Overriden property in a custom attribute crashes runtime

[MichalStrehovsky]

This crashes with internal CLR error.

using System;
using System.Reflection;

Console.WriteLine(typeof(Program).GetCustomAttribute<DerivedAttribute>().Prop);

[Derived(Prop = 123)]
partial class Program;

class BaseAttribute : Attribute
{
    public virtual object Prop { get; set; }
}

class DerivedAttribute : BaseAttribute
{
    public override Enum Prop { get => (Enum)base.Prop; }
}

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[MihuBot]

I'm a bot. Here is a possible related and/or duplicate issue (I may be wrong):

• System.Reflection.CustomAttributeFormatException thrown on a retrieval of derived attribute with overridden property getter #110412

[Gaurav26061999]

I took a look at the code you shared, and the crash you’re seeing is caused by an invalid property override in your attribute classes. Specifically, in your DerivedAttribute class, you’re overriding the Prop property which is defined as type object in the base class, but in the derived class, you’ve changed its type to Enum.

In C#, when you override a property, the type must exactly match the base property’s type. Changing the return type from object to Enum breaks this rule, which leads to a violation of the CLR’s type safety. This mismatch causes the runtime to crash with an internal error when it tries to handle the attribute via reflection.

To fix this, you should keep the property type consistent override Prop as object in the derived class as well, and if you need to work with it as an Enum, add a separate property or method to perform the casting safely.

[jkotas]

In C#, when you override a property, the type must exactly match the base property’s type.

That's not correct since C# 9.0. See https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-9.0/covariant-returns

[Windows10CE]

This seems like at least partially a Roslyn issue to me. Iif you do [Derived(Prop= "some value")] here, it produces a blob that has a NamedArg that looks like PROPERTY ELEMENT_TYPE_ENUM SerString("System.Enum, System.Runtime, ...") SerString("Prop") SerString("some value"), which seems nonsensical. It is acting like it is assigning to the overriden property (which has the wrong type for the value being assigned), when its actually using the setter from the virtual definition in the base class and no setter even exists on the property it looks like it is trying to write to from the blob.

[MichalStrehovsky]

This seems like at least partially a Roslyn issue to me.

Yeah, the problem is that Roslyn has some idea of what setter this should call, the reflection stack has some other idea, and there is some mismatch.

This is somewhat related to #117584 that is out for review now (and I ran into this as part of a "I wonder what this will do" experiment). There is a mismatch between what Roslyn expects and what reflection implements. I think if we want to make changes in the area, it would be better to define what should be supported first. A possible outcome could be that some of the things that compile as valid C# now shouldn't actually compile.

Cc @AaronRobinsonMSFT