allow `[UnmanagedCallersOnly]` on interface's static abstract method

[GUZZ07]

Background and Motivation

Assuming we have such native method:

[DllImport("somelib")]
public static extern void RegisterSomeStrategy(Guid identifier, [MarshalAs(UnmanagedType.LPWStr)] string displayName, delegate* unmanaged<nint, nint, nint, void> callback);

Then someone in team says "can you write a wrapper for RegisterSomeStrategy? other teammates do not want to touch this pinvoke directly, we just want to implements an interface, write strategy details in its method and call a void RegisterStrategy(InterfaceImplType)".

So I write a wrapper like following code

public interface IXXStrategy
{
    public static abstract Guid Identifier { get; }
    public static abstract string DisplayName { get; }
    public static abstract void StrategyImpl(nint para0, nint para1, nint para2);
}
public static void RegisterSomeStragety<TStrategy>() where TStrategy : IXXStrategy
{
    unsafe
    {
        RegisterSomeStrategy(TStrategy.Identifier, TStrategy.DisplayName, &TStrategy.StrategyImpl); // CS8786: Calling convention of 'void IXXStrategy.StrategyImpl(nint para0, nint para1, nint para2)' is not compatible with 'Unmanaged'.
    }
}

Then I found there is no way to require every implementation of IXXStrategy must provide a StrategyImpl that is [UnmanagedCallersOnly] and let TStrategy knows it.

• If adding [UnmanagedCallersOnly] to implementation class, I will get a "CS8786: Calling convention of 'void IXXStrategy.StrategyImpl(nint para0, nint para1, nint para2)' is not compatible with 'Unmanaged'.
  " in RegisterSomeStragety<TStrategy>, and a "CS8932: 'UnmanagedCallersOnly' method cannot implement interface member in type" in implementation class ;
• If adding [UnmanagedCallersOnly] to interface, I will get a "'UnmanagedCallersOnly' can only be applied to ordinary static non-abstract, non-virtual methods or static local functions." in interface.

Proposal

Since method with [UnmanagedCallersOnly] cannot implementation interface, is it part of method signature?
If it is part of method signature, can it be used in static abstract method in interface like following?

public interface IXXStrategy
{
    // ...
    public static abstract unmanaged void StrategyImpl(nint para0, nint para1, nint para2);
}

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[huoyaoyuan]

The standard practice of passing [UnmanagedCallersOnly] methods is passing them as raw function pointers:

public struct XXStrategy
{
    public Guid Identifier;
    public string DisplayName;
    public unsafe delegate*<nint, nint, nint, void> Impl;
}

Or simply define a static virtual property that returns function pointer:

public static abstract unsafe delegate*<nint, nint, nint, void> StrategyImpl { get; }

Since method with [UnmanagedCallersOnly] cannot implementation interface, is it part of method signature?
If it is part of method signature, can it be used in static abstract method in interface like following?

As the word "only" in the feature implies, UCO methods are NOT compatible with most managed type system features.

[AaronRobinsonMSFT]

@GUZZ07 Thanks for the suggestion. @huoyaoyuan's reply, #129883 (comment), aligns with how the interop team thinks about this issue.

The UnmanagedCallersOnly is explicitly designed around exposing a function to an unmanaged context. Originally for exposing an unmanaged function from the final assembly. Trying to reconcile that with an interface experience, whether static or instance, is beyond the intended scope of use. The key phrase to take away from the comment is:

UCO methods are NOT compatible with most managed type system features.