Fix CustomAttributeTypedArgument.ToString() throwing an exception on Mono

[Venkad000]

CustomAttributeTypedArgument is structured differently in Mono and CoreClr and the function ToString() only takes into consideration how CoreClr's CustomAttributeTypedArgument is implemented and does a cast accordingly which causes it to fail with an InvalidCastException on Mono. So the casts should be runtime specific for it to not throw an InvalidCastException.

For Example, this is a standalone program which throws an InvalidCastException on Mono but not on CoreClr:

using System;
using System.Collections;
using System.Reflection;

using System.Runtime.InteropServices;

var attr = typeof(C).CustomAttributes.Single(d => d.AttributeType == typeof(A));
var arg = attr.ConstructorArguments.Single();
Console.WriteLine(arg.ToString());
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 { }

Output on Mono without the fix:

Unhandled Exception:
System.InvalidCastException: Specified cast is not valid.
   at System.Reflection.CustomAttributeTypedArgument.ToString(Boolean typed)
   at System.Reflection.CustomAttributeTypedArgument.ToString()
   at Program.<Main>$(String[] args) in /home/venkad/extracts/Program.cs:line 9
[ERROR] FATAL UNHANDLED EXCEPTION: System.InvalidCastException: Specified cast is not valid.
   at System.Reflection.CustomAttributeTypedArgument.ToString(Boolean typed)
   at System.Reflection.CustomAttributeTypedArgument.ToString()
   at Program.<Main>$(String[] args) in /home/venkad/extracts/Program.cs:line 9

Output on CoreClr:

new B`1+E[[System.Void()[], System.Private.CoreLib, Version=10.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]][1] { 0 }

Output on Mono with the fix:

new B`1+E[[System.Void()[], System.Private.CoreLib, Version=10.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]][1] { 0 }

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[jkotas]

Value is documented to return ReadOnlyCollection<CustomAttributeTypedArgument> for arrays: https://learn.microsoft.com/en-us/dotnet/api/system.reflection.customattributetypedargument.value?view=net-9.0#remarks

Should this issue be fixed in the implementation of Value property on Mono instead?

[Venkad000]

@jkotas I couldn't figure out where to change the implementation of Value. Could you maybe help me point to it?

[jkotas]

I would expect that the fix is needed in ves_icall_System_Reflection_RuntimeCustomAttributeData_ResolveArgumentsInternal or one of the methods that it calls.

[jkotas]

I have assigned this to copilot: #119293. Let's see whether it can figure it out.

[jkotas]

You can take a look at the commits in #119293 for some inspiration where to fix this. (The changes in that PR are not quite right - if nothing else the changes in the shared code need to be under MONO ifdef.)

[Venkad000]

@jkotas I can't seem to figure out a way to get this working. The change makes mono crash irrespective of whether I do it in the C# code or C code.

[Venkad000]

@jkotas Can we make the code under a MONO ifdef?

[jkotas]

The fix should be fixing the implementation of Value property to follow the documented behavior.

I am ok with some under MONO ifdef if there is no better way.