Question

I am trying to implement a generic Wrapper-Class for Qt’s class system using C#’s DynamicObject. However, I want to write the following code:

dynamic obj = new SomeWrapperClass(....); // This extends DynamicObject
obj.OnMyEvent += (Action)(() => Console.WriteLine("DO something!"));

The above is valid code according to VS2010 (the explicit cast to Action is required), but how exactly do i “catch” that statement using DynamicObject’s methods?

I tried implementing TryGetMember() and it gets called for the statement, but I have no idea what I have to return to make it work.

Any hints?

Answer 1

Reflector is your friend on this one. The code generated for your second line looks something like this (approximately):

if(Binder.IsEvent("OnMyEvent", typeof(SomeWrapperClass)))
{
    Binder.InvokeMember("add_OnMyEvent", obj, myAction);
}
else
{
    var e = Binder.GetMember("OnMyEvent", obj);
    var ae = Binder.BinaryOperation(ExpressionType.AddAssign, e, myAction);
    Binder.SetMember("OnMyEvent", obj, ae);
}

If you can’t use a real event for OnMyEvent (in which case you can lean on the default DynamicObject implementation), then you’ll need to return something that implements AddAssign returning something like a multicast delegate. I’d suggest the former, if possible…

For fun, here’s a hackish example that dynamically binds OnMyEvent to OnMyOtherEvent:

public class SomeWrapperClass : DynamicObject
{
    public event Action OnMyOtherEvent;

    public override bool TryGetMember(GetMemberBinder binder, out object result)
    {
        if (binder.Name == "OnMyEvent")
        {
            result = OnMyOtherEvent;
            return true;
        }
        return base.TryGetMember(binder, out result);
    }

    public override bool TrySetMember(SetMemberBinder binder, object value)
    {
        if (binder.Name == "OnMyEvent" && value is Action)
        {
            OnMyOtherEvent = (Action)value;
            return true;
        }
        return TrySetMember(binder, value);
    }

    public void Test()
    {
        if (OnMyOtherEvent != null)
            OnMyOtherEvent();
    }

    private static void TestEventHandling()
    {
        dynamic obj = new SomeWrapperClass(); // This extends DynamicObject
        obj.OnMyEvent += (Action)(() => Console.WriteLine("DO something!"));
        obj.Test();
    }
}