Breaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11) - #53409

Merged
AaronRobinsonMSFT merged 6 commits into
mainfrom
copilot/fix-nullable-getunderlyingtype-issue
Apr 29, 2026
Merged

Breaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11)#53409
AaronRobinsonMSFT merged 6 commits into
mainfrom
copilot/fix-nullable-getunderlyingtype-issue

Conversation

CopilotAI commented Apr 28, 2026•
edited by github-actions Bot
Loading

Copy link
Copy Markdown
Contributor

Closes#53407

In .NET 11 Preview 4, Nullable.GetUnderlyingType was changed to delegate to the new Type.GetNullableUnderlyingType() virtual. The base implementation throws NotSupportedException, so custom Type subclasses that don't override it now throw where they previously returned null.

Changes

  • New articledocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md — covers previous/new behavior, reason (fixes MetadataLoadContext nullable detection bug), and three override patterns:
    • Return null (subclass never represents Nullable<T>)
    • Inspect generic type definition and return the type argument
    • Delegate to an inner wrapped Type
  • docs/core/compatibility/toc.yml — adds entry under .NET 11 / Core .NET libraries
  • docs/core/compatibility/11.md — adds row to the Core .NET libraries breaking changes table

Affected APIs

  • System.Nullable.GetUnderlyingType(System.Type)
  • System.Type.GetNullableUnderlyingType (new virtual)

Internal previews

📄 File🔗 Preview link
docs/core/compatibility/11.mdBreaking changes in .NET 11
docs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.mdNullable.GetUnderlyingType throws for custom Type subclasses
docs/core/compatibility/toc.ymldocs/core/compatibility/toc

CopilotAI linked an issue Apr 28, 2026 that may be closed by this pull request
[Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType #53407
Closed
1 task
@gewarren
Add breaking change article: Nullable.GetUnderlyingType throws for cu…
3167202
…stom Type subclasses
Agent-Logs-Url: https://github.com/dotnet/docs/sessions/2c45f021-3ef8-4caf-855a-7ea80054cb39
Co-authored-by: gewarren <24882762+gewarren@users.noreply.github.com>
CopilotAI changed the title [WIP] Fix Nullable.GetUnderlyingType to handle custom Type subclassesBreaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11)Apr 28, 2026
CopilotAI requested a review from gewarrenApril 28, 2026 21:29
jkotas
jkotas reviewed Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md Outdated
jkotas
jkotas reviewed Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md Outdated
jkotas
jkotas reviewed Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md Outdated
@AaronRobinsonMSFT@jkotas
Apply suggestions from code review
6d724ac
Co-authored-by: Jan Kotas <jkotas@microsoft.com>
AaronRobinsonMSFT
AaronRobinsonMSFT reviewed Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md Outdated
@AaronRobinsonMSFT

AaronRobinsonMSFT commented Apr 29, 2026

Copy link
Copy Markdown
Member

@gewarren@jkotas These changes have been merged. I think it will be going into .NET 11 Preview 4 - dotnet/runtime#126905.

@gewarren
gewarren marked this pull request as ready for review April 29, 2026 16:26
@gewarren
gewarren requested a review from a team as a code ownerApril 29, 2026 16:26
CopilotAI review requested due to automatic review settings April 29, 2026 16:26
CopilotAI reviewed Apr 29, 2026
View reviewed changes

CopilotAI left a comment

Copy link
Copy Markdown
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Pull request overview

Adds a new .NET 11 breaking change article documenting that Nullable.GetUnderlyingType(Type) now forwards to Type.GetNullableUnderlyingType() and can throw NotSupportedException for custom Type subclasses that don’t override the new virtual.

Changes:

  • Adds a new breaking change article describing the behavior change, rationale, and recommended override patterns.
  • Updates the .NET compatibility TOC to include the new article under .NET 11 core libraries.
  • Updates the .NET 11 breaking changes index table to reference the new article.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

FileDescription
docs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.mdNew breaking change article documenting the new forwarding behavior and mitigation guidance.
docs/core/compatibility/toc.ymlAdds the new breaking change article to the navigation tree.
docs/core/compatibility/11.mdAdds the new breaking change entry to the .NET 11 index table.

Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md Outdated
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md Outdated
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md Outdated
@AaronRobinsonMSFT
Apply suggestions from code review
3774b56
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
@AaronRobinsonMSFT

AaronRobinsonMSFT commented Apr 29, 2026

Copy link
Copy Markdown
Member

@gewarren I'm seeing

Cross reference not found: 'System.Type.GetNullableUnderlyingType'.

in https://buildapi.docs.microsoft.com/Output/PullRequest/a3bda507-3390-de91-8f7f-26f90f4e5fc8/202604291851254639-53409/BuildReport?accessString=aa4585a1989a22ee47264fec513db0a7ccd24ebb715277e92091f33bdc1d25d9. Is this a real error?

@gewarren

gewarren commented Apr 29, 2026

Copy link
Copy Markdown
Collaborator

@gewarren I'm seeing

Cross reference not found: 'System.Type.GetNullableUnderlyingType'.

in https://buildapi.docs.microsoft.com/Output/PullRequest/a3bda507-3390-de91-8f7f-26f90f4e5fc8/202604291851254639-53409/BuildReport?accessString=aa4585a1989a22ee47264fec513db0a7ccd24ebb715277e92091f33bdc1d25d9. Is this a real error?

@AaronRobinsonMSFT Yes, it's a real error because that method isn't published in the .NET API reference. I assume this is a new API for Preview 4? We can either wait to merge this until the Preview 4 APIs are published, or we can just put the API name in backticks for now (instead of an xref).

AaronRobinsonMSFT
AaronRobinsonMSFT reviewed Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md Outdated
AaronRobinsonMSFT
AaronRobinsonMSFT reviewed Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md Outdated
@AaronRobinsonMSFT
Apply suggestions from code review
37f987d
Co-authored-by: Aaron R Robinson <arobins@microsoft.com>
@AaronRobinsonMSFT
AaronRobinsonMSFT merged commit 081c4ba into mainApr 29, 2026
11 checks passed
@AaronRobinsonMSFT
AaronRobinsonMSFT deleted the copilot/fix-nullable-getunderlyingtype-issue branch April 29, 2026 21:45
Sign up for freeto join this conversation on GitHub. Already have an account? Sign in to comment

Reviewers

@jkotasjkotasjkotas left review comments
Copilot code reviewCopilot
Copilot review effort, defaults to Lite
Applies to this pull request for everyone.Learn more about Copilot code review.
Copilot left review comments
@AaronRobinsonMSFTAaronRobinsonMSFTAaronRobinsonMSFT approved these changes
+1 more reviewer
@gewarrengewarrengewarren approved these changes
Reviewers whose approvals may not affect merge requirements

Assignees

@gewarrengewarren

Copilot code reviewCopilot

Labels

dotnet-fundamentals/svc

Projects

None yet

Milestone

No milestone

Development

Successfully merging this pull request may close these issues.

[Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType

5 participants

@AaronRobinsonMSFT@gewarren@jkotas
, 'i'); if (__m === '*' || __re.test(location.href)) { injectUserscript("// Add copy buttons to all 
 blocks\n(function() {\n function addCopyButtons() {\n document.querySelectorAll('pre code').forEach(function(codeBlock) {\n if (codeBlock.parentElement.hasAttribute('data-copy-added')) return;\n codeBlock.parentElement.setAttribute('data-copy-added', 'true');\n \n var btn = document.createElement('button');\n btn.textContent = 'Copy';\n btn.style.cssText = 'position:absolute;top:4px;right:4px;padding:2px 8px;font-size:11px;background:#4ecdc4;border:none;border-radius:4px;color:#1a1a2e;cursor:pointer;opacity:0.7;transition:opacity 0.2s;';\n btn.onmouseover = function() { this.style.opacity = '1'; };\n btn.onmouseout = function() { this.style.opacity = '0.7'; };\n btn.onclick = function() {\n navigator.clipboard.writeText(codeBlock.textContent).then(function() {\n btn.textContent = 'Copied!';\n setTimeout(function() { btn.textContent = 'Copy'; }, 1500);\n });\n };\n codeBlock.parentElement.style.position = 'relative';\n codeBlock.parentElement.appendChild(btn);\n });\n }\n \n addCopyButtons();\n \n // Re-run on dynamic content\n var observer = new MutationObserver(addCopyButtons);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Add Copy Buttons to Code Blocks");
}
} catch(__e) { console.warn('[Userscript:Add Copy Buttons to Code Blocks]', __e); }
})();
(function(){
try {
var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '

Skip to content
Breaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11) - #53409
Merged
AaronRobinsonMSFT merged 6 commits into
mainfrom 
copilot/fix-nullable-getunderlyingtype-issue
Apr 29, 2026
Merged
Breaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11)#53409
AaronRobinsonMSFT merged 6 commits into
mainfrom 
copilot/fix-nullable-getunderlyingtype-issue
Conversation
CopilotAI
commented
Apr 28, 2026•

edited by github-actions
Bot
Loading

Copy link


Copy Markdown

Contributor
Closes#53407
In .NET 11 Preview 4, Nullable.GetUnderlyingType was changed to delegate to the new Type.GetNullableUnderlyingType() virtual. The base implementation throws NotSupportedException, so custom Type subclasses that don't override it now throw where they previously returned null.
Changes
  • New articledocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md — covers previous/new behavior, reason (fixes MetadataLoadContext nullable detection bug), and three override patterns:

    • Return null (subclass never represents Nullable<T>)
    • Inspect generic type definition and return the type argument
    • Delegate to an inner wrapped Type
  • docs/core/compatibility/toc.yml — adds entry under .NET 11 / Core .NET libraries
  • docs/core/compatibility/11.md — adds row to the Core .NET libraries breaking changes table
Affected APIs
  • System.Nullable.GetUnderlyingType(System.Type)
  • System.Type.GetNullableUnderlyingType (new virtual)
Internal previews
📄 File🔗 Preview link
docs/core/compatibility/11.mdBreaking changes in .NET 11
docs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.mdNullable.GetUnderlyingType throws for custom Type subclasses
docs/core/compatibility/toc.ymldocs/core/compatibility/toc
CopilotAI
linked an issue
Apr 28, 2026
that may be
closed
by this pull request


[Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType
#53407

Closed

1 task
@gewarren
Add breaking change article: Nullable.GetUnderlyingType throws for cu…
3167202
…stom Type subclasses
Agent-Logs-Url: https://github.com/dotnet/docs/sessions/2c45f021-3ef8-4caf-855a-7ea80054cb39
Co-authored-by: gewarren <24882762+gewarren@users.noreply.github.com>
CopilotAI
changed the title
[WIP] Fix Nullable.GetUnderlyingType to handle custom Type subclassesBreaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11)Apr 28, 2026
CopilotAI
requested a review
from gewarrenApril 28, 2026 21:29
jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT@jkotas
Apply suggestions from code review
6d724ac
Co-authored-by: Jan Kotas <jkotas@microsoft.com>
AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
AaronRobinsonMSFT
commented
Apr 29, 2026

Copy link


Copy Markdown

Member
@gewarren@jkotas These changes have been merged. I think it will be going into .NET 11 Preview 4 - dotnet/runtime#126905.
@gewarren
gewarren
marked this pull request as ready for review
April 29, 2026 16:26
@gewarren
gewarren
requested a review
from a team
as a code ownerApril 29, 2026 16:26
CopilotAI
review requested due to automatic review settings
April 29, 2026 16:26
CopilotAI
reviewed
Apr 29, 2026
View reviewed changes
CopilotAI
left a comment


Copy link


Copy Markdown

Contributor
Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Pull request overview
Adds a new .NET 11 breaking change article documenting that Nullable.GetUnderlyingType(Type) now forwards to Type.GetNullableUnderlyingType() and can throw NotSupportedException for custom Type subclasses that don’t override the new virtual.
Changes:
  • Adds a new breaking change article describing the behavior change, rationale, and recommended override patterns.
  • Updates the .NET compatibility TOC to include the new article under .NET 11 core libraries.
  • Updates the .NET 11 breaking changes index table to reference the new article.
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.
FileDescription
docs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.mdNew breaking change article documenting the new forwarding behavior and mitigation guidance.
docs/core/compatibility/toc.ymlAdds the new breaking change article to the navigation tree.
docs/core/compatibility/11.mdAdds the new breaking change entry to the .NET 11 index table.
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
Apply suggestions from code review
3774b56
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
@AaronRobinsonMSFT
AaronRobinsonMSFT
commented
Apr 29, 2026

Copy link


Copy Markdown

Member
@gewarren I'm seeing
Cross reference not found: 'System.Type.GetNullableUnderlyingType'.
in https://buildapi.docs.microsoft.com/Output/PullRequest/a3bda507-3390-de91-8f7f-26f90f4e5fc8/202604291851254639-53409/BuildReport?accessString=aa4585a1989a22ee47264fec513db0a7ccd24ebb715277e92091f33bdc1d25d9. Is this a real error?
@gewarren
gewarren
commented
Apr 29, 2026

Copy link


Copy Markdown

Collaborator
@gewarren I'm seeing
Cross reference not found: 'System.Type.GetNullableUnderlyingType'.
in https://buildapi.docs.microsoft.com/Output/PullRequest/a3bda507-3390-de91-8f7f-26f90f4e5fc8/202604291851254639-53409/BuildReport?accessString=aa4585a1989a22ee47264fec513db0a7ccd24ebb715277e92091f33bdc1d25d9. Is this a real error?
@AaronRobinsonMSFT Yes, it's a real error because that method isn't published in the .NET API reference. I assume this is a new API for Preview 4? We can either wait to merge this until the Preview 4 APIs are published, or we can just put the API name in backticks for now (instead of an xref).
AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
Apply suggestions from code review
37f987d
Co-authored-by: Aaron R Robinson <arobins@microsoft.com>
@AaronRobinsonMSFT
AaronRobinsonMSFT
merged commit 081c4ba
into
mainApr 29, 2026

11 checks passed

@AaronRobinsonMSFT
AaronRobinsonMSFT
deleted the

copilot/fix-nullable-getunderlyingtype-issue

branch
April 29, 2026 21:45
Sign up for freeto join this conversation on GitHub.
Already have an account?
Sign in to comment

Reviewers

@jkotasjkotasjkotas left review comments
Copilot code reviewCopilot
Copilot review effort, defaults to Lite
Applies to this pull request for everyone.Learn more about Copilot code review.
Copilot left review comments
@AaronRobinsonMSFTAaronRobinsonMSFTAaronRobinsonMSFT approved these changes

+1 more reviewer

@gewarrengewarrengewarren approved these changes
Reviewers whose approvals may not affect merge requirements

Assignees

@gewarrengewarren
Copilot code reviewCopilot

Labels

dotnet-fundamentals/svc

Projects


None yet


Milestone


No milestone


Development

Successfully merging this pull request may close these issues.

[Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType


5 participants

@AaronRobinsonMSFT@gewarren@jkotas

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Force GitHub README to respect dark mode\n(function() {\n var style = document.createElement('style');\n style.textContent = '\n .markdown-body {\n color-scheme: dark light;\n }\n .markdown-body pre { background: #161b22 !important; }\n .markdown-body code { background: rgba(110, 118, 129, 0.4) !important; }\n .markdown-body table th, .markdown-body table td { border-color: #30363d !important; }\n .markdown-body img { background: #0d1117; }\n .markdown-body blockquote { border-left-color: #8b949e; }\n .markdown-body hr { border-color: #30363d; }\n ';\n document.head.appendChild(style);\n})();", "GitHub Dark Mode README Fix");
}
} catch(__e) { console.warn('[Userscript:GitHub Dark Mode README Fix]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
Breaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11) - #53409
Merged
AaronRobinsonMSFT merged 6 commits into
mainfrom 
copilot/fix-nullable-getunderlyingtype-issue
Apr 29, 2026
Merged
Breaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11)#53409
AaronRobinsonMSFT merged 6 commits into
mainfrom 
copilot/fix-nullable-getunderlyingtype-issue
Conversation
CopilotAI
commented
Apr 28, 2026•

edited by github-actions
Bot
Loading

Copy link


Copy Markdown

Contributor
Closes#53407
In .NET 11 Preview 4, Nullable.GetUnderlyingType was changed to delegate to the new Type.GetNullableUnderlyingType() virtual. The base implementation throws NotSupportedException, so custom Type subclasses that don't override it now throw where they previously returned null.
Changes
  • New articledocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md — covers previous/new behavior, reason (fixes MetadataLoadContext nullable detection bug), and three override patterns:

    • Return null (subclass never represents Nullable<T>)
    • Inspect generic type definition and return the type argument
    • Delegate to an inner wrapped Type
  • docs/core/compatibility/toc.yml — adds entry under .NET 11 / Core .NET libraries
  • docs/core/compatibility/11.md — adds row to the Core .NET libraries breaking changes table
Affected APIs
  • System.Nullable.GetUnderlyingType(System.Type)
  • System.Type.GetNullableUnderlyingType (new virtual)
Internal previews
📄 File🔗 Preview link
docs/core/compatibility/11.mdBreaking changes in .NET 11
docs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.mdNullable.GetUnderlyingType throws for custom Type subclasses
docs/core/compatibility/toc.ymldocs/core/compatibility/toc
CopilotAI
linked an issue
Apr 28, 2026
that may be
closed
by this pull request


[Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType
#53407

Closed

1 task
@gewarren
Add breaking change article: Nullable.GetUnderlyingType throws for cu…
3167202
…stom Type subclasses
Agent-Logs-Url: https://github.com/dotnet/docs/sessions/2c45f021-3ef8-4caf-855a-7ea80054cb39
Co-authored-by: gewarren <24882762+gewarren@users.noreply.github.com>
CopilotAI
changed the title
[WIP] Fix Nullable.GetUnderlyingType to handle custom Type subclassesBreaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11)Apr 28, 2026
CopilotAI
requested a review
from gewarrenApril 28, 2026 21:29
jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT@jkotas
Apply suggestions from code review
6d724ac
Co-authored-by: Jan Kotas <jkotas@microsoft.com>
AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
AaronRobinsonMSFT
commented
Apr 29, 2026

Copy link


Copy Markdown

Member
@gewarren@jkotas These changes have been merged. I think it will be going into .NET 11 Preview 4 - dotnet/runtime#126905.
@gewarren
gewarren
marked this pull request as ready for review
April 29, 2026 16:26
@gewarren
gewarren
requested a review
from a team
as a code ownerApril 29, 2026 16:26
CopilotAI
review requested due to automatic review settings
April 29, 2026 16:26
CopilotAI
reviewed
Apr 29, 2026
View reviewed changes
CopilotAI
left a comment


Copy link


Copy Markdown

Contributor
Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Pull request overview
Adds a new .NET 11 breaking change article documenting that Nullable.GetUnderlyingType(Type) now forwards to Type.GetNullableUnderlyingType() and can throw NotSupportedException for custom Type subclasses that don’t override the new virtual.
Changes:
  • Adds a new breaking change article describing the behavior change, rationale, and recommended override patterns.
  • Updates the .NET compatibility TOC to include the new article under .NET 11 core libraries.
  • Updates the .NET 11 breaking changes index table to reference the new article.
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.
FileDescription
docs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.mdNew breaking change article documenting the new forwarding behavior and mitigation guidance.
docs/core/compatibility/toc.ymlAdds the new breaking change article to the navigation tree.
docs/core/compatibility/11.mdAdds the new breaking change entry to the .NET 11 index table.
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
Apply suggestions from code review
3774b56
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
@AaronRobinsonMSFT
AaronRobinsonMSFT
commented
Apr 29, 2026

Copy link


Copy Markdown

Member
@gewarren I'm seeing
Cross reference not found: 'System.Type.GetNullableUnderlyingType'.
in https://buildapi.docs.microsoft.com/Output/PullRequest/a3bda507-3390-de91-8f7f-26f90f4e5fc8/202604291851254639-53409/BuildReport?accessString=aa4585a1989a22ee47264fec513db0a7ccd24ebb715277e92091f33bdc1d25d9. Is this a real error?
@gewarren
gewarren
commented
Apr 29, 2026

Copy link


Copy Markdown

Collaborator
@gewarren I'm seeing
Cross reference not found: 'System.Type.GetNullableUnderlyingType'.
in https://buildapi.docs.microsoft.com/Output/PullRequest/a3bda507-3390-de91-8f7f-26f90f4e5fc8/202604291851254639-53409/BuildReport?accessString=aa4585a1989a22ee47264fec513db0a7ccd24ebb715277e92091f33bdc1d25d9. Is this a real error?
@AaronRobinsonMSFT Yes, it's a real error because that method isn't published in the .NET API reference. I assume this is a new API for Preview 4? We can either wait to merge this until the Preview 4 APIs are published, or we can just put the API name in backticks for now (instead of an xref).
AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
Apply suggestions from code review
37f987d
Co-authored-by: Aaron R Robinson <arobins@microsoft.com>
@AaronRobinsonMSFT
AaronRobinsonMSFT
merged commit 081c4ba
into
mainApr 29, 2026

11 checks passed

@AaronRobinsonMSFT
AaronRobinsonMSFT
deleted the

copilot/fix-nullable-getunderlyingtype-issue

branch
April 29, 2026 21:45
Sign up for freeto join this conversation on GitHub.
Already have an account?
Sign in to comment

Reviewers

@jkotasjkotasjkotas left review comments
Copilot code reviewCopilot
Copilot review effort, defaults to Lite
Applies to this pull request for everyone.Learn more about Copilot code review.
Copilot left review comments
@AaronRobinsonMSFTAaronRobinsonMSFTAaronRobinsonMSFT approved these changes

+1 more reviewer

@gewarrengewarrengewarren approved these changes
Reviewers whose approvals may not affect merge requirements

Assignees

@gewarrengewarren
Copilot code reviewCopilot

Labels

dotnet-fundamentals/svc

Projects


None yet


Milestone


No milestone


Development

Successfully merging this pull request may close these issues.

[Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType


5 participants

@AaronRobinsonMSFT@gewarren@jkotas

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Highlight search terms from Google/DuckDuckGo/Bing referrer\n(function() {\n var ref = document.referrer;\n var terms = [];\n \n if (ref.includes('google.com') || ref.includes('duckduckgo.com') || ref.includes('bing.com')) {\n var url = new URL(ref);\n var q = url.searchParams.get('q') || url.searchParams.get('p');\n if (q) {\n terms = q.split(/\\s+/).filter(function(t) { return t.length > 2; });\n }\n }\n \n if (terms.length === 0) return;\n \n var style = document.createElement('style');\n style.textContent = '.userscript-highlight { background: #fbbf24; color: #1a1a2e; padding: 1px 3px; border-radius: 2px; }';\n document.head.appendChild(style);\n \n function highlight(node) {\n if (node.nodeType === 3) { // text node\n var text = node.textContent;\n var found = false;\n terms.forEach(function(term) {\n var regex = new RegExp('(' + term.replace(/[.*+?^${}()|[\\]\\\\]/g, '\\\\') + ')', 'gi');\n if (regex.test(text)) {\n found = true;\n var frag = document.createDocumentFragment();\n var parts = text.split(regex);\n parts.forEach(function(part, i) {\n if (i % 2 === 0) {\n frag.appendChild(document.createTextNode(part));\n } else {\n var span = document.createElement('span');\n span.className = 'userscript-highlight';\n span.textContent = part;\n frag.appendChild(span);\n }\n });\n node.parentNode.replaceChild(frag, node);\n }\n });\n } else if (node.nodeType === 1 && node.childNodes) { // element\n var skipTags = ['SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT', 'SELECT'];\n if (!skipTags.includes(node.tagName)) {\n Array.from(node.childNodes).forEach(highlight);\n }\n }\n }\n \n highlight(document.body);\n \n // Re-highlight on dynamic content\n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1 || node.nodeType === 3) highlight(node);\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Highlight Search Terms");
}
} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
Breaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11) - #53409
Merged
AaronRobinsonMSFT merged 6 commits into
mainfrom 
copilot/fix-nullable-getunderlyingtype-issue
Apr 29, 2026
Merged
Breaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11)#53409
AaronRobinsonMSFT merged 6 commits into
mainfrom 
copilot/fix-nullable-getunderlyingtype-issue
Conversation
CopilotAI
commented
Apr 28, 2026•

edited by github-actions
Bot
Loading

Copy link


Copy Markdown

Contributor
Closes#53407
In .NET 11 Preview 4, Nullable.GetUnderlyingType was changed to delegate to the new Type.GetNullableUnderlyingType() virtual. The base implementation throws NotSupportedException, so custom Type subclasses that don't override it now throw where they previously returned null.
Changes
  • New articledocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md — covers previous/new behavior, reason (fixes MetadataLoadContext nullable detection bug), and three override patterns:

    • Return null (subclass never represents Nullable<T>)
    • Inspect generic type definition and return the type argument
    • Delegate to an inner wrapped Type
  • docs/core/compatibility/toc.yml — adds entry under .NET 11 / Core .NET libraries
  • docs/core/compatibility/11.md — adds row to the Core .NET libraries breaking changes table
Affected APIs
  • System.Nullable.GetUnderlyingType(System.Type)
  • System.Type.GetNullableUnderlyingType (new virtual)
Internal previews
📄 File🔗 Preview link
docs/core/compatibility/11.mdBreaking changes in .NET 11
docs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.mdNullable.GetUnderlyingType throws for custom Type subclasses
docs/core/compatibility/toc.ymldocs/core/compatibility/toc
CopilotAI
linked an issue
Apr 28, 2026
that may be
closed
by this pull request


[Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType
#53407

Closed

1 task
@gewarren
Add breaking change article: Nullable.GetUnderlyingType throws for cu…
3167202
…stom Type subclasses
Agent-Logs-Url: https://github.com/dotnet/docs/sessions/2c45f021-3ef8-4caf-855a-7ea80054cb39
Co-authored-by: gewarren <24882762+gewarren@users.noreply.github.com>
CopilotAI
changed the title
[WIP] Fix Nullable.GetUnderlyingType to handle custom Type subclassesBreaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11)Apr 28, 2026
CopilotAI
requested a review
from gewarrenApril 28, 2026 21:29
jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT@jkotas
Apply suggestions from code review
6d724ac
Co-authored-by: Jan Kotas <jkotas@microsoft.com>
AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
AaronRobinsonMSFT
commented
Apr 29, 2026

Copy link


Copy Markdown

Member
@gewarren@jkotas These changes have been merged. I think it will be going into .NET 11 Preview 4 - dotnet/runtime#126905.
@gewarren
gewarren
marked this pull request as ready for review
April 29, 2026 16:26
@gewarren
gewarren
requested a review
from a team
as a code ownerApril 29, 2026 16:26
CopilotAI
review requested due to automatic review settings
April 29, 2026 16:26
CopilotAI
reviewed
Apr 29, 2026
View reviewed changes
CopilotAI
left a comment


Copy link


Copy Markdown

Contributor
Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Pull request overview
Adds a new .NET 11 breaking change article documenting that Nullable.GetUnderlyingType(Type) now forwards to Type.GetNullableUnderlyingType() and can throw NotSupportedException for custom Type subclasses that don’t override the new virtual.
Changes:
  • Adds a new breaking change article describing the behavior change, rationale, and recommended override patterns.
  • Updates the .NET compatibility TOC to include the new article under .NET 11 core libraries.
  • Updates the .NET 11 breaking changes index table to reference the new article.
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.
FileDescription
docs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.mdNew breaking change article documenting the new forwarding behavior and mitigation guidance.
docs/core/compatibility/toc.ymlAdds the new breaking change article to the navigation tree.
docs/core/compatibility/11.mdAdds the new breaking change entry to the .NET 11 index table.
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
Apply suggestions from code review
3774b56
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
@AaronRobinsonMSFT
AaronRobinsonMSFT
commented
Apr 29, 2026

Copy link


Copy Markdown

Member
@gewarren I'm seeing
Cross reference not found: 'System.Type.GetNullableUnderlyingType'.
in https://buildapi.docs.microsoft.com/Output/PullRequest/a3bda507-3390-de91-8f7f-26f90f4e5fc8/202604291851254639-53409/BuildReport?accessString=aa4585a1989a22ee47264fec513db0a7ccd24ebb715277e92091f33bdc1d25d9. Is this a real error?
@gewarren
gewarren
commented
Apr 29, 2026

Copy link


Copy Markdown

Collaborator
@gewarren I'm seeing
Cross reference not found: 'System.Type.GetNullableUnderlyingType'.
in https://buildapi.docs.microsoft.com/Output/PullRequest/a3bda507-3390-de91-8f7f-26f90f4e5fc8/202604291851254639-53409/BuildReport?accessString=aa4585a1989a22ee47264fec513db0a7ccd24ebb715277e92091f33bdc1d25d9. Is this a real error?
@AaronRobinsonMSFT Yes, it's a real error because that method isn't published in the .NET API reference. I assume this is a new API for Preview 4? We can either wait to merge this until the Preview 4 APIs are published, or we can just put the API name in backticks for now (instead of an xref).
AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
Apply suggestions from code review
37f987d
Co-authored-by: Aaron R Robinson <arobins@microsoft.com>
@AaronRobinsonMSFT
AaronRobinsonMSFT
merged commit 081c4ba
into
mainApr 29, 2026

11 checks passed

@AaronRobinsonMSFT
AaronRobinsonMSFT
deleted the

copilot/fix-nullable-getunderlyingtype-issue

branch
April 29, 2026 21:45
Sign up for freeto join this conversation on GitHub.
Already have an account?
Sign in to comment

Reviewers

@jkotasjkotasjkotas left review comments
Copilot code reviewCopilot
Copilot review effort, defaults to Lite
Applies to this pull request for everyone.Learn more about Copilot code review.
Copilot left review comments
@AaronRobinsonMSFTAaronRobinsonMSFTAaronRobinsonMSFT approved these changes

+1 more reviewer

@gewarrengewarrengewarren approved these changes
Reviewers whose approvals may not affect merge requirements

Assignees

@gewarrengewarren
Copilot code reviewCopilot

Labels

dotnet-fundamentals/svc

Projects


None yet


Milestone


No milestone


Development

Successfully merging this pull request may close these issues.

[Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType


5 participants

@AaronRobinsonMSFT@gewarren@jkotas

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Strip utm_, fbclid, gclid, etc. from all links on page\n(function() {\n var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',\n 'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',\n 'ref', 'ref_src', 'source', 'medium', 'campaign'];\n \n function cleanUrl(url) {\n try {\n var u = new URL(url, window.location.origin);\n var changed = false;\n trackingParams.forEach(function(p) {\n if (u.searchParams.has(p)) {\n u.searchParams.delete(p);\n changed = true;\n }\n });\n return changed ? u.toString() : url;\n } catch (e) {\n return url;\n }\n }\n \n function cleanLinks() {\n document.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n \n cleanLinks();\n \n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1) {\n if (node.tagName === 'A') cleanLinks();\n node.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Remove Tracking Parameters from Links");
}
} catch(__e) { console.warn('[Userscript:Remove Tracking Parameters from Links]', __e); }
})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '

Skip to content
Breaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11) - #53409
Merged
AaronRobinsonMSFT merged 6 commits into
mainfrom 
copilot/fix-nullable-getunderlyingtype-issue
Apr 29, 2026
Merged
Breaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11)#53409
AaronRobinsonMSFT merged 6 commits into
mainfrom 
copilot/fix-nullable-getunderlyingtype-issue
Conversation
CopilotAI
commented
Apr 28, 2026•

edited by github-actions
Bot
Loading

Copy link


Copy Markdown

Contributor
Closes#53407
In .NET 11 Preview 4, Nullable.GetUnderlyingType was changed to delegate to the new Type.GetNullableUnderlyingType() virtual. The base implementation throws NotSupportedException, so custom Type subclasses that don't override it now throw where they previously returned null.
Changes
  • New articledocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md — covers previous/new behavior, reason (fixes MetadataLoadContext nullable detection bug), and three override patterns:

    • Return null (subclass never represents Nullable<T>)
    • Inspect generic type definition and return the type argument
    • Delegate to an inner wrapped Type
  • docs/core/compatibility/toc.yml — adds entry under .NET 11 / Core .NET libraries
  • docs/core/compatibility/11.md — adds row to the Core .NET libraries breaking changes table
Affected APIs
  • System.Nullable.GetUnderlyingType(System.Type)
  • System.Type.GetNullableUnderlyingType (new virtual)
Internal previews
📄 File🔗 Preview link
docs/core/compatibility/11.mdBreaking changes in .NET 11
docs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.mdNullable.GetUnderlyingType throws for custom Type subclasses
docs/core/compatibility/toc.ymldocs/core/compatibility/toc
CopilotAI
linked an issue
Apr 28, 2026
that may be
closed
by this pull request


[Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType
#53407

Closed

1 task
@gewarren
Add breaking change article: Nullable.GetUnderlyingType throws for cu…
3167202
…stom Type subclasses
Agent-Logs-Url: https://github.com/dotnet/docs/sessions/2c45f021-3ef8-4caf-855a-7ea80054cb39
Co-authored-by: gewarren <24882762+gewarren@users.noreply.github.com>
CopilotAI
changed the title
[WIP] Fix Nullable.GetUnderlyingType to handle custom Type subclassesBreaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11)Apr 28, 2026
CopilotAI
requested a review
from gewarrenApril 28, 2026 21:29
jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT@jkotas
Apply suggestions from code review
6d724ac
Co-authored-by: Jan Kotas <jkotas@microsoft.com>
AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
AaronRobinsonMSFT
commented
Apr 29, 2026

Copy link


Copy Markdown

Member
@gewarren@jkotas These changes have been merged. I think it will be going into .NET 11 Preview 4 - dotnet/runtime#126905.
@gewarren
gewarren
marked this pull request as ready for review
April 29, 2026 16:26
@gewarren
gewarren
requested a review
from a team
as a code ownerApril 29, 2026 16:26
CopilotAI
review requested due to automatic review settings
April 29, 2026 16:26
CopilotAI
reviewed
Apr 29, 2026
View reviewed changes
CopilotAI
left a comment


Copy link


Copy Markdown

Contributor
Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Pull request overview
Adds a new .NET 11 breaking change article documenting that Nullable.GetUnderlyingType(Type) now forwards to Type.GetNullableUnderlyingType() and can throw NotSupportedException for custom Type subclasses that don’t override the new virtual.
Changes:
  • Adds a new breaking change article describing the behavior change, rationale, and recommended override patterns.
  • Updates the .NET compatibility TOC to include the new article under .NET 11 core libraries.
  • Updates the .NET 11 breaking changes index table to reference the new article.
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.
FileDescription
docs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.mdNew breaking change article documenting the new forwarding behavior and mitigation guidance.
docs/core/compatibility/toc.ymlAdds the new breaking change article to the navigation tree.
docs/core/compatibility/11.mdAdds the new breaking change entry to the .NET 11 index table.
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
Apply suggestions from code review
3774b56
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
@AaronRobinsonMSFT
AaronRobinsonMSFT
commented
Apr 29, 2026

Copy link


Copy Markdown

Member
@gewarren I'm seeing
Cross reference not found: 'System.Type.GetNullableUnderlyingType'.
in https://buildapi.docs.microsoft.com/Output/PullRequest/a3bda507-3390-de91-8f7f-26f90f4e5fc8/202604291851254639-53409/BuildReport?accessString=aa4585a1989a22ee47264fec513db0a7ccd24ebb715277e92091f33bdc1d25d9. Is this a real error?
@gewarren
gewarren
commented
Apr 29, 2026

Copy link


Copy Markdown

Collaborator
@gewarren I'm seeing
Cross reference not found: 'System.Type.GetNullableUnderlyingType'.
in https://buildapi.docs.microsoft.com/Output/PullRequest/a3bda507-3390-de91-8f7f-26f90f4e5fc8/202604291851254639-53409/BuildReport?accessString=aa4585a1989a22ee47264fec513db0a7ccd24ebb715277e92091f33bdc1d25d9. Is this a real error?
@AaronRobinsonMSFT Yes, it's a real error because that method isn't published in the .NET API reference. I assume this is a new API for Preview 4? We can either wait to merge this until the Preview 4 APIs are published, or we can just put the API name in backticks for now (instead of an xref).
AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
Apply suggestions from code review
37f987d
Co-authored-by: Aaron R Robinson <arobins@microsoft.com>
@AaronRobinsonMSFT
AaronRobinsonMSFT
merged commit 081c4ba
into
mainApr 29, 2026

11 checks passed

@AaronRobinsonMSFT
AaronRobinsonMSFT
deleted the

copilot/fix-nullable-getunderlyingtype-issue

branch
April 29, 2026 21:45
Sign up for freeto join this conversation on GitHub.
Already have an account?
Sign in to comment

Reviewers

@jkotasjkotasjkotas left review comments
Copilot code reviewCopilot
Copilot review effort, defaults to Lite
Applies to this pull request for everyone.Learn more about Copilot code review.
Copilot left review comments
@AaronRobinsonMSFTAaronRobinsonMSFTAaronRobinsonMSFT approved these changes

+1 more reviewer

@gewarrengewarrengewarren approved these changes
Reviewers whose approvals may not affect merge requirements

Assignees

@gewarrengewarren
Copilot code reviewCopilot

Labels

dotnet-fundamentals/svc

Projects


None yet


Milestone


No milestone


Development

Successfully merging this pull request may close these issues.

[Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType


5 participants

@AaronRobinsonMSFT@gewarren@jkotas

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Auto-enable theater mode on YouTube\n(function() {\n function tryTheater() {\n var btn = document.querySelector('button[aria-label=\"Theater mode\"], ytd-player #player button[title=\"Theater mode\"]');\n if (btn && !btn.classList.contains('activated')) {\n btn.click();\n }\n }\n \n // Try immediately\n tryTheater();\n \n // Try after navigation (SPA)\n var lastUrl = location.href;\n setInterval(function() {\n if (location.href !== lastUrl) {\n lastUrl = location.href;\n setTimeout(tryTheater, 500);\n }\n }, 1000);\n \n // Also try on player load\n var observer = new MutationObserver(tryTheater);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "YouTube Theater Mode Default");
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
Breaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11) - #53409
Merged
AaronRobinsonMSFT merged 6 commits into
mainfrom 
copilot/fix-nullable-getunderlyingtype-issue
Apr 29, 2026
Merged
Breaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11)#53409
AaronRobinsonMSFT merged 6 commits into
mainfrom 
copilot/fix-nullable-getunderlyingtype-issue
Conversation
CopilotAI
commented
Apr 28, 2026•

edited by github-actions
Bot
Loading

Copy link


Copy Markdown

Contributor
Closes#53407
In .NET 11 Preview 4, Nullable.GetUnderlyingType was changed to delegate to the new Type.GetNullableUnderlyingType() virtual. The base implementation throws NotSupportedException, so custom Type subclasses that don't override it now throw where they previously returned null.
Changes
  • New articledocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md — covers previous/new behavior, reason (fixes MetadataLoadContext nullable detection bug), and three override patterns:

    • Return null (subclass never represents Nullable<T>)
    • Inspect generic type definition and return the type argument
    • Delegate to an inner wrapped Type
  • docs/core/compatibility/toc.yml — adds entry under .NET 11 / Core .NET libraries
  • docs/core/compatibility/11.md — adds row to the Core .NET libraries breaking changes table
Affected APIs
  • System.Nullable.GetUnderlyingType(System.Type)
  • System.Type.GetNullableUnderlyingType (new virtual)
Internal previews
📄 File🔗 Preview link
docs/core/compatibility/11.mdBreaking changes in .NET 11
docs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.mdNullable.GetUnderlyingType throws for custom Type subclasses
docs/core/compatibility/toc.ymldocs/core/compatibility/toc
CopilotAI
linked an issue
Apr 28, 2026
that may be
closed
by this pull request


[Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType
#53407

Closed

1 task
@gewarren
Add breaking change article: Nullable.GetUnderlyingType throws for cu…
3167202
…stom Type subclasses
Agent-Logs-Url: https://github.com/dotnet/docs/sessions/2c45f021-3ef8-4caf-855a-7ea80054cb39
Co-authored-by: gewarren <24882762+gewarren@users.noreply.github.com>
CopilotAI
changed the title
[WIP] Fix Nullable.GetUnderlyingType to handle custom Type subclassesBreaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11)Apr 28, 2026
CopilotAI
requested a review
from gewarrenApril 28, 2026 21:29
jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT@jkotas
Apply suggestions from code review
6d724ac
Co-authored-by: Jan Kotas <jkotas@microsoft.com>
AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
AaronRobinsonMSFT
commented
Apr 29, 2026

Copy link


Copy Markdown

Member
@gewarren@jkotas These changes have been merged. I think it will be going into .NET 11 Preview 4 - dotnet/runtime#126905.
@gewarren
gewarren
marked this pull request as ready for review
April 29, 2026 16:26
@gewarren
gewarren
requested a review
from a team
as a code ownerApril 29, 2026 16:26
CopilotAI
review requested due to automatic review settings
April 29, 2026 16:26
CopilotAI
reviewed
Apr 29, 2026
View reviewed changes
CopilotAI
left a comment


Copy link


Copy Markdown

Contributor
Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Pull request overview
Adds a new .NET 11 breaking change article documenting that Nullable.GetUnderlyingType(Type) now forwards to Type.GetNullableUnderlyingType() and can throw NotSupportedException for custom Type subclasses that don’t override the new virtual.
Changes:
  • Adds a new breaking change article describing the behavior change, rationale, and recommended override patterns.
  • Updates the .NET compatibility TOC to include the new article under .NET 11 core libraries.
  • Updates the .NET 11 breaking changes index table to reference the new article.
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.
FileDescription
docs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.mdNew breaking change article documenting the new forwarding behavior and mitigation guidance.
docs/core/compatibility/toc.ymlAdds the new breaking change article to the navigation tree.
docs/core/compatibility/11.mdAdds the new breaking change entry to the .NET 11 index table.
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
Apply suggestions from code review
3774b56
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
@AaronRobinsonMSFT
AaronRobinsonMSFT
commented
Apr 29, 2026

Copy link


Copy Markdown

Member
@gewarren I'm seeing
Cross reference not found: 'System.Type.GetNullableUnderlyingType'.
in https://buildapi.docs.microsoft.com/Output/PullRequest/a3bda507-3390-de91-8f7f-26f90f4e5fc8/202604291851254639-53409/BuildReport?accessString=aa4585a1989a22ee47264fec513db0a7ccd24ebb715277e92091f33bdc1d25d9. Is this a real error?
@gewarren
gewarren
commented
Apr 29, 2026

Copy link


Copy Markdown

Collaborator
@gewarren I'm seeing
Cross reference not found: 'System.Type.GetNullableUnderlyingType'.
in https://buildapi.docs.microsoft.com/Output/PullRequest/a3bda507-3390-de91-8f7f-26f90f4e5fc8/202604291851254639-53409/BuildReport?accessString=aa4585a1989a22ee47264fec513db0a7ccd24ebb715277e92091f33bdc1d25d9. Is this a real error?
@AaronRobinsonMSFT Yes, it's a real error because that method isn't published in the .NET API reference. I assume this is a new API for Preview 4? We can either wait to merge this until the Preview 4 APIs are published, or we can just put the API name in backticks for now (instead of an xref).
AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
Apply suggestions from code review
37f987d
Co-authored-by: Aaron R Robinson <arobins@microsoft.com>
@AaronRobinsonMSFT
AaronRobinsonMSFT
merged commit 081c4ba
into
mainApr 29, 2026

11 checks passed

@AaronRobinsonMSFT
AaronRobinsonMSFT
deleted the

copilot/fix-nullable-getunderlyingtype-issue

branch
April 29, 2026 21:45
Sign up for freeto join this conversation on GitHub.
Already have an account?
Sign in to comment

Reviewers

@jkotasjkotasjkotas left review comments
Copilot code reviewCopilot
Copilot review effort, defaults to Lite
Applies to this pull request for everyone.Learn more about Copilot code review.
Copilot left review comments
@AaronRobinsonMSFTAaronRobinsonMSFTAaronRobinsonMSFT approved these changes

+1 more reviewer

@gewarrengewarrengewarren approved these changes
Reviewers whose approvals may not affect merge requirements

Assignees

@gewarrengewarren
Copilot code reviewCopilot

Labels

dotnet-fundamentals/svc

Projects


None yet


Milestone


No milestone


Development

Successfully merging this pull request may close these issues.

[Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType


5 participants

@AaronRobinsonMSFT@gewarren@jkotas

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Remove or un-stick sticky/fixed headers that block content\n(function() {\n function unstick() {\n document.querySelectorAll('header, nav, [role=\"banner\"], .header, .navbar, .sticky, .fixed-top, [style*=\"position: fixed\"], [style*=\"position:sticky\"]').forEach(function(el) {\n if (el.style.position === 'fixed' || el.style.position === 'sticky' || \n getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {\n el.style.position = 'static';\n el.style.top = 'auto';\n el.style.zIndex = 'auto';\n }\n });\n }\n \n unstick();\n \n var observer = new MutationObserver(unstick);\n observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['style', 'class'] });\n})();", "Kill Sticky Headers");
}
} catch(__e) { console.warn('[Userscript:Kill Sticky Headers]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
Breaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11) - #53409
Merged
AaronRobinsonMSFT merged 6 commits into
mainfrom 
copilot/fix-nullable-getunderlyingtype-issue
Apr 29, 2026
Merged
Breaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11)#53409
AaronRobinsonMSFT merged 6 commits into
mainfrom 
copilot/fix-nullable-getunderlyingtype-issue
Conversation
CopilotAI
commented
Apr 28, 2026•

edited by github-actions
Bot
Loading

Copy link


Copy Markdown

Contributor
Closes#53407
In .NET 11 Preview 4, Nullable.GetUnderlyingType was changed to delegate to the new Type.GetNullableUnderlyingType() virtual. The base implementation throws NotSupportedException, so custom Type subclasses that don't override it now throw where they previously returned null.
Changes
  • New articledocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md — covers previous/new behavior, reason (fixes MetadataLoadContext nullable detection bug), and three override patterns:

    • Return null (subclass never represents Nullable<T>)
    • Inspect generic type definition and return the type argument
    • Delegate to an inner wrapped Type
  • docs/core/compatibility/toc.yml — adds entry under .NET 11 / Core .NET libraries
  • docs/core/compatibility/11.md — adds row to the Core .NET libraries breaking changes table
Affected APIs
  • System.Nullable.GetUnderlyingType(System.Type)
  • System.Type.GetNullableUnderlyingType (new virtual)
Internal previews
📄 File🔗 Preview link
docs/core/compatibility/11.mdBreaking changes in .NET 11
docs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.mdNullable.GetUnderlyingType throws for custom Type subclasses
docs/core/compatibility/toc.ymldocs/core/compatibility/toc
CopilotAI
linked an issue
Apr 28, 2026
that may be
closed
by this pull request


[Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType
#53407

Closed

1 task
@gewarren
Add breaking change article: Nullable.GetUnderlyingType throws for cu…
3167202
…stom Type subclasses
Agent-Logs-Url: https://github.com/dotnet/docs/sessions/2c45f021-3ef8-4caf-855a-7ea80054cb39
Co-authored-by: gewarren <24882762+gewarren@users.noreply.github.com>
CopilotAI
changed the title
[WIP] Fix Nullable.GetUnderlyingType to handle custom Type subclassesBreaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11)Apr 28, 2026
CopilotAI
requested a review
from gewarrenApril 28, 2026 21:29
jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT@jkotas
Apply suggestions from code review
6d724ac
Co-authored-by: Jan Kotas <jkotas@microsoft.com>
AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
AaronRobinsonMSFT
commented
Apr 29, 2026

Copy link


Copy Markdown

Member
@gewarren@jkotas These changes have been merged. I think it will be going into .NET 11 Preview 4 - dotnet/runtime#126905.
@gewarren
gewarren
marked this pull request as ready for review
April 29, 2026 16:26
@gewarren
gewarren
requested a review
from a team
as a code ownerApril 29, 2026 16:26
CopilotAI
review requested due to automatic review settings
April 29, 2026 16:26
CopilotAI
reviewed
Apr 29, 2026
View reviewed changes
CopilotAI
left a comment


Copy link


Copy Markdown

Contributor
Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Pull request overview
Adds a new .NET 11 breaking change article documenting that Nullable.GetUnderlyingType(Type) now forwards to Type.GetNullableUnderlyingType() and can throw NotSupportedException for custom Type subclasses that don’t override the new virtual.
Changes:
  • Adds a new breaking change article describing the behavior change, rationale, and recommended override patterns.
  • Updates the .NET compatibility TOC to include the new article under .NET 11 core libraries.
  • Updates the .NET 11 breaking changes index table to reference the new article.
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.
FileDescription
docs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.mdNew breaking change article documenting the new forwarding behavior and mitigation guidance.
docs/core/compatibility/toc.ymlAdds the new breaking change article to the navigation tree.
docs/core/compatibility/11.mdAdds the new breaking change entry to the .NET 11 index table.
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
Apply suggestions from code review
3774b56
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
@AaronRobinsonMSFT
AaronRobinsonMSFT
commented
Apr 29, 2026

Copy link


Copy Markdown

Member
@gewarren I'm seeing
Cross reference not found: 'System.Type.GetNullableUnderlyingType'.
in https://buildapi.docs.microsoft.com/Output/PullRequest/a3bda507-3390-de91-8f7f-26f90f4e5fc8/202604291851254639-53409/BuildReport?accessString=aa4585a1989a22ee47264fec513db0a7ccd24ebb715277e92091f33bdc1d25d9. Is this a real error?
@gewarren
gewarren
commented
Apr 29, 2026

Copy link


Copy Markdown

Collaborator
@gewarren I'm seeing
Cross reference not found: 'System.Type.GetNullableUnderlyingType'.
in https://buildapi.docs.microsoft.com/Output/PullRequest/a3bda507-3390-de91-8f7f-26f90f4e5fc8/202604291851254639-53409/BuildReport?accessString=aa4585a1989a22ee47264fec513db0a7ccd24ebb715277e92091f33bdc1d25d9. Is this a real error?
@AaronRobinsonMSFT Yes, it's a real error because that method isn't published in the .NET API reference. I assume this is a new API for Preview 4? We can either wait to merge this until the Preview 4 APIs are published, or we can just put the API name in backticks for now (instead of an xref).
AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
Apply suggestions from code review
37f987d
Co-authored-by: Aaron R Robinson <arobins@microsoft.com>
@AaronRobinsonMSFT
AaronRobinsonMSFT
merged commit 081c4ba
into
mainApr 29, 2026

11 checks passed

@AaronRobinsonMSFT
AaronRobinsonMSFT
deleted the

copilot/fix-nullable-getunderlyingtype-issue

branch
April 29, 2026 21:45
Sign up for freeto join this conversation on GitHub.
Already have an account?
Sign in to comment

Reviewers

@jkotasjkotasjkotas left review comments
Copilot code reviewCopilot
Copilot review effort, defaults to Lite
Applies to this pull request for everyone.Learn more about Copilot code review.
Copilot left review comments
@AaronRobinsonMSFTAaronRobinsonMSFTAaronRobinsonMSFT approved these changes

+1 more reviewer

@gewarrengewarrengewarren approved these changes
Reviewers whose approvals may not affect merge requirements

Assignees

@gewarrengewarren
Copilot code reviewCopilot

Labels

dotnet-fundamentals/svc

Projects


None yet


Milestone


No milestone


Development

Successfully merging this pull request may close these issues.

[Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType


5 participants

@AaronRobinsonMSFT@gewarren@jkotas

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Universal Dark Mode - works on any site\n(function() {\n var enabled = true;\n \n function applyDarkMode() {\n if (!enabled) return;\n \n // Create style element if it doesn't exist\n var style = document.getElementById('universal-dark-mode-style');\n if (!style) {\n style = document.createElement('style');\n style.id = 'universal-dark-mode-style';\n document.head.appendChild(style);\n }\n \n // Dark mode CSS - inverts colors but preserves images/video\n style.textContent = '\n /* Invert everything except media */\n html {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #1a1a2e !important;\n }\n \n /* Restore images, videos, iframes, canvas */\n img, video, iframe, canvas, svg, picture, [style*=\"background-image\"] {\n filter: invert(1) hue-rotate(180deg) !important;\n }\n \n /* Preserve specific elements that should not be inverted */\n .no-dark-mode, .no-dark-mode *,\n [data-theme=\"light\"], [data-theme=\"light\"],\n .ace_editor, .ace_editor *,\n .CodeMirror, .CodeMirror *,\n .monaco-editor, .monaco-editor *,\n .markdown-body pre, .markdown-body pre *,\n .highlight, .highlight *,\n pre code, pre code * {\n filter: none !important;\n }\n \n /* Fix common UI elements */\n .modal, .popup, .dropdown-menu, .tooltip, .popover {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #2d2d44 !important;\n border-color: #444 !important;\n }\n \n /* Scrollbars */\n ::-webkit-scrollbar { background: #1a1a2e !important; }\n ::-webkit-scrollbar-thumb { background: #444 !important; }\n ::-webkit-scrollbar-thumb:hover { background: #555 !important; }\n \n /* Selection */\n ::selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ::-moz-selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ';\n }\n \n function removeDarkMode() {\n var style = document.getElementById('universal-dark-mode-style');\n if (style) style.remove();\n }\n \n // Toggle with Alt+Shift+D\n document.addEventListener('keydown', function(e) {\n if (e.altKey && e.shiftKey && e.key === 'D') {\n e.preventDefault();\n enabled = !enabled;\n if (enabled) {\n applyDarkMode();\n console.log('[Universal Dark Mode] Enabled');\n } else {\n removeDarkMode();\n console.log('[Universal Dark Mode] Disabled');\n }\n }\n });\n \n // Apply on load\n applyDarkMode();\n \n // Re-apply on dynamic content\n var observer = new MutationObserver(function(mutations) {\n if (enabled && !document.getElementById('universal-dark-mode-style')) {\n applyDarkMode();\n }\n });\n observer.observe(document.head, { childList: true });\n \n console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');\n})();", "Universal Dark Mode");
}
} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
})();

Skip to content
Breaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11) - #53409
Merged
AaronRobinsonMSFT merged 6 commits into
mainfrom 
copilot/fix-nullable-getunderlyingtype-issue
Apr 29, 2026
Merged
Breaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11)#53409
AaronRobinsonMSFT merged 6 commits into
mainfrom 
copilot/fix-nullable-getunderlyingtype-issue
Conversation
CopilotAI
commented
Apr 28, 2026•

edited by github-actions
Bot
Loading

Copy link


Copy Markdown

Contributor
Closes#53407
In .NET 11 Preview 4, Nullable.GetUnderlyingType was changed to delegate to the new Type.GetNullableUnderlyingType() virtual. The base implementation throws NotSupportedException, so custom Type subclasses that don't override it now throw where they previously returned null.
Changes
  • New articledocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md — covers previous/new behavior, reason (fixes MetadataLoadContext nullable detection bug), and three override patterns:

    • Return null (subclass never represents Nullable<T>)
    • Inspect generic type definition and return the type argument
    • Delegate to an inner wrapped Type
  • docs/core/compatibility/toc.yml — adds entry under .NET 11 / Core .NET libraries
  • docs/core/compatibility/11.md — adds row to the Core .NET libraries breaking changes table
Affected APIs
  • System.Nullable.GetUnderlyingType(System.Type)
  • System.Type.GetNullableUnderlyingType (new virtual)
Internal previews
📄 File🔗 Preview link
docs/core/compatibility/11.mdBreaking changes in .NET 11
docs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.mdNullable.GetUnderlyingType throws for custom Type subclasses
docs/core/compatibility/toc.ymldocs/core/compatibility/toc
CopilotAI
linked an issue
Apr 28, 2026
that may be
closed
by this pull request


[Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType
#53407

Closed

1 task
@gewarren
Add breaking change article: Nullable.GetUnderlyingType throws for cu…
3167202
…stom Type subclasses
Agent-Logs-Url: https://github.com/dotnet/docs/sessions/2c45f021-3ef8-4caf-855a-7ea80054cb39
Co-authored-by: gewarren <24882762+gewarren@users.noreply.github.com>
CopilotAI
changed the title
[WIP] Fix Nullable.GetUnderlyingType to handle custom Type subclassesBreaking change doc: Nullable.GetUnderlyingType throws for custom Type subclasses (.NET 11)Apr 28, 2026
CopilotAI
requested a review
from gewarrenApril 28, 2026 21:29
jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

jkotas
jkotas
reviewed
Apr 28, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT@jkotas
Apply suggestions from code review
6d724ac
Co-authored-by: Jan Kotas <jkotas@microsoft.com>
AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
AaronRobinsonMSFT
commented
Apr 29, 2026

Copy link


Copy Markdown

Member
@gewarren@jkotas These changes have been merged. I think it will be going into .NET 11 Preview 4 - dotnet/runtime#126905.
@gewarren
gewarren
marked this pull request as ready for review
April 29, 2026 16:26
@gewarren
gewarren
requested a review
from a team
as a code ownerApril 29, 2026 16:26
CopilotAI
review requested due to automatic review settings
April 29, 2026 16:26
CopilotAI
reviewed
Apr 29, 2026
View reviewed changes
CopilotAI
left a comment


Copy link


Copy Markdown

Contributor
Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Pull request overview
Adds a new .NET 11 breaking change article documenting that Nullable.GetUnderlyingType(Type) now forwards to Type.GetNullableUnderlyingType() and can throw NotSupportedException for custom Type subclasses that don’t override the new virtual.
Changes:
  • Adds a new breaking change article describing the behavior change, rationale, and recommended override patterns.
  • Updates the .NET compatibility TOC to include the new article under .NET 11 core libraries.
  • Updates the .NET 11 breaking changes index table to reference the new article.
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.
FileDescription
docs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.mdNew breaking change article documenting the new forwarding behavior and mitigation guidance.
docs/core/compatibility/toc.ymlAdds the new breaking change article to the navigation tree.
docs/core/compatibility/11.mdAdds the new breaking change entry to the .NET 11 index table.
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
Apply suggestions from code review
3774b56
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
@AaronRobinsonMSFT
AaronRobinsonMSFT
commented
Apr 29, 2026

Copy link


Copy Markdown

Member
@gewarren I'm seeing
Cross reference not found: 'System.Type.GetNullableUnderlyingType'.
in https://buildapi.docs.microsoft.com/Output/PullRequest/a3bda507-3390-de91-8f7f-26f90f4e5fc8/202604291851254639-53409/BuildReport?accessString=aa4585a1989a22ee47264fec513db0a7ccd24ebb715277e92091f33bdc1d25d9. Is this a real error?
@gewarren
gewarren
commented
Apr 29, 2026

Copy link


Copy Markdown

Collaborator
@gewarren I'm seeing
Cross reference not found: 'System.Type.GetNullableUnderlyingType'.
in https://buildapi.docs.microsoft.com/Output/PullRequest/a3bda507-3390-de91-8f7f-26f90f4e5fc8/202604291851254639-53409/BuildReport?accessString=aa4585a1989a22ee47264fec513db0a7ccd24ebb715277e92091f33bdc1d25d9. Is this a real error?
@AaronRobinsonMSFT Yes, it's a real error because that method isn't published in the .NET API reference. I assume this is a new API for Preview 4? We can either wait to merge this until the Preview 4 APIs are published, or we can just put the API name in backticks for now (instead of an xref).
AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

AaronRobinsonMSFT
AaronRobinsonMSFT
reviewed
Apr 29, 2026
View reviewed changes
Comment threaddocs/core/compatibility/core-libraries/11/nullable-getunderlyingtype-throws.md
Outdated

@AaronRobinsonMSFT
Apply suggestions from code review
37f987d
Co-authored-by: Aaron R Robinson <arobins@microsoft.com>
@AaronRobinsonMSFT
AaronRobinsonMSFT
merged commit 081c4ba
into
mainApr 29, 2026

11 checks passed

@AaronRobinsonMSFT
AaronRobinsonMSFT
deleted the

copilot/fix-nullable-getunderlyingtype-issue

branch
April 29, 2026 21:45
Sign up for freeto join this conversation on GitHub.
Already have an account?
Sign in to comment

Reviewers

@jkotasjkotasjkotas left review comments
Copilot code reviewCopilot
Copilot review effort, defaults to Lite
Applies to this pull request for everyone.Learn more about Copilot code review.
Copilot left review comments
@AaronRobinsonMSFTAaronRobinsonMSFTAaronRobinsonMSFT approved these changes

+1 more reviewer

@gewarrengewarrengewarren approved these changes
Reviewers whose approvals may not affect merge requirements

Assignees

@gewarrengewarren
Copilot code reviewCopilot

Labels

dotnet-fundamentals/svc

Projects


None yet


Milestone


No milestone


Development

Successfully merging this pull request may close these issues.

[Breaking change]: Nullable.GetUnderlyingType throws on custom Type subclasses that don't override Type.GetNullableUnderlyingType


5 participants

@AaronRobinsonMSFT@gewarren@jkotas