Implement compiler support for System.Runtime.InteropServices.ExtendedLayoutAttribute #78741

jkoritzinsky · 2025-05-28T23:50:52Z

The interop team has decided to add a new mechanism to specify type layouts, designed similarly to how the UnmanagedCallersOnly feature was designed. In particular, the team has decided to use the last remaining value in the "Layout Kind" mask of a type def's attributes to represent "extended layout". When this value it set, the runtime will look for the System.Runtime.InteropServices.ExtendedLayoutAttribute to provide all layout information.

As part of the design process, we noticed that Roslyn blocks emitting a type with the last remaining value of the System.Runtime.InteropServices.LayoutKind enumeration in the StructLayout pseudo-attribute as the compiler can't map it to a valid TypeAttributes value.

The System.Runtime.InteropServices.ExtendedLayoutAttribute type was approved in dotnet/runtime#100896 and has the first part of runtime implementation (the TypeAttributes bit and the LayoutKind value) in dotnet/runtime#116082.

The support in C# is to allow the user to specify the ExtendedLayoutAttribute on a type. When they do so, the correct layout bit will be set. The user cannot manually specify the new "extended layout" value. The only way in C# to specify that value is to put this attribute on the type.

The ExtendedLayoutAttribute will be embedded in NoPia scenarios as the layout bits are embedded in NoPia scenarios.

This should go into .NET 11

…tribute Some error handling is still missing. NoPia tests are still TBD

jaredpar · 2025-05-29T16:19:23Z

@jkoritzinsky think we'll need a small specification document for this contribution.

jkoritzinsky · 2025-05-29T16:36:46Z

I'll put one together. Where is the best place for it to go?

jaredpar · 2025-05-29T22:06:22Z

I'll put one together. Where is the best place for it to go?

dotnet/csharplang. @333fred can you think of a good example that would be similar to this?

jkoritzinsky · 2025-05-29T22:09:09Z

I considered doing a proposal in csharplang, but there's zero specifications of StructLayoutAttribute or FieldOffset on csharplang or csharpstandard.

333fred · 2025-05-29T22:56:35Z

I would agree with Jeremy, C# the language doesn't care about those attributes, just Roslyn the compiler. Perhaps under docs/features? Also, what support does VB have for these attributes, and what will we need to update in its support?

jaredpar · 2025-05-29T23:13:43Z

Hmm ... for some reason I thought the spec discussed StructLayout but looking now it does not. Let's just drop a spec doc/features then.

jkoritzinsky · 2025-05-30T18:30:05Z

I'll drop a spec in docs/features and add VB support.

…ther layout or layout-like attributes.

jkoritzinsky · 2025-05-30T21:24:19Z

I've added a spec and added VB support

docs/features/ExtendedLayoutAttribute.md

…Kind to be defined in InternalSpecialType to enforce the "defined in CoreLib" requirement.

…nch 'main' of github.com:dotnet/roslyn into extended-layout

333fred · 2025-12-18T23:52:44Z

This is generally looking ok to me. @dotnet/roslyn-compiler for a second review.

jcouv · 2025-12-21T05:58:01Z

src/Compilers/Core/Portable/WellKnownMembers.cs

                // System_Runtime_CompilerServices_AsyncStateMachineAttribute__ctor
                (byte)MemberFlags.Constructor,                                                                              // Flags
-                (byte)WellKnownType.System_Runtime_CompilerServices_AsyncStateMachineAttribute,                             // DeclaringTypeId
+                (byte)WellKnownType.ExtSentinel, (byte)(WellKnownType.System_Runtime_CompilerServices_AsyncStateMachineAttribute - WellKnownType.ExtSentinel), // DeclaringTypeId// DeclaringTypeId


// DeclaringTypeId

duplicated // DeclaringTypeId #Closed

jcouv · 2025-12-21T05:58:44Z

src/Compilers/Core/Portable/SpecialMembers.cs

                    (byte)SignatureTypeCode.TypeHandle, (byte)InternalSpecialType.System_Threading_Tasks_Task_T,
                    1,
                    (byte)SignatureTypeCode.TypeHandle, (byte)SpecialType.System_Int32,
+                // System_Runtime_InteropServices_ExtendedLayoutAttribute__ctor


nit: consider also adding a line break above #Closed

jcouv · 2025-12-21T06:10:37Z

src/Compilers/CSharp/Portable/Symbols/Source/SourceAssemblySymbol.cs

                    }

-                    if (containingType.HasStructLayoutAttribute || containingType.HasInlineArrayAttribute(out _))
+                    if (containingType.HasStructLayoutAttribute || containingType.HasExtendedLayoutAttribute || containingType.HasInlineArrayAttribute(out _))


There's another usage of HasStructLayoutAttribute below at line 2768. Should it also be updated? #Closed

jcouv · 2025-12-21T06:42:43Z

src/Compilers/Core/Portable/MetadataReader/PEModule.cs

                        return default(TypeLayout);

                    default:
+                        if ((def.Attributes & TypeAttributes.LayoutMask) == TypeAttributes.ExtendedLayout)


ExtendedLayout is emitted as an attribute, unlike StructLayout, We should decide what to do when the extended flag and the [ExtendedLayout(...)] attribute don't agree, in metadata.
I couldn't find any place where the compiler cares about the value from Layout from metadata, outside of verifying that the information round-trips through metadata. Is there any other scenario where it matters?
In any case, let's add a test (you can use CreateCompilationWithIL).

Btw, do we have a test observing that ExtendedLayout attribute is emitted? #Closed

There's no other case that Roslyn cares about the Layout metadata. I'll add some IL tests to validate behavior when the attribute and the metadata bits disagree.

jcouv · 2025-12-21T06:49:37Z

src/Compilers/CSharp/Test/Emit3/Attributes/AttributeTests_StructLayout.cs

+            CreateEmptyCompilation(
+                source,
+                references: [s_extendedLayoutAttributeMinimalCoreLibrary])
+            .VerifyDiagnostics(


Let's use VerifyEmitDiagnostics for all tests that error. It pulls a bit further in the compilation pipeline and we sometimes find issues with later pipeline phases that have trouble dealing with unexpected configuration of symbols. #Closed

There is no VerifyEmitDiagnostics when using CompileAndVerify. It looks like VerifyDiagnostics on the CompilationVerifier looks at the emit diagnostics though. Is that okay?

Right. If you just create a compilation and verify diagnostics, then use VerifyEmitDiagnostics. If you use CompileAndVerify (which already emits) then use VerifyDiagnostics on that.

jcouv · 2025-12-21T07:02:32Z

src/Compilers/CSharp/Test/Emit3/Attributes/AttributeTests_StructLayout.cs

+            CreateEmptyCompilation(
+                source,
+                references: [s_extendedLayoutAttributeMinimalCoreLibrary])
+            .VerifyDiagnostics(


Let's also verify what the symbols report in this error scenario #Closed

I tried moving this to use CompileAndVerify, but because it has error diagnostics, it fails to emit. I'm going to write this as Assert calls on top of the returned compilation, ok? Or is there another way I should do this?

Yes, you can get symbols from the compilation. var comp = CreateCompiation(...); var e = comp.GlobalNamespace.GetTypeMember("E"); ... assert on e ...

jcouv · 2025-12-21T07:06:09Z

src/Compilers/VisualBasic/Portable/Symbols/Source/SourceNamedTypeSymbol.vb

+
+            If HasExtendedLayoutAttribute And Not ContainingAssembly.RuntimeSupportsExtendedLayout Then
+                diagnostics = BindingDiagnosticBag.GetInstance()
+                diagnostics.Add(ERRID.ERR_RuntimeDoesNotSupportExtendedLayoutTypes, GetFirstLocation(), Me)


~~Do we have a test for this in VB?~~ Found it: ExtendedLayoutAttribute_DefinedInOtherAssembly_Errors #Closed

jcouv · 2025-12-21T07:09:23Z

src/Compilers/VisualBasic/Portable/Symbols/Source/SourceNamedTypeSymbol.vb

+                If diagnostics Is Nothing Then
+                    diagnostics = BindingDiagnosticBag.GetInstance()
+                End If
+                diagnostics.Add(ERRID.ERR_StructLayoutAndExtendedLayout, GetFirstLocation())


~~Do we have a test for this in VB?~~ Found it: ExtendedLayout_OtherLayoutKind_Errors #Closed

jcouv · 2025-12-21T07:14:29Z

src/Compilers/VisualBasic/Portable/Symbols/Source/SourceNamedTypeSymbol.vb

                    If Me.IsGenericType Then
                        diagnostics.Add(ERRID.ERR_StructLayoutAttributeNotAllowed, arguments.AttributeSyntaxOpt.GetLocation(), Me)
                    End If
+                ElseIf attrData.IsTargetAttribute(AttributeDescription.ExtendedLayoutAttribute) Then


StructLayout appears disallowed on generic types (a few lines above). Should the same restriction apply to ExtendedLayout? #Closed

That's a VB-only restriction, C# does not block it. Additionally, I think that that restriction is both not necessary for ExtendedLayout and would block key scenarios if we did it across the board. If we wanted to block it just for VB though, I personally don't really care one way or the other. I don't expect anyone to use this from VB.

jcouv · 2025-12-21T07:20:42Z

src/Compilers/VisualBasic/Test/Emit/Attributes/AttributeTests_StructLayout.vb

+Structure C
+          ~
+]]>)
+        End Sub


Do we have a test for partial types in VB? #Closed

There's no warning from the VB compiler when partial members are split across types for types that are explicitly LayoutKind.Sequential, so I didn't add the same feature for extended layout (and as a result did not add a test for partials). I can add one if you'd like just to validate the metadata.

jcouv · 2025-12-21T07:22:34Z

docs/features/ExtendedLayoutAttribute.md

+To provide the user experience requested by the runtime team, the C# and VB compilers will have the following support for this attribute:
+
+- If a type has the `ExtendedLayoutAttribute` applied, the compiler will emit the `TypeAttributes.ExtendedLayout` value in the type's `TypeAttributes` flags.
+- If a type has the `ExtendedLayoutAttribute` applied, the compiler will not allow the `StructLayoutAttribute` to be applied to the type.


Not related to this PR: The linked API proposal wasn't updated to clarify that ExtendedLayout attribute implies StructLayout(Extended). The code samples still show explicit [StructLayout(Extended)].

jcouv · 2025-12-21T07:26:29Z

docs/features/ExtendedLayoutAttribute.md

@@ -0,0 +1,16 @@
+System.Runtime.InteropServices.ExtendedLayoutAttribute Compiler Support


Not related to this PR: will the F# compiler need to be updated as well? Consider filing an issue #Closed

F# issue opened as well: dotnet/fsharp#19190

F# shouldn't need any changes to consume, but it will need changes to correctly produce types with the right bits set.

jcouv

Done with review pass (commit 28)

src/Compilers/CSharp/Test/Emit3/Attributes/AttributeTests_StructLayout.cs

jcouv

LGTM Thanks (commit 30) modulo unnecessary using

…ctLayout.cs

… extended-layout

jkoritzinsky added 4 commits February 21, 2025 16:09

Implement support for System.Runtime.InteropServices.ExtendedLayoutAt…

72be215

…tribute Some error handling is still missing. NoPia tests are still TBD

Implement NoPia support for importing ExtendedLayoutAttribute on types

140c7f8

Update test

00c5e58

Merge branch 'main' of github.com:dotnet/roslyn into extended-layout

23b2105

jkoritzinsky requested a review from a team as a code owner May 28, 2025 23:50

github-actions bot added the Area-Compilers label May 28, 2025

jkoritzinsky added 3 commits May 30, 2025 13:03

Fix test failures and add validation for mixing ExtendedLayout with o…

8da6a1b

…ther layout or layout-like attributes.

Add ExtendedLayoutAttribute support to VB

838698c

Add a speclet for this feature.

3857e79

jaredpar reviewed May 30, 2025

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jkoritzinsky added 6 commits June 2, 2025 18:12

Per offline feedback, move ExtendedLayoutAttribute and ExtendedLayout…

4468b16

…Kind to be defined in InternalSpecialType to enforce the "defined in CoreLib" requirement.

Adjust VB tests for SpecialType changes

0e2370a

Update ErrorFacts

ffc2a59

Fix various test failures

98922c4

Account for missing types in InternalSpecialType

9797dbe

Merge branch 'main' of github.com:dotnet/roslyn into extended-layout

a610c4b

jkoritzinsky mentioned this pull request Jun 17, 2025

Add support for "extended" layouts in the runtimes and add CStruct layout dotnet/runtime#116082

Merged

jkoritzinsky added 5 commits June 20, 2025 12:48

Don't run verify on the extendedlayout emit test. it will fail

57f5c77

Set debug format to match test default.

fd4d41f

Merge branch 'main' of github.com:dotnet/roslyn into extended-layout

91e5314

Merge branch 'main' of github.com:dotnet/roslyn into extended-layout

e873123

Update tests

23be26d

Merge branch 'extended-layout' of github.com:jkoritzinsky/roslyn; bra…

ccdbb3d

…nch 'main' of github.com:dotnet/roslyn into extended-layout

jcouv self-assigned this Dec 19, 2025

jcouv reviewed Dec 21, 2025

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jkoritzinsky added 2 commits January 5, 2026 14:25

PR feedback

48c9ebd

Merge branch 'main' of github.com:dotnet/roslyn into extended-layout

e277233

jkoritzinsky mentioned this pull request Jan 5, 2026

Add support for the new System.Runtime.InteropServices.ExtendedLayoutAttribute dotnet/fsharp#19190

Open

jkoritzinsky requested a review from jcouv January 5, 2026 22:35

jcouv reviewed Jan 6, 2026

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src/Compilers/CSharp/Test/Emit3/Attributes/AttributeTests_StructLayout.cs Outdated Show resolved Hide resolved

jcouv approved these changes Jan 6, 2026

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jkoritzinsky and others added 3 commits January 6, 2026 11:42

Update src/Compilers/CSharp/Test/Emit3/Attributes/AttributeTests_Stru…

32ae68f

…ctLayout.cs

Fix formatting

0cf1cfb

Merge branch 'extended-layout' of github.com:jkoritzinsky/roslyn into…

d8ca2da

… extended-layout

jkoritzinsky enabled auto-merge (squash) January 6, 2026 20:17

jkoritzinsky merged commit 85d6c4a into dotnet:main Jan 6, 2026
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dotnet-maestro bot mentioned this pull request Jan 7, 2026

[main] Source code updates from dotnet/roslyn dotnet/dotnet#4084

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dotnet-bot mentioned this pull request Jan 7, 2026

[Automated] PRs inserted in VS build main-11407.07 #81901

Closed

		@@ -0,0 +1,16 @@
		System.Runtime.InteropServices.ExtendedLayoutAttribute Compiler Support

Implement compiler support for System.Runtime.InteropServices.ExtendedLayoutAttribute #78741

Implement compiler support for System.Runtime.InteropServices.ExtendedLayoutAttribute #78741

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