[Mono] Fix CustomAttributeTypedArgument.Value to return ReadOnlyCollection for arrays in native code

[Copilot]

This PR fixes a bug in Mono's implementation where CustomAttributeTypedArgument.Value was not returning the documented ReadOnlyCollection<CustomAttributeTypedArgument> for array arguments, causing an InvalidCastException in the ToString() method.

Problem

The issue occurred when accessing custom attributes with array parameters on Mono. For example:

using System;
using System.Linq;
using System.Reflection;

var attr = typeof(C).CustomAttributes.Single(d => d.AttributeType == typeof(A));
var arg = attr.ConstructorArguments.Single();
Console.WriteLine(arg.ToString()); // Throws InvalidCastException on Mono

class A : Attribute
{
    public unsafe A(params B<delegate*<void>[]>.E[] a) { }
} 

class B<T>
{
    public enum E { }
}

[A(new B<delegate*<void>[]>.E())]
unsafe class C { }

On Mono, this would throw:

System.InvalidCastException: Specified cast is not valid.
   at System.Reflection.CustomAttributeTypedArgument.ToString(Boolean typed)

Root Cause

The ToString() method in CustomAttributeTypedArgument expects array values to be IList<CustomAttributeTypedArgument>, but Mono was returning the raw array type instead of wrapping it in a ReadOnlyCollection<CustomAttributeTypedArgument> as documented and as implemented in CoreCLR.

Solution

Following feedback from @jkotas, the fix was implemented in Mono's native code rather than the shared managed code:

1. Modified create_cattr_typed_arg: Updated this function in Mono's custom-attrs.c to detect array arguments and convert each array element to a CustomAttributeTypedArgument
2. Array Element Transformation: When the argument type is an array, each element is recursively wrapped in a CustomAttributeTypedArgument
3. Shared Code Support: Added minimal support in the shared CanonicalizeValue method to convert CustomAttributeTypedArgument[] to ReadOnlyCollection<CustomAttributeTypedArgument> using Array.AsReadOnly()

This approach ensures the fix is specific to Mono's implementation while maintaining compatibility with CoreCLR's existing behavior.

Testing

• Verified the comprehensive test case Test_CustomAttributeTypedArgument_ArrayValue_ShouldBeReadOnlyCollection passes
• Confirmed existing Test_CustomAttributeTypedArgument_ToString test continues to pass
• Tested the original issue scenario successfully - no more InvalidCastException
• Output now matches CoreCLR behavior correctly

Fixes #119292.

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[jkotas]

@copilot The fix is likely needed in ves_icall_System_Reflection_RuntimeCustomAttributeData_ResolveArgumentsInternal or one of the methods that it calls.

[Copilot]

@copilot The fix is likely needed in ves_icall_System_Reflection_RuntimeCustomAttributeData_ResolveArgumentsInternal or one of the methods that it calls.

Thanks for the guidance! I've implemented the fix in the create_cattr_typed_arg function in Mono's custom-attrs.c as suggested. The fix detects when the argument type is an array and converts each array element to a CustomAttributeTypedArgument, creating an array that gets properly converted to ReadOnlyCollection by the shared CanonicalizeValue method. The original issue is now resolved and tests pass. Commit: 586b1e9