DispatchProxy does not allow for building collectible assemblies.

[cablooshe]

Currently when you create a DispatchProxy, it makes the DynamicAssembly that generates the IL an noncollectable assembly (it's hardcoded to use AssemblyBuildAccess.Run here). This makes it impossible to create DispatchProxies inside collectible AssemblyLoadContexts.

If you try and use this create within a collectible AssemblyLoadContext, it fails with System.NotSupportedException: 'A non-collectible assembly may not reference a collectible assembly.'

public class ExampleDispatch<I> : DispatchProxy
{
    public object instance;
    internal static I Create<Dispatch>(Type type)
    {
        object proxy = Create<I, ExampleDispatch<I>>(); //Error throws here
        ((ExampleDispatch<I>)proxy).SetParameters(type, new Type[] { }, new object[] { });
        return (I)proxy;
    }

    private void SetParameters(Type instanceType, Type[] argTypes, object[] constructorArgs)
    {
        instance = instanceType.GetConstructor(argTypes).Invoke(constructorArgs);
    }

    protected override object Invoke(MethodInfo targetMethod, object[] args)
    {
        return targetMethod.Invoke(instance, args);
    }
}

A few options to fix this could be to change DispatchProxy's dynamic assembly to run with AssemblyBuildAccess.RunAndCollect, or allow for manually entering an option in DispatchProxy.Create<T, TProxy>() to decide if it should be collectible or not.

[steveharter]

or allow for manually entering an option in DispatchProxy.Create<T, TProxy>() to decide if it should be collectible or not.

This seems to be a viable option since it won't break existing users.

However, DispatchProxy investment is not our roadmap for 5.0. If there are additional demand for this, we can reconsider.

cc @GrabYourPitchforks