Add test for custom attribute with generic enum array argument #123445

Copilot · 2026-01-21T20:31:26Z

Description

Adds regression test for custom attribute decoding with generic enum arguments involving arrays. This test validates the fix for a Mono runtime crash caused by incorrect MonoClass resolution for MONO_TYPE_GENERICINST when loading custom attribute values (see #123439).

Note: This PR contains only the regression test. The actual Mono runtime fix is in PR #123439. This PR is meant to land after #123439 to provide test coverage.

Changes Made

Test Addition (System.Reflection.Tests/CustomAttributeTests.cs):

Added CustomAttributeCtor_WithGenericEnumArgument_DecodesCorrectly test method
Verifies custom attribute with constructor parameter GenericClassForEnum<int[]>.E decodes successfully
Ensures no crash when accessing CustomAttributes and ConstructorArguments on decorated types
Validates ArgumentType matches the expected generic enum type typeof(GenericClassForEnum<int[]>.E)
Validates Value equals the default enum value using Convert.ToInt32() to properly compare the boxed enum value
Provides regression coverage for the MONO_TYPE_GENERICINST handling issue
Includes [ActiveIssue] attribute for a related bug in System.Reflection.Metadata/NativeAOT (CustomAttribute.DecodeValue doesn't handle generic type instantiations #123878)

private class GenericEnumAttributeWithArray : Attribute
{
    public GenericEnumAttributeWithArray(GenericClassForEnum<int[]>.E e) { }
}

private class GenericClassForEnum<T>
{
    public enum E { }
}

[GenericEnumAttributeWithArray(default)]
private class ClassWithGenericEnumAttribute { }

[Fact]
[ActiveIssue("https://github.com/dotnet/runtime/issues/123878", typeof(PlatformDetection), nameof(PlatformDetection.IsNativeAot))]
public void CustomAttributeCtor_WithGenericEnumArgument_DecodesCorrectly()
{
    // This test ensures correct reflection behavior when loading a custom attribute
    // whose constructor argument is a generic enum instance involving an array.
    // This scenario was causing a crash in Mono (see https://github.com/dotnet/runtime/pull/123439)
    var attr = typeof(ClassWithGenericEnumAttribute).CustomAttributes.Single(d => d.AttributeType == typeof(GenericEnumAttributeWithArray));
    var arg = attr.ConstructorArguments.Single();

    // Verify that we can successfully decode the attribute without crashing
    Assert.Equal(typeof(CustomAttributeTypedArgument), arg.GetType());

    // Verify the argument type is the expected generic enum type
    Assert.Equal(typeof(GenericClassForEnum<int[]>.E), arg.ArgumentType);

    // Verify the value is the default enum value (0)
    Assert.Equal(0, Convert.ToInt32(arg.Value));
}

This test provides regression coverage for the issue, ensuring the crash doesn't reoccur after the fix in PR #123439 is merged.

Original prompt

This adds a test to System.Reflection.Tests/CustomAttributeTests.cs that covers loading a custom attribute whose constructor argument is a generic enum instance involving a function pointer array, as in the Mono crash reported in #123439. This ensures correct reflection behavior without crashing or incorrect MonoClass resolution for MONO_TYPE_GENERICINST.

The test, CustomAttributeCtor_WithGenericEnumArgument_DecodesCorrectly, uses the code example from the PR description as scenario coverage.

The new test method is placed inside the CustomAttributeTests class to match repository convention.

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lewing · 2026-01-21T23:20:24Z

@copilot merge #123439 into this pr to verify the fix

Copilot · 2026-01-21T23:28:18Z

@copilot merge #123439 into this pr to verify the fix

Merged the Mono fix from PR #123439 in commit 5542a1f. The test now covers both the regression scenario and validates the fix works correctly.

lewing · 2026-01-31T00:08:00Z

this is meant to land after #123439 as a test for the mono crash.

Copilot

Pull request overview

This PR adds both a Mono runtime fix and a regression test for a crash that occurred when loading custom attributes with generic enum constructor arguments involving function pointer arrays.

Changes:

Fixed Mono runtime crash in custom attribute decoding for generic enum types with function pointer arrays
Added regression test to prevent future occurrences of this issue

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/mono/mono/metadata/custom-attrs.c	Defers `tklass` initialization for `MONO_TYPE_GENERICINST` types and updates assertion to handle generic instance cases, preventing crash during custom attribute value loading
src/libraries/System.Runtime/tests/System.Reflection.Tests/CustomAttributeTests.cs	Adds `CustomAttributeCtor_WithGenericEnumArgument_DecodesCorrectly` test to verify custom attributes with generic enum arguments involving function pointer arrays decode without crashing

Copilot · 2026-01-31T00:17:51Z

src/libraries/System.Runtime/tests/System.Reflection.Tests/CustomAttributeTests.cs

+            var arg = attr.ConstructorArguments.Single();
+            Assert.Equal(typeof(CustomAttributeTypedArgument), arg.GetType());


Consider adding more comprehensive assertions to validate the actual argument value and type, similar to the BoxedEnumAttributes() test pattern on lines 259-277. For example, you could assert that arg.ArgumentType equals the expected generic enum type typeof(GenericClassForEnum<delegate*<void>[]>.E) and verify the value is the default enum value (0). This would provide stronger regression coverage beyond just ensuring the code doesn't crash.

@copilot apply this feedback

Enhanced the test assertions in commit c7189a0. The test now validates:

ArgumentType matches the expected generic enum type GenericClassForEnum<delegate*<void>[]>.E

Value is the default enum value (0)

This provides stronger regression coverage beyond just ensuring the code doesn't crash.

src/libraries/System.Runtime/tests/System.Reflection.Tests/CustomAttributeTests.cs

MichalStrehovsky · 2026-02-02T11:41:44Z

@copilot this doesn't build because of src/libraries/System.Runtime/tests/System.Reflection.Tests/CustomAttributeTests.cs(319,47): error CS0214: (NETCORE_ENGINEERING_TELEMETRY=Build) Pointers and fixed size buffers may only be used in an unsafe context

Copilot · 2026-02-02T11:46:29Z

@copilot this doesn't build because of `src/libraries/System.Runtime/tests/System.Reflection.Tests/CustomAttributeTests.cs(319,47): error CS0214: (NETCORE_ENGINEERING_TELEMETRY=Build) Pointers and fix...

Fixed in commit d05fc87. Marked the test method as unsafe to allow the function pointer type usage in the assertions.

MichalStrehovsky · 2026-02-02T14:43:23Z

@copilot test failure:

      Assert.Equal() Failure: Values differ
      Expected: 0
      Actual:   0
      Stack Trace:
        /_/src/libraries/System.Runtime/tests/System.Reflection.Tests/CustomAttributeTests.cs(319,0): at System.Reflection.Tests.CustomAttributeTests.CustomAttributeCtor_WithGenericEnumArgument_DecodesCorrectly()

Copilot · 2026-02-02T14:47:45Z

@copilot test failure:

...

Fixed in commit db2bfe7. The issue was comparing a boxed enum value directly with a cast enum type. Changed to use Convert.ToInt32(arg.Value) to compare the underlying integer value instead.

lewing · 2026-02-02T20:45:40Z

I'm guessing the mono failures are #90308 now

MichalStrehovsky · 2026-02-02T22:00:13Z

I'm guessing the mono failures are #90308 now

Do we need a function pointer array for this regression test? Wouldn't we hit the problem with something like

public unsafe GenericEnumAttributeWithFunctionPointer(GenericClassForEnum<int[]>.E e) { }

too? It would be enough to hit the System.Reflection.Metadata bug. Function pointers might hit more bonus bugs too, but I'd cross that bridge when we get there.

Add regression test for Mono crash when decoding custom attributes with generic enum arguments involving arrays (PR #123439). Uses GenericClassForEnum<int[]>.E to test the MONO_TYPE_GENERICINST handling in custom attribute decoding. Co-authored-by: Michal Strehovsky <MichalStrehovsky@users.noreply.github.com>

lewing · 2026-02-03T19:12:27Z

@copilot update the description to reflect the current state of the pr

Copilot · 2026-02-03T19:16:09Z

@copilot update the description to reflect the current state of the pr

Updated the PR description. The current PR now only contains the regression test (not the Mono runtime fix, which is in PR #123439). The test was also simplified to use GenericClassForEnum<int[]>.E instead of function pointer arrays, making it cleaner and no longer requiring unsafe context.

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Copilot AI changed the title ~~Add test for custom attribute with generic enum function pointer array argument~~ Add test and Mono fix for custom attribute with generic enum function pointer array argument Jan 21, 2026

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Avoid incorrectly resolving a MonoClass for MONO_TYPE_GENERICINST when loading custom attribute values. #123439

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Add an azdo failure skill #123913

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		var arg = attr.ConstructorArguments.Single();
		Assert.Equal(typeof(CustomAttributeTypedArgument), arg.GetType());

Add test for custom attribute with generic enum array argument #123445

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