Mocking grain interfaces returned from AsReference for unit testing

[TimoSchmechel]

When writing unit tests for a class that performs some grain calls its currently impossible if the extension method AsReference is used.
This problem doesnt arise when just using GetGrain to access the grain as there we can easily return a mocked version of the grain interface.

Ideally I would like to be able to unit test services that rely on grain calls easily without having to use an in memory cluster or any other complicated mock setups. The fact orleans relies on interfaces makes this very easy except for when relying on AsReference casts to get an IAddressable.

Below is a simple example that demonstrates the problem

[TestFixture]
public class UnitTestExample
{
    [Test]
    public async Task GrainMocking()
    {
        var fixture = new Fixture();

        var sut = fixture.Create<IExampleService>();

        var result = await sut.MethodWithGrainCall();

        result.Should().NotBeNull();
    }
    
    [Test]
    public async Task AddressableMocking()
    {
        var fixture = new Fixture();

        var sut = fixture.Create<IExampleService>();

        //this fails
        var result = await sut.MethodWithGrainCallViaAsReference();

        result.Should().NotBeNull();
    }
}

public interface IExampleService
{
    Task<string> MethodWithGrainCall();
    Task<string> MethodWithGrainCallViaAsReference();
}

class ExampleService : IExampleService
{
    private IGrainFactory _grainFactory;

    public ExampleService(IGrainFactory grainFactory)
    {
        _grainFactory = grainFactory;
    }

    public async Task<string> MethodWithGrainCall()
    {
        var grain = _grainFactory.GetGrain<IExampleGrain>(0);

        var grainResult = await grain.GetString();

        return grainResult;
    }
    
    public async Task<string> MethodWithGrainCallViaAsReference()
    {
        var grain = _grainFactory.GetGrain<IExampleGrain>(0).AsReference<IExampleAddressable>();

        var grainResult = await grain.GetStringAsAddressable();

        return grainResult;
    }
}

internal interface IExampleAddressable : IAddressable
{
    Task<string> GetStringAsAddressable();
}

internal interface IExampleGrain : IGrainWithIntegerKey
{
    Task<string> GetString();
}