Binary Compatibility Shim

[TonyValenti]

RE: #61281

Many times I have heard people say "Such-and-such would be nice, but we can't do it because it would be a binary breaking change".

In the case of the above, is it possible to enhance the .NET runtime so that it can be told about different missing-methods and be provided an alternative?

It would be really nice if there were a way to say "Such and Such method has moved/changed/whatever and, if it is ever called, HERE is an alternative implementation of it".

I would imagine something like this:

//The type we're fixing up.
[MissingMethodHandler(IDictionary<,>)]
internal static class IDictionaryFixup {
   //A method that matches the missing signature exactly.
    internal static TValue GetValue<TKey, TValue>(IDictionary<TKey, Value> Instance, TKey Key) {
      return ((IReadOnlyDictionary<TKey, TValue>)Instance).GetValue(Key);
    }
}

[FiniteReality]

I imagine implementing something like this would make method lookup very expensive, especially using this approach. I'd probably reverse the attribute, so instead if looks something like this:

[MissingMethodHandler(typeof(IDictionaryAdditionalMethods))]
interface IDictionary<TKey, TValue>
{
    // ...
}

That way you don't have to scan the entire set of loaded assemblies for types that may be annotated that may be targeted for the type you're looking for - you just have to look at the target type's attributes.