>).
• RuntimeConstructedGenericTypeInfo.cs (NativeAOT): override for constructed generics.

Reflection subclasses

• TypeDelegator.cs: Override forwarding to typeImpl.GetNullableUnderlyingType().
• SignatureType.cs / SignatureConstructedGenericType.cs / SignatureModifiedType.cs: Overrides that delegate through the generic definition.
• ModifiedType.cs: Override delegating through the unmodified type.

Reflection.Emit

• TypeBuilder.cs / EnumBuilder.cs / GenericTypeParameterBuilder.cs / TypeBuilderInstantiation.cs: Overrides returning null (or appropriate result for instantiations).
• SymbolType.cs: Override returning null so Nullable.GetUnderlyingType doesn't throw on MakeArrayType/MakePointerType/MakeByRefType results from TypeBuilder.

MetadataLoadContext

• RoType.cs: Override using CoreType.NullableT identity comparison; uses GetGenericArguments()[0] so the open Nullable<> returns the MLC-projected generic parameter rather than indexing empty GenericTypeArguments.
• RoModifiedType.cs: Override delegating through the unmodified type (required because RoModifiedType.GetGenericTypeDefinition() throws).

Tests

• NullableTests.cs: Coverage for RuntimeType (constructed + open-generic) and TypeDelegator.
• SignatureTypes.cs: Coverage for SignatureConstructedGenericType and SignatureModifiedType.
• ModifiedTypeTests.cs: New NullableModifiedTypeHolder (uses volatile delegate*<int?> to obtain a ModifiedType wrapping Nullable<int>) and tests for modified Nullable / non-Nullable.
• TypeBuilderGetNullableUnderlyingType.cs (new): Coverage for TypeBuilder, EnumBuilder, GenericTypeParameterBuilder, TypeBuilderInstantiation, and SymbolType (Array / multi-dim Array / Pointer / ByRef).
• TypeTests.Nullable.cs (MetadataLoadContext): Coverage for RoType (constructed + open-generic).

Note

This PR description was updated with assistance from GitHub Copilot.

[Copilot]

Pull request overview

This PR introduces a new public virtual Type.GetNullableUnderlyingType() API so non-RuntimeTypeType providers (notably MetadataLoadContext’s RoType) can correctly identify closed Nullable<T> types, and updates Nullable.GetUnderlyingType(Type) to delegate to this new virtual.

Changes:

• Add Type.GetNullableUnderlyingType() and implement/override it for RuntimeType (CoreCLR/Mono), NativeAOT RuntimeType, TypeDelegator, and MetadataLoadContext’s RoType.
• Change Nullable.GetUnderlyingType(Type) to forward to Type.GetNullableUnderlyingType().
• Add System.Runtime tests covering both RuntimeType and MetadataLoadContext behavior, plus a test project reference to System.Reflection.MetadataLoadContext.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds the new public virtual GetNullableUnderlyingType() method.
src/libraries/System.Private.CoreLib/src/System/RuntimeType.cs	Overrides GetNullableUnderlyingType() for runtime types.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	Adds NativeAOT override of GetNullableUnderlyingType().
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual through TypeDelegator.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Implements nullable detection for MLC RoType.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Updates Nullable.GetUnderlyingType to delegate to the new virtual.
src/libraries/System.Runtime/ref/System.Runtime.cs	Updates the ref assembly surface area for the new API and TypeDelegator override.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Switches Mono’s internal nullable check to use GetNullableUnderlyingType().
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds tests for Type.GetNullableUnderlyingType() and MLC scenarios.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System.Runtime.Tests.csproj	Adds test-time project reference to System.Reflection.MetadataLoadContext.

Copilot's findings

• Files reviewed: 10/10 changed files
• Comments generated: 3

[Copilot]

Pull request overview

Adds a new virtual Type.GetNullableUnderlyingType() so non-RuntimeType implementations (notably MetadataLoadContext’s RoType) can correctly detect Nullable<T>, and updates Nullable.GetUnderlyingType(Type) to forward to the virtual.

Changes:

• Introduces Type.GetNullableUnderlyingType() and wires Nullable.GetUnderlyingType() to call it.
• Implements overrides for CoreCLR, Mono, NativeAOT RuntimeType, and key wrapper types (TypeDelegator, modified/signature types).
• Adds tests for RuntimeType and MetadataLoadContext behavior.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds new virtual GetNullableUnderlyingType() API.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Forwards Nullable.GetUnderlyingType to Type.GetNullableUnderlyingType().
src/coreclr/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs	Implements CoreCLR RuntimeType override using MethodTable fast-path + fallback.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Implements Mono RuntimeType override; updates IsNullableOfT to use it.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	Implements NativeAOT RuntimeType override.
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual to the underlying Type.
src/libraries/System.Private.CoreLib/src/System/Reflection/ModifiedType.cs	Forwards the new virtual to the unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureModifiedType.cs	Forwards the new virtual to the unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureConstructedGenericType.cs	Explicitly returns null for the new virtual.
src/libraries/System.Private.CoreLib/src/System/Reflection/Emit/TypeBuilderInstantiation.cs	Explicitly returns null for the new virtual.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Implements MLC RoType override via CoreType.NullableT identity.
src/libraries/System.Runtime/ref/System.Runtime.cs	Adds the new API to the ref assembly and TypeDelegator override surface.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds tests for Type.GetNullableUnderlyingType() on runtime types.
src/libraries/System.Reflection.MetadataLoadContext/tests/src/Tests/Type/TypeTests.Nullable.cs	Adds MLC-specific tests for nullable detection, including open-generic case.
src/libraries/System.Reflection.MetadataLoadContext/tests/System.Reflection.MetadataLoadContext.Tests.csproj	Includes the new MLC test file in compilation.

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs:99

• The new GetNullableUnderlyingType test cases don't cover the open generic definition typeof(Nullable<>). Given the API contract is "closed generic Nullable only", add a case asserting typeof(Nullable<>).GetNullableUnderlyingType() returns null to prevent regressions (and to catch the current behavior in the runtime overrides).

 [Theory]
[InlineData(typeof(int?), typeof(int))]
[InlineData(typeof(int), null)]
[InlineData(typeof(G<int>), null)]
public static void GetNullableUnderlyingType_RuntimeType(Type type, Type? expected)
{
Assert.Equal(expected, type.GetNullableUnderlyingType());
}

• Files reviewed: 15/15 changed files
• Comments generated: 4

[Copilot]

Pull request overview

Adds a new public virtual Type.GetNullableUnderlyingType() API so non-RuntimeType implementations (notably MetadataLoadContext’s RoType) can correctly recognize closed Nullable<T> and provide the underlying T. Nullable.GetUnderlyingType(Type) is updated to delegate to this virtual, mirroring the existing Enum.GetUnderlyingType() → Type.GetEnumUnderlyingType() pattern.

Changes:

• Introduce Type.GetNullableUnderlyingType() (virtual) and wire Nullable.GetUnderlyingType(Type) to call it for constructed generic types.
• Implement/forward the virtual across CoreCLR, Mono, NativeAOT, MetadataLoadContext (RoType), and common wrapper types (TypeDelegator, modified/signature types, TypeBuilderInstantiation).
• Add tests covering both RuntimeType and MetadataLoadContext behavior.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds the new public virtual GetNullableUnderlyingType() API and docs.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Updates Nullable.GetUnderlyingType to delegate to Type.GetNullableUnderlyingType().
src/coreclr/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs	CoreCLR override that recognizes Nullable<T> via method table fast-path and fallback.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Mono override implementation + internal IsNullableOfT updated to use the new virtual.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	NativeAOT override implementation.
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual to the delegated typeImpl.
src/libraries/System.Private.CoreLib/src/System/Reflection/ModifiedType.cs	Forwards the new virtual to the underlying unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureModifiedType.cs	Forwards the new virtual to the underlying unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureConstructedGenericType.cs	Sealed override returning null to preserve existing signature-type semantics.
src/libraries/System.Private.CoreLib/src/System/Reflection/Emit/TypeBuilderInstantiation.cs	Adds override to surface underlying T for constructed Nullable<T> in Reflection.Emit instantiations.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Adds RoType override (guarded) using core-type identity comparison for Nullable<T>.
src/libraries/System.Runtime/ref/System.Runtime.cs	Updates ref surface area for Type and TypeDelegator.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds direct Type.GetNullableUnderlyingType() runtime tests.
src/libraries/System.Reflection.MetadataLoadContext/tests/src/Tests/Type/TypeTests.Nullable.cs	Adds MLC tests validating both Nullable.GetUnderlyingType and the direct virtual call.
src/libraries/System.Reflection.MetadataLoadContext/tests/System.Reflection.MetadataLoadContext.Tests.csproj	Includes the new MLC test file in the test project.

Copilot's findings

• Files reviewed: 15/15 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 22/22 changed files
• Comments generated: 1

[dotnet-policy-service]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.

[Copilot]

Copilot's findings

• Files reviewed: 22/22 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 28/28 changed files
• Comments generated: 2

[AaronRobinsonMSFT]

Note

This comment was generated with assistance from GitHub Copilot.

Filed the breaking-change documentation issue: dotnet/docs#53407.

Remaining checklist item from the policy bot above:

• Email a link to the docs issue to the .NET Breaking Change Notification DL.

[AaronRobinsonMSFT]

Closes#125388
Fixes#124216

Breaking change documentation: dotnet/docs#53407

Summary

Adds a new public virtual Type.GetNullableUnderlyingType() method so that Type subclasses (e.g. MetadataLoadContext's RoType) can correctly identify Nullable types. Nullable.GetUnderlyingType() now forwards to this virtual.

This follows the same pattern as Enum.GetUnderlyingType() forwarding to Type.GetEnumUnderlyingType().

Contract

• Type.GetNullableUnderlyingType() returns a non-null result for both:
  • a constructed Nullable<T> (returns T), and
  • the generic type definition typeof(Nullable<>) (returns the generic type parameter T).
• Nullable.GetUnderlyingType(Type) continues to return null for the generic type definition for COMPAT.
• The base Type.GetNullableUnderlyingType() throws NotSupportedException. Custom Type subclasses outside the BCL must override it; this is a behavioral breaking change documented in [Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType docs#53407.

Changes

Public API

• Type.cs: New public virtual Type? GetNullableUnderlyingType() that throws NotSupportedException(SR.NotSupported_SubclassOverride) (matches IsByRefLike pattern per @MichalStrehovsky's feedback). XML doc documents that the open generic Nullable<> is treated as nullable and yields the generic type parameter.
• System.Runtime.cs / System.Reflection.Emit.cs (ref assemblies): New API + TypeDelegator/TypeBuilder/EnumBuilder/GenericTypeParameterBuilder overrides.

Nullable.GetUnderlyingType rewire

• Nullable.cs: Now delegates to the new virtual after preserving the IsGenericTypeDefinition COMPAT short-circuit.

Runtime overrides (all three runtimes)

• RuntimeType.CoreCLR.cs / RuntimeType.Mono.cs: Override that handles both constructed and open-generic cases. Open Nullable<> returns GetGenericArguments()[0] since the native fast-path can't yield a MethodTable for the formal type parameter T.
• RuntimeType.NativeAot.cs: Same handling for constructed Nullable<X> via the EEType fast-path.
• RuntimeTypeInfo.cs (NativeAOT): Added public virtual returning null (per @jkotas's feedback).
• NativeFormatRuntimeNamedTypeInfo.cs (NativeAOT): Sealed override that returns the generic parameter only when the type is typeof(Nullable<>).
• RuntimeConstructedGenericTypeInfo.cs (NativeAOT): override for constructed generics.

Reflection subclasses

• TypeDelegator.cs: Override forwarding to typeImpl.GetNullableUnderlyingType().
• SignatureType.cs / SignatureConstructedGenericType.cs / SignatureModifiedType.cs: Overrides that delegate through the generic definition.
• ModifiedType.cs: Override delegating through the unmodified type.

Reflection.Emit

• TypeBuilder.cs / EnumBuilder.cs / GenericTypeParameterBuilder.cs / TypeBuilderInstantiation.cs: Overrides returning null (or appropriate result for instantiations).
• SymbolType.cs: Override returning null so Nullable.GetUnderlyingType doesn't throw on MakeArrayType/MakePointerType/MakeByRefType results from TypeBuilder.

MetadataLoadContext

• RoType.cs: Override using CoreType.NullableT identity comparison; uses GetGenericArguments()[0] so the open Nullable<> returns the MLC-projected generic parameter rather than indexing empty GenericTypeArguments.
• RoModifiedType.cs: Override delegating through the unmodified type (required because RoModifiedType.GetGenericTypeDefinition() throws).

Tests

• NullableTests.cs: Coverage for RuntimeType (constructed + open-generic) and TypeDelegator.
• SignatureTypes.cs: Coverage for SignatureConstructedGenericType and SignatureModifiedType.
• ModifiedTypeTests.cs: New NullableModifiedTypeHolder (uses volatile delegate*<int?> to obtain a ModifiedType wrapping Nullable<int>) and tests for modified Nullable / non-Nullable.
• TypeBuilderGetNullableUnderlyingType.cs (new): Coverage for TypeBuilder, EnumBuilder, GenericTypeParameterBuilder, TypeBuilderInstantiation, and SymbolType (Array / multi-dim Array / Pointer / ByRef).
• TypeTests.Nullable.cs (MetadataLoadContext): Coverage for RoType (constructed + open-generic).

Note

This PR description was updated with assistance from GitHub Copilot.

[Copilot]

Pull request overview

This PR introduces a new public virtual Type.GetNullableUnderlyingType() API so non-RuntimeTypeType providers (notably MetadataLoadContext’s RoType) can correctly identify closed Nullable<T> types, and updates Nullable.GetUnderlyingType(Type) to delegate to this new virtual.

Changes:

• Add Type.GetNullableUnderlyingType() and implement/override it for RuntimeType (CoreCLR/Mono), NativeAOT RuntimeType, TypeDelegator, and MetadataLoadContext’s RoType.
• Change Nullable.GetUnderlyingType(Type) to forward to Type.GetNullableUnderlyingType().
• Add System.Runtime tests covering both RuntimeType and MetadataLoadContext behavior, plus a test project reference to System.Reflection.MetadataLoadContext.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds the new public virtual GetNullableUnderlyingType() method.
src/libraries/System.Private.CoreLib/src/System/RuntimeType.cs	Overrides GetNullableUnderlyingType() for runtime types.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	Adds NativeAOT override of GetNullableUnderlyingType().
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual through TypeDelegator.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Implements nullable detection for MLC RoType.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Updates Nullable.GetUnderlyingType to delegate to the new virtual.
src/libraries/System.Runtime/ref/System.Runtime.cs	Updates the ref assembly surface area for the new API and TypeDelegator override.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Switches Mono’s internal nullable check to use GetNullableUnderlyingType().
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds tests for Type.GetNullableUnderlyingType() and MLC scenarios.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System.Runtime.Tests.csproj	Adds test-time project reference to System.Reflection.MetadataLoadContext.

Copilot's findings

• Files reviewed: 10/10 changed files
• Comments generated: 3

[Copilot]

Pull request overview

Adds a new virtual Type.GetNullableUnderlyingType() so non-RuntimeType implementations (notably MetadataLoadContext’s RoType) can correctly detect Nullable<T>, and updates Nullable.GetUnderlyingType(Type) to forward to the virtual.

Changes:

• Introduces Type.GetNullableUnderlyingType() and wires Nullable.GetUnderlyingType() to call it.
• Implements overrides for CoreCLR, Mono, NativeAOT RuntimeType, and key wrapper types (TypeDelegator, modified/signature types).
• Adds tests for RuntimeType and MetadataLoadContext behavior.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds new virtual GetNullableUnderlyingType() API.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Forwards Nullable.GetUnderlyingType to Type.GetNullableUnderlyingType().
src/coreclr/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs	Implements CoreCLR RuntimeType override using MethodTable fast-path + fallback.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Implements Mono RuntimeType override; updates IsNullableOfT to use it.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	Implements NativeAOT RuntimeType override.
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual to the underlying Type.
src/libraries/System.Private.CoreLib/src/System/Reflection/ModifiedType.cs	Forwards the new virtual to the unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureModifiedType.cs	Forwards the new virtual to the unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureConstructedGenericType.cs	Explicitly returns null for the new virtual.
src/libraries/System.Private.CoreLib/src/System/Reflection/Emit/TypeBuilderInstantiation.cs	Explicitly returns null for the new virtual.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Implements MLC RoType override via CoreType.NullableT identity.
src/libraries/System.Runtime/ref/System.Runtime.cs	Adds the new API to the ref assembly and TypeDelegator override surface.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds tests for Type.GetNullableUnderlyingType() on runtime types.
src/libraries/System.Reflection.MetadataLoadContext/tests/src/Tests/Type/TypeTests.Nullable.cs	Adds MLC-specific tests for nullable detection, including open-generic case.
src/libraries/System.Reflection.MetadataLoadContext/tests/System.Reflection.MetadataLoadContext.Tests.csproj	Includes the new MLC test file in compilation.

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs:99

• The new GetNullableUnderlyingType test cases don't cover the open generic definition typeof(Nullable<>). Given the API contract is "closed generic Nullable only", add a case asserting typeof(Nullable<>).GetNullableUnderlyingType() returns null to prevent regressions (and to catch the current behavior in the runtime overrides).

 [Theory]
[InlineData(typeof(int?), typeof(int))]
[InlineData(typeof(int), null)]
[InlineData(typeof(G<int>), null)]
public static void GetNullableUnderlyingType_RuntimeType(Type type, Type? expected)
{
Assert.Equal(expected, type.GetNullableUnderlyingType());
}

• Files reviewed: 15/15 changed files
• Comments generated: 4

[Copilot]

Pull request overview

Adds a new public virtual Type.GetNullableUnderlyingType() API so non-RuntimeType implementations (notably MetadataLoadContext’s RoType) can correctly recognize closed Nullable<T> and provide the underlying T. Nullable.GetUnderlyingType(Type) is updated to delegate to this virtual, mirroring the existing Enum.GetUnderlyingType() → Type.GetEnumUnderlyingType() pattern.

Changes:

• Introduce Type.GetNullableUnderlyingType() (virtual) and wire Nullable.GetUnderlyingType(Type) to call it for constructed generic types.
• Implement/forward the virtual across CoreCLR, Mono, NativeAOT, MetadataLoadContext (RoType), and common wrapper types (TypeDelegator, modified/signature types, TypeBuilderInstantiation).
• Add tests covering both RuntimeType and MetadataLoadContext behavior.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds the new public virtual GetNullableUnderlyingType() API and docs.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Updates Nullable.GetUnderlyingType to delegate to Type.GetNullableUnderlyingType().
src/coreclr/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs	CoreCLR override that recognizes Nullable<T> via method table fast-path and fallback.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Mono override implementation + internal IsNullableOfT updated to use the new virtual.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	NativeAOT override implementation.
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual to the delegated typeImpl.
src/libraries/System.Private.CoreLib/src/System/Reflection/ModifiedType.cs	Forwards the new virtual to the underlying unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureModifiedType.cs	Forwards the new virtual to the underlying unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureConstructedGenericType.cs	Sealed override returning null to preserve existing signature-type semantics.
src/libraries/System.Private.CoreLib/src/System/Reflection/Emit/TypeBuilderInstantiation.cs	Adds override to surface underlying T for constructed Nullable<T> in Reflection.Emit instantiations.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Adds RoType override (guarded) using core-type identity comparison for Nullable<T>.
src/libraries/System.Runtime/ref/System.Runtime.cs	Updates ref surface area for Type and TypeDelegator.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds direct Type.GetNullableUnderlyingType() runtime tests.
src/libraries/System.Reflection.MetadataLoadContext/tests/src/Tests/Type/TypeTests.Nullable.cs	Adds MLC tests validating both Nullable.GetUnderlyingType and the direct virtual call.
src/libraries/System.Reflection.MetadataLoadContext/tests/System.Reflection.MetadataLoadContext.Tests.csproj	Includes the new MLC test file in the test project.

Copilot's findings

• Files reviewed: 15/15 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 22/22 changed files
• Comments generated: 1

[dotnet-policy-service]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.

[Copilot]

Copilot's findings

• Files reviewed: 22/22 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 28/28 changed files
• Comments generated: 2

[AaronRobinsonMSFT]

Note

This comment was generated with assistance from GitHub Copilot.

Filed the breaking-change documentation issue: dotnet/docs#53407.

Remaining checklist item from the policy bot above:

• Email a link to the docs issue to the .NET Breaking Change Notification DL.

[AaronRobinsonMSFT]

Closes#125388
Fixes#124216

Breaking change documentation: dotnet/docs#53407

Summary

Adds a new public virtual Type.GetNullableUnderlyingType() method so that Type subclasses (e.g. MetadataLoadContext's RoType) can correctly identify Nullable types. Nullable.GetUnderlyingType() now forwards to this virtual.

This follows the same pattern as Enum.GetUnderlyingType() forwarding to Type.GetEnumUnderlyingType().

Contract

• Type.GetNullableUnderlyingType() returns a non-null result for both:
  • a constructed Nullable<T> (returns T), and
  • the generic type definition typeof(Nullable<>) (returns the generic type parameter T).
• Nullable.GetUnderlyingType(Type) continues to return null for the generic type definition for COMPAT.
• The base Type.GetNullableUnderlyingType() throws NotSupportedException. Custom Type subclasses outside the BCL must override it; this is a behavioral breaking change documented in [Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType docs#53407.

Changes

Public API

• Type.cs: New public virtual Type? GetNullableUnderlyingType() that throws NotSupportedException(SR.NotSupported_SubclassOverride) (matches IsByRefLike pattern per @MichalStrehovsky's feedback). XML doc documents that the open generic Nullable<> is treated as nullable and yields the generic type parameter.
• System.Runtime.cs / System.Reflection.Emit.cs (ref assemblies): New API + TypeDelegator/TypeBuilder/EnumBuilder/GenericTypeParameterBuilder overrides.

Nullable.GetUnderlyingType rewire

• Nullable.cs: Now delegates to the new virtual after preserving the IsGenericTypeDefinition COMPAT short-circuit.

Runtime overrides (all three runtimes)

• RuntimeType.CoreCLR.cs / RuntimeType.Mono.cs: Override that handles both constructed and open-generic cases. Open Nullable<> returns GetGenericArguments()[0] since the native fast-path can't yield a MethodTable for the formal type parameter T.
• RuntimeType.NativeAot.cs: Same handling for constructed Nullable<X> via the EEType fast-path.
• RuntimeTypeInfo.cs (NativeAOT): Added public virtual returning null (per @jkotas's feedback).
• NativeFormatRuntimeNamedTypeInfo.cs (NativeAOT): Sealed override that returns the generic parameter only when the type is typeof(Nullable<>).
• RuntimeConstructedGenericTypeInfo.cs (NativeAOT): override for constructed generics.

Reflection subclasses

• TypeDelegator.cs: Override forwarding to typeImpl.GetNullableUnderlyingType().
• SignatureType.cs / SignatureConstructedGenericType.cs / SignatureModifiedType.cs: Overrides that delegate through the generic definition.
• ModifiedType.cs: Override delegating through the unmodified type.

Reflection.Emit

• TypeBuilder.cs / EnumBuilder.cs / GenericTypeParameterBuilder.cs / TypeBuilderInstantiation.cs: Overrides returning null (or appropriate result for instantiations).
• SymbolType.cs: Override returning null so Nullable.GetUnderlyingType doesn't throw on MakeArrayType/MakePointerType/MakeByRefType results from TypeBuilder.

MetadataLoadContext

• RoType.cs: Override using CoreType.NullableT identity comparison; uses GetGenericArguments()[0] so the open Nullable<> returns the MLC-projected generic parameter rather than indexing empty GenericTypeArguments.
• RoModifiedType.cs: Override delegating through the unmodified type (required because RoModifiedType.GetGenericTypeDefinition() throws).

Tests

• NullableTests.cs: Coverage for RuntimeType (constructed + open-generic) and TypeDelegator.
• SignatureTypes.cs: Coverage for SignatureConstructedGenericType and SignatureModifiedType.
• ModifiedTypeTests.cs: New NullableModifiedTypeHolder (uses volatile delegate*<int?> to obtain a ModifiedType wrapping Nullable<int>) and tests for modified Nullable / non-Nullable.
• TypeBuilderGetNullableUnderlyingType.cs (new): Coverage for TypeBuilder, EnumBuilder, GenericTypeParameterBuilder, TypeBuilderInstantiation, and SymbolType (Array / multi-dim Array / Pointer / ByRef).
• TypeTests.Nullable.cs (MetadataLoadContext): Coverage for RoType (constructed + open-generic).

Note

This PR description was updated with assistance from GitHub Copilot.

[Copilot]

Pull request overview

This PR introduces a new public virtual Type.GetNullableUnderlyingType() API so non-RuntimeTypeType providers (notably MetadataLoadContext’s RoType) can correctly identify closed Nullable<T> types, and updates Nullable.GetUnderlyingType(Type) to delegate to this new virtual.

Changes:

• Add Type.GetNullableUnderlyingType() and implement/override it for RuntimeType (CoreCLR/Mono), NativeAOT RuntimeType, TypeDelegator, and MetadataLoadContext’s RoType.
• Change Nullable.GetUnderlyingType(Type) to forward to Type.GetNullableUnderlyingType().
• Add System.Runtime tests covering both RuntimeType and MetadataLoadContext behavior, plus a test project reference to System.Reflection.MetadataLoadContext.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds the new public virtual GetNullableUnderlyingType() method.
src/libraries/System.Private.CoreLib/src/System/RuntimeType.cs	Overrides GetNullableUnderlyingType() for runtime types.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	Adds NativeAOT override of GetNullableUnderlyingType().
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual through TypeDelegator.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Implements nullable detection for MLC RoType.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Updates Nullable.GetUnderlyingType to delegate to the new virtual.
src/libraries/System.Runtime/ref/System.Runtime.cs	Updates the ref assembly surface area for the new API and TypeDelegator override.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Switches Mono’s internal nullable check to use GetNullableUnderlyingType().
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds tests for Type.GetNullableUnderlyingType() and MLC scenarios.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System.Runtime.Tests.csproj	Adds test-time project reference to System.Reflection.MetadataLoadContext.

Copilot's findings

• Files reviewed: 10/10 changed files
• Comments generated: 3

[Copilot]

Pull request overview

Adds a new virtual Type.GetNullableUnderlyingType() so non-RuntimeType implementations (notably MetadataLoadContext’s RoType) can correctly detect Nullable<T>, and updates Nullable.GetUnderlyingType(Type) to forward to the virtual.

Changes:

• Introduces Type.GetNullableUnderlyingType() and wires Nullable.GetUnderlyingType() to call it.
• Implements overrides for CoreCLR, Mono, NativeAOT RuntimeType, and key wrapper types (TypeDelegator, modified/signature types).
• Adds tests for RuntimeType and MetadataLoadContext behavior.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds new virtual GetNullableUnderlyingType() API.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Forwards Nullable.GetUnderlyingType to Type.GetNullableUnderlyingType().
src/coreclr/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs	Implements CoreCLR RuntimeType override using MethodTable fast-path + fallback.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Implements Mono RuntimeType override; updates IsNullableOfT to use it.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	Implements NativeAOT RuntimeType override.
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual to the underlying Type.
src/libraries/System.Private.CoreLib/src/System/Reflection/ModifiedType.cs	Forwards the new virtual to the unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureModifiedType.cs	Forwards the new virtual to the unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureConstructedGenericType.cs	Explicitly returns null for the new virtual.
src/libraries/System.Private.CoreLib/src/System/Reflection/Emit/TypeBuilderInstantiation.cs	Explicitly returns null for the new virtual.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Implements MLC RoType override via CoreType.NullableT identity.
src/libraries/System.Runtime/ref/System.Runtime.cs	Adds the new API to the ref assembly and TypeDelegator override surface.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds tests for Type.GetNullableUnderlyingType() on runtime types.
src/libraries/System.Reflection.MetadataLoadContext/tests/src/Tests/Type/TypeTests.Nullable.cs	Adds MLC-specific tests for nullable detection, including open-generic case.
src/libraries/System.Reflection.MetadataLoadContext/tests/System.Reflection.MetadataLoadContext.Tests.csproj	Includes the new MLC test file in compilation.

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs:99

• The new GetNullableUnderlyingType test cases don't cover the open generic definition typeof(Nullable<>). Given the API contract is "closed generic Nullable only", add a case asserting typeof(Nullable<>).GetNullableUnderlyingType() returns null to prevent regressions (and to catch the current behavior in the runtime overrides).

 [Theory]
[InlineData(typeof(int?), typeof(int))]
[InlineData(typeof(int), null)]
[InlineData(typeof(G<int>), null)]
public static void GetNullableUnderlyingType_RuntimeType(Type type, Type? expected)
{
Assert.Equal(expected, type.GetNullableUnderlyingType());
}

• Files reviewed: 15/15 changed files
• Comments generated: 4

[Copilot]

Pull request overview

Adds a new public virtual Type.GetNullableUnderlyingType() API so non-RuntimeType implementations (notably MetadataLoadContext’s RoType) can correctly recognize closed Nullable<T> and provide the underlying T. Nullable.GetUnderlyingType(Type) is updated to delegate to this virtual, mirroring the existing Enum.GetUnderlyingType() → Type.GetEnumUnderlyingType() pattern.

Changes:

• Introduce Type.GetNullableUnderlyingType() (virtual) and wire Nullable.GetUnderlyingType(Type) to call it for constructed generic types.
• Implement/forward the virtual across CoreCLR, Mono, NativeAOT, MetadataLoadContext (RoType), and common wrapper types (TypeDelegator, modified/signature types, TypeBuilderInstantiation).
• Add tests covering both RuntimeType and MetadataLoadContext behavior.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Type.cs	Adds the new public virtual GetNullableUnderlyingType() API and docs.
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Updates Nullable.GetUnderlyingType to delegate to Type.GetNullableUnderlyingType().
src/coreclr/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs	CoreCLR override that recognizes Nullable<T> via method table fast-path and fallback.
src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs	Mono override implementation + internal IsNullableOfT updated to use the new virtual.
src/coreclr/nativeaot/System.Private.CoreLib/src/System/RuntimeType.NativeAot.cs	NativeAOT override implementation.
src/libraries/System.Private.CoreLib/src/System/Reflection/TypeDelegator.cs	Forwards the new virtual to the delegated typeImpl.
src/libraries/System.Private.CoreLib/src/System/Reflection/ModifiedType.cs	Forwards the new virtual to the underlying unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureModifiedType.cs	Forwards the new virtual to the underlying unmodified type.
src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureConstructedGenericType.cs	Sealed override returning null to preserve existing signature-type semantics.
src/libraries/System.Private.CoreLib/src/System/Reflection/Emit/TypeBuilderInstantiation.cs	Adds override to surface underlying T for constructed Nullable<T> in Reflection.Emit instantiations.
src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoType.cs	Adds RoType override (guarded) using core-type identity comparison for Nullable<T>.
src/libraries/System.Runtime/ref/System.Runtime.cs	Updates ref surface area for Type and TypeDelegator.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds direct Type.GetNullableUnderlyingType() runtime tests.
src/libraries/System.Reflection.MetadataLoadContext/tests/src/Tests/Type/TypeTests.Nullable.cs	Adds MLC tests validating both Nullable.GetUnderlyingType and the direct virtual call.
src/libraries/System.Reflection.MetadataLoadContext/tests/System.Reflection.MetadataLoadContext.Tests.csproj	Includes the new MLC test file in the test project.

Copilot's findings

• Files reviewed: 15/15 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 22/22 changed files
• Comments generated: 1

[dotnet-policy-service]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.

[Copilot]

Copilot's findings

• Files reviewed: 22/22 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 28/28 changed files
• Comments generated: 2

[AaronRobinsonMSFT]

Note

This comment was generated with assistance from GitHub Copilot.

Filed the breaking-change documentation issue: dotnet/docs#53407.

Remaining checklist item from the policy bot above:

• Email a link to the docs issue to the .NET Breaking Change Notification DL.