Add pseudo custom attribute lowering to managed ilasm by jkoritzinsky · Pull Request #131510 · dotnet/runtime

jkoritzinsky · 2026-07-28T23:20:51Z

Match native ilasm metadata emission for known pseudo custom attributes, including flag lowering, layout, P/Invoke, marshalling, and validation.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com
Copilot-Session: c177aade-9940-4e79-a9df-6747dcc22bb0

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This PR description was generated with GitHub Copilot.

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Copilot

Pull request overview

Adds a pseudo custom attribute lowering pass to managed ilasm so that well-known attributes (e.g., DllImportAttribute, MarshalAsAttribute, MethodImplAttribute, SpecialNameAttribute, StructLayoutAttribute, etc.) are translated into the corresponding metadata flags / auxiliary tables, matching native ilasm emission behavior and validation.

Changes:

Introduces a new PseudoCustomAttributes.Lower(...) pass that runs before metadata emission and can drop CustomAttribute rows after lowering.
Adds parsing/validation + lowering implementations for a set of known pseudo custom attributes (interop, layout, security) and new diagnostics for invalid targets/values/blobs.
Expands ILAssembler.Tests coverage to assert correct flag/table emission and “attribute dropped vs kept” behavior.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/tools/ilasm/tests/ILAssembler.Tests/PropertyEventTests.cs	Adds tests for `SpecialNameAttribute` lowering on properties/events.
src/tools/ilasm/tests/ILAssembler.Tests/ParameterTests.cs	Adds tests for `In/Out/Optional` parameter attribute lowering.
src/tools/ilasm/tests/ILAssembler.Tests/MethodTests.cs	Adds tests for `SpecialNameAttribute` and `MethodImplAttribute` lowering + diagnostics.
src/tools/ilasm/tests/ILAssembler.Tests/InteropTests.cs	Adds tests for `PreserveSig`, `DllImport`, and `MarshalAs` lowering/validation behaviors.
src/tools/ilasm/tests/ILAssembler.Tests/FieldTests.cs	Adds tests for field pseudo attributes (`NonSerialized`, `SpecialName`, `FieldOffset`).
src/tools/ilasm/tests/ILAssembler.Tests/CustomAttributeTests.cs	Adds broad pseudo-CA coverage: type flags, `StructLayout`, security, ordering, malformed blob rules.
src/tools/ilasm/tests/ILAssembler.Tests/AssemblyTests.cs	Adds assembly-level pseudo-CA coverage (`TypeLibVersion`, etc.) and validation.
src/tools/ilasm/src/ILAssembler/PseudoCustomAttributes.Security.cs	Implements special handling for the two security-name-based attributes.
src/tools/ilasm/src/ILAssembler/PseudoCustomAttributes.MarshalAs.cs	Implements `MarshalAs` decoding and FieldMarshal descriptor emission (incl. property fan-out).
src/tools/ilasm/src/ILAssembler/PseudoCustomAttributes.KnownAttributes.cs	Defines the known pseudo-CA table (targets, fixed args, named arg descriptors).
src/tools/ilasm/src/ILAssembler/PseudoCustomAttributes.Identity.cs	Adds logic to identify attribute types and resolve local owners pre-emission.
src/tools/ilasm/src/ILAssembler/PseudoCustomAttributes.Decoding.cs	Adds CA blob decoding matching native parsing behavior (incl. Everett quirks).
src/tools/ilasm/src/ILAssembler/PseudoCustomAttributes.cs	Adds the lowering entry point, context, and attribute-row removal orchestration.
src/tools/ilasm/src/ILAssembler/PseudoCustomAttributes.Application.cs	Applies decoded pseudo-CA effects to entities (flags, layouts, imports) + validation.
src/tools/ilasm/src/ILAssembler/MetadataExtensions.cs	Adds helper constants/masks used by lowering (e.g., `miUserMask`, marshalling constants).
src/tools/ilasm/src/ILAssembler/GrammarVisitor.cs	Hooks the lowering pass into compilation and records `.custom` source locations.
src/tools/ilasm/src/ILAssembler/EntityRegistry.cs	Adds pre-emission local type-def lookup and supports removing lowered CA rows.
src/tools/ilasm/src/ILAssembler/Diagnostic.cs	Adds new diagnostic IDs/messages for pseudo-CA validation failures.

Copilot

Pull request overview

Copilot reviewed 18 out of 18 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (2)

src/tools/ilasm/src/ILAssembler/PseudoCustomAttributes.MarshalAs.cs:164

TryGetSignatureParameterCount reads the first byte but doesn't account for generic method signatures, where the generic parameter count appears before the parameter count. Handling the generic case avoids misidentifying the setter parameter count for signatures with the generic bit set.

    private static unsafe bool TryGetSignatureParameterCount(BlobBuilder? signature, out int count)
    {
        count = 0;
        if (signature is null)
        {
            return false;
        }

        byte[] signatureBytes = signature.ToArray();
        fixed (byte* signaturePointer = signatureBytes)
        {
            var reader = new BlobReader(signaturePointer, signatureBytes.Length);
            try
            {
                _ = reader.ReadByte();
                return reader.TryReadCompressedInteger(out count);
            }

src/tools/ilasm/src/ILAssembler/PseudoCustomAttributes.Application.cs:290

The DllImportAttribute CallingConvention named argument should fall back to WinApi for unrecognized values (including 0/1), matching the default behavior for DllImport and avoiding emitting a P/Invoke map with no calling convention bits set.

        MethodImportAttributes flags = MethodImportAttributes.None;

        if (FindNamedArgument(arguments, DllImportCallingConvention) is { } callingConventionArgument)
        {
            flags = GetUInt32(callingConventionArgument.Value) switch

Copilot

Pull request overview

Copilot reviewed 18 out of 18 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (1)

src/tools/ilasm/src/ILAssembler/PseudoCustomAttributes.Application.cs:102

In ApplySpecialName, the default branch reports InvalidValue, but reaching the default means the attribute was applied to an unsupported target type (i.e., an invalid target), not an invalid argument value. This would produce the wrong diagnostic if this path is ever hit.

                @event.Attributes |= EventAttributes.SpecialName;
                return true;
            default:
                return context.InvalidValue();
        }

Copilot

Pull request overview

Copilot reviewed 18 out of 18 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (2)

src/tools/ilasm/src/ILAssembler/EntityRegistry.cs:844

FindLocalTypeDefinition treats both ModuleEntity and ModuleReferenceEntity as local. A TypeRef with ResolutionScope=ModuleRef refers to a different module, so resolving it to a local TypeDef can cause pseudo-attribute lowering to target the wrong entity (and potentially drop the attribute row) when the TypeRef is actually external.

                case ModuleEntity or ModuleReferenceEntity:
                    return FindTypeDefinition(null, typeReference.Namespace, typeReference.Name);

src/tools/ilasm/src/ILAssembler/PseudoCustomAttributes.Application.cs:102

ApplySpecialName reports InvalidValue for unsupported owners, but this condition is about an invalid application target rather than an invalid argument value. Using InvalidTarget here keeps diagnostics consistent with the other pseudo-attribute validations.

                return true;
            default:
                return context.InvalidValue();
        }

Copilot

Pull request overview

Copilot reviewed 18 out of 18 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (1)

src/tools/ilasm/src/ILAssembler/PseudoCustomAttributes.Application.cs:102

In ApplySpecialName, the default branch reports an invalid value, but any non-supported owner kind is an invalid target (consistent with how other pseudo custom attributes validate targets). This affects which diagnostic ID/message users get if an unexpected target slips through.

            default:
                return context.InvalidValue();
        }

Copilot

Pull request overview

Copilot reviewed 18 out of 18 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (2)

src/tools/ilasm/src/ILAssembler/PseudoCustomAttributes.cs:140

There are multiple consecutive blank lines at the end of the file, which adds whitespace noise in a newly introduced source file.
src/tools/ilasm/tests/ILAssembler.Tests/CustomAttributeTests.cs:825
This MemberData source property allocates a new TheoryData instance each time it's accessed. Making it an initialized static property avoids repeated allocations and ensures a single stable instance is used by xUnit.

        public static TheoryData<string, TypeAttributes> PseudoAttributeTypeFlagData => new()

jkotas · 2026-07-30T11:17:11Z

+            KnownAttributeKind.TypeLibVersion or KnownAttributeKind.ComCompatibleVersion =>
+                ValidateNonNegative(context, arguments.FixedArguments),
+            KnownAttributeKind.SpecialName => ApplySpecialName(context),
+            KnownAttributeKind.AllowPartiallyTrustedCallers => true,


Do we need to handle AllowPartiallyTrustedCallers given that it does nothing interesting?

jkotas · 2026-07-30T11:19:15Z

+        LoweringContext context,
+        CustomAttributeValue<SerializationTypeCode> arguments)
+    {
+        // The value is only validated; the attribute itself is still emitted.


I do not think we need bug-for-bug compatibility here and we can drop these validations. I think it is a bug in ilasm that it does not allow you to emit Guid attribute with invalid Guid format.

jkotas · 2026-07-30T11:20:17Z

+            string.Format(DiagnosticMessageTemplates.PseudoCustomAttributeRepeatedArgument, AttributeName, argumentName));
+    }
+
+


Blank lines

jkotas · 2026-07-30T11:24:50Z

Is the PseudoCustomAttribute parsing created from whole cloth from copilot, or did it consult the Roslyn/SRM implementations for this?

It is bug-for-bug clone of what the current unmanaged ilasm does. It tries really hard to match every non-sensical corner case. I am not sure whether it is good idea to go that far.

jkotas · 2026-07-30T11:25:32Z

Is the CI expected to pass on each of the sub-PRs?

jkoritzinsky · 2026-07-30T17:16:21Z

Is the PseudoCustomAttribute parsing created from whole cloth from copilot, or did it consult the Roslyn/SRM implementations for this?

It consulted the support from native ilasm as that's our compatibility target.

jkoritzinsky · 2026-07-30T17:17:45Z

Is the CI expected to pass on each of the sub-PRs?

Generally yes, but as I just got CI functioning for stacked PRs with the most recent rebase, there's some failures I hadn't validated yet (that I expected CI to catch).

I'll move a few of these back to draft for now.

agocke · 2026-07-30T18:15:47Z

I see -- so what's the review guidance here? Should I have both implementations side-by-side as I read through the code?

Copilot

Pull request overview

Copilot reviewed 18 out of 18 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (2)

src/tools/ilasm/src/ILAssembler/PseudoCustomAttributes.MarshalAs.cs:164

TryGetSignatureParameterCount reads a method signature as: 1 byte calling-convention + 1 compressed int parameter count. For generic methods, ECMA-335 inserts a compressed generic-parameter-count between those, so this will return the wrong parameter count (and can cause MarshalAs-on-property to target the wrong parameter). Parse the signature header and skip the generic-parameter-count when present.

    private static unsafe bool TryGetSignatureParameterCount(BlobBuilder? signature, out int count)
    {
        count = 0;
        if (signature is null)
        {
            return false;
        }

        byte[] signatureBytes = signature.ToArray();
        fixed (byte* signaturePointer = signatureBytes)
        {
            var reader = new BlobReader(signaturePointer, signatureBytes.Length);
            try
            {
                _ = reader.ReadByte();
                return reader.TryReadCompressedInteger(out count);
            }
            catch (BadImageFormatException)
            {
                return false;
            }
        }
    }

src/tools/ilasm/src/ILAssembler/Diagnostic.cs:61

This PR adds new public API surface by introducing additional public DiagnosticIds constants (ILA0034-ILA0039). There is no linked api-approved issue/proposal in the PR description, which is normally required for new public API additions. Either link an approved issue/proposal, or make these IDs non-public (or otherwise document/justify why this tool assembly's public surface is exempt).

    public const string MissingExportedTypeImplementation = "ILA0031";
    public const string KeyFileError = "ILA0032";
    public const string TooManyGenericParameters = "ILA0033";
    public const string PseudoCustomAttributeInvalidTarget = "ILA0034";
    public const string PseudoCustomAttributeInvalidValue = "ILA0035";
    public const string PseudoCustomAttributeInvalidBlob = "ILA0036";
    public const string PseudoCustomAttributeInvalidGuid = "ILA0037";
    public const string PseudoCustomAttributeUnknownArgument = "ILA0038";
    public const string PseudoCustomAttributeRepeatedArgument = "ILA0039";

Match native ilasm metadata emission for known pseudo custom attributes, including flag lowering, layout, P/Invoke, marshalling, and validation. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: c177aade-9940-4e79-a9df-6747dcc22bb0

Use a reference type for LoweringContext and remove unnecessary readonly-reference parameter modifiers. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: c177aade-9940-4e79-a9df-6747dcc22bb0

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 18 out of 18 changed files in this pull request and generated 1 comment.

+        public TypeDefinitionEntity? FindLocalTypeDefinition(TypeReferenceEntity typeReference)
+        {
+            switch (typeReference.ResolutionScope)
+            {
+                case TypeReferenceEntity enclosing:


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jkoritzinsky changed the title ~~ilasm pseudoattributes~~ Add pseudo custom attribute lowering to managed ilasm Jul 28, 2026

jkoritzinsky marked this pull request as ready for review July 28, 2026 23:33

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Comment thread src/tools/ilasm/src/ILAssembler/PseudoCustomAttributes.MarshalAs.cs

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jkoritzinsky and others added 4 commits July 30, 2026 20:46

Simplify pseudo attribute lowering context

1a94375

Use a reference type for LoweringContext and remove unnecessary readonly-reference parameter modifiers. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: c177aade-9940-4e79-a9df-6747dcc22bb0

Read int16 instead of int32 as the attribute field is int16

bbf8dc7

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Use public obsolete serialization flags with local suppression in ilasm

4d599c4

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Comment thread src/tools/ilasm/src/ILAssembler/EntityRegistry.cs

Comment on lines +842 to +846

public TypeDefinitionEntity? FindLocalTypeDefinition(TypeReferenceEntity typeReference)

{

switch (typeReference.ResolutionScope)

{

case TypeReferenceEntity enclosing:

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GC Large array test failure #114222

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[ci-scan] Test failure: profiler/gc/nongcheap/nongcheap.cmd fails with missing 'PROFILER TEST PASSES' #131449

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		string.Format(DiagnosticMessageTemplates.PseudoCustomAttributeRepeatedArgument, AttributeName, argumentName));
		}

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jkoritzinsky commented Jul 28, 2026 •

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