Null reference on equality with nested record

[Szer]

Two snippets below fails with NRE

1. Equality

type Bar = { b: obj }
type Foo = { f: Bar | null }
let a = { f = null }
let b = { f = null }
a = b

Error

System.NullReferenceException: Object reference not set to an instance of an object.
 at FSI_0045.Foo.Equals(Foo obj, IEqualityComparer comp)
 at <StartupCode$FSI_0048>.$FSI_0048.main@()
 at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
 at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
Stopped due to error

2. Hash code

type Bar = { b: obj }
type Foo = { f: Bar | null }
let a = { f = null }
a.GetHashCode()

Error

System.NullReferenceException: Object reference not set to an instance of an object.
 at FSI_0017.Foo.GetHashCode(IEqualityComparer comp)
 at FSI_0017.Foo.GetHashCode()
 at <StartupCode$FSI_0019>.$FSI_0019.main@()
 at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
 at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
Stopped due to error

Known workarounds

change Bar to be NON record

type Bar = class end
type Foo = { f: Bar | null }
let a = { f = null }
let b = { f = null }
a = b // works now

Related information

• Windows 10
• Microsoft (R) F# Interactive version 12.9.101.0 for F# 9.0

dotnet --list-sdks
5.0.408 [C:\Program Files\dotnet\sdk]
6.0.203 [C:\Program Files\dotnet\sdk]
6.0.321 [C:\Program Files\dotnet\sdk]
6.0.428 [C:\Program Files\dotnet\sdk]
7.0.120 [C:\Program Files\dotnet\sdk]
7.0.203 [C:\Program Files\dotnet\sdk]
7.0.317 [C:\Program Files\dotnet\sdk]
7.0.410 [C:\Program Files\dotnet\sdk]
8.0.112 [C:\Program Files\dotnet\sdk]
8.0.308 [C:\Program Files\dotnet\sdk]
8.0.405 [C:\Program Files\dotnet\sdk]
9.0.102 [C:\Program Files\dotnet\sdk]

[vzarytovskii]

@T-Gro equality codegen is not accounting for the nullability now it seems, before we would've had when trying to assign null directly, you would've had

 error FS0043: The type 'Bar' does not have 'null' as a proper value
 error FS0043: The type 'Bar' does not have 'null' as a proper value

Although you would've hit NRE when using unchecked default value, this makes it easier to achieve.

Since now we allow | null for records, nullability will be more explicit and used. Should we generate a defensive nullcheck? I think any performance hit will be negligible.

[Szer]

This code also fails with NRE

type Bar = { b: obj }
type Foo = { f: Bar | null }
let a = { f = null }
a.GetHashCode()

System.NullReferenceException: Object reference not set to an instance of an object.
 at FSI_0017.Foo.GetHashCode(IEqualityComparer comp)
 at FSI_0017.Foo.GetHashCode()
 at <StartupCode$FSI_0019>.$FSI_0019.main@()
 at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
 at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
Stopped due to error

Added that to the first post

[vzarytovskii]

If someone wants to fix it, it's a really easy one, for equality, there needs to be a tester added for each field, before .Equals(that) call is generated, around here (around when we mapping over the fields), and made a part of expression:

fsharp/src/Compiler/Checking/AugmentWithHashCompare.fs

Lines 355 to 364 in 58560f8

	let expr = mkEqualsTestConjuncts g m (List.map mkTest fields)
	let expr =
	if tycon.IsStructOrEnumTycon then
	expr
	else
	mkBindThatNullEquals g m thise thataddre expr
	let thatv, expr = mkThatVarBind g m ty thataddrv expr
	thisv, thatv, expr

Hashing will be somewhere around there as well.