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.

333fred

Done an initial review pass.

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

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

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

src/Compilers/Test/Core/MinimalCoreLibBuilder.cs

src/Compilers/Core/Portable/WellKnownTypes.cs

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

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

jjonescz · 2025-12-12T10:21:02Z

docs/features/ExtendedLayoutAttribute.md

Consider adding an entry to https://github.com/dotnet/roslyn/blob/main/docs/Language%20Feature%20Status.md for this feature.

@jkoritzinsky Add the following line beneath "extension indexers" in the feature status page.
| [ExtendedLayoutAttribute](https://github.com/dotnet/runtime/issues/100896) | main | [Merged into 18.3](https://github.com/dotnet/roslyn/pull/78741) | [jkoritzinsky](https://github.com/jkoritzinsky) | [333fred](https://github.com/333fred), [jcouv](https://github.com/jcouv) | | |

333fred

Done review pass (commit 25)

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

src/Compilers/Core/Portable/PEWriter/MetadataWriter.cs

src/Compilers/CSharp/Portable/Symbols/Metadata/PE/PETypeParameterSymbol.cs

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

333fred · 2025-12-12T19:17:16Z

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

                    public float f;
                }
-
+                


Nit: extra whitespace

src/Compilers/CSharp/Test/Symbol/Symbols/Metadata/PE/NoPia.cs

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

…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

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

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?

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?

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.

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

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?

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?

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

jcouv

Done with review pass (commit 28)

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

View reviewed changes

docs/features/ExtendedLayoutAttribute.md Show resolved Hide resolved

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

jkoritzinsky added 3 commits December 1, 2025 13:29

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

7317277

Add missing commas

bfafeef

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

a918ed4

333fred reviewed Dec 6, 2025

View reviewed changes

jkoritzinsky added 2 commits December 10, 2025 17:58

PR feedback

e97e4d9

Add inverted order tests, remove MinimalCoreLibBuilder

f7343d6

jkoritzinsky requested a review from 333fred December 11, 2025 02:04

Fix debug info format setting for Unix runs

6f08876

jkoritzinsky linked an issue Dec 11, 2025 that may be closed by this pull request

C# compiler integration with new System.Runtime.InteropServices.ExtendedLayoutAttribute #74705

Open

jjonescz reviewed Dec 12, 2025

View reviewed changes

333fred reviewed Dec 12, 2025

View reviewed changes

jkoritzinsky added 2 commits December 15, 2025 12:44

Revert moving AllowByRefLike bit to also use an extension

9900fe3

Use helpers for creating compilations everywhere.

7451867

jkoritzinsky requested a review from 333fred December 15, 2025 21:14

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

View reviewed changes

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

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

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

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