Question

This compiles:

public interface IMyInterface
{
    event Action<dynamic> OnSomeEvent;
}

class MyInterface : IMyInterface
{
    public event Action<dynamic> OnSomeEvent;
}

But when i separate the interface and the implementation to different projects, i get:

Accessor
‘TestProject2.MyInterface.OnSomeEvent.remove’
cannot implement interface member
‘InterfaceNamespace.IMyInterface.remove_OnSomeEvent(System.Action)’
for type ‘TestProject2.MyInterface’.
Use an explicit interface
implementation.

This occurs only with a dynamic parameter…

Answer 1

Good catch. This looks like it’s possibly a bug in the C# compiler – I’ll ping Eric Lippert to see what he thinks. (dynamic can be a bit tricksy; there may well be a perfectly good but non-obvious reason for this error.)

EDIT: The code below appears not to work after all. I could have sworn I had it working this morning… I’m very confused as to what’s going on. As per Simon’s comments, the code fails with a message saying it’s not supported by the language.

Note that if you do use explicit interface implementation, it appears to compile just fine:

// Doesn't actually compile - see edit above
class MyInterface : IMyInterface
{
    private Action<dynamic> foo;

    event Action<dynamic> IMyInterface.OnSomeEvent
    {
        // TODO (potentially): thread safety
        add { foo += value; }
        remove { foo -= value; }
    }
}

---

EDIT: The rest of this answer still stands…

Note that you can’t specify a field-like event as an explicitly-implemented event, i.e. this doesn’t work:

event Action<dynamic> IMyInterface.OnSomeEvent;

It gives the following error message:

Test.cs(15,39): error CS0071: An explicit interface implementation of an event must use event accessor syntax

And if you just try to change to the event accessor syntax, you get the same error as the
original code.

Note that changing the event to a property works fine with an auto-implemented property implementation.