How can I implement event acessors with DynamicObject in C#

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?

dahlbyk · Accepted Answer · 2010-11-24 08:20:37Z

2

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();
    }
}

edited Nov 24, 2010 at 8:20

answered Nov 24, 2010 at 8:06

dahlbyk

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2 Comments

shartte Over a year ago

First of all, thanks a LOT for the hint about just looking at the IL it generates (well or a rather high-level representation). What I find quite interesting is how it handles the return value of GetMember. Couldn't I just use a static "mock" event to return from TryGetMember for all my events, then do the actual binding in TryInvokeMember? (for add_XYZEvent)?

dahlbyk Over a year ago

That's what I would try, yes. I have no idea if it would work well, but if you get it figured out please share. :)

Cheng Chen · Accepted Answer · 2010-11-24 08:31:32Z

0

Invoke your Action with reflection:

dynamic o = new SomeWrapperClass();
o.OnMyEvent += (Action)(() => Console.WriteLine("DO something!"));
var a = typeof(SomeWrapperClass).GetField("OnMyEvent", BindingFlags.Instance | BindingFlags.NonPublic);
(a.GetValue(o) as Action).Invoke();

Output: DO something!

answered Nov 24, 2010 at 8:31

Cheng Chen

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Comments

Dean Chalk · Accepted Answer · 2010-11-24 09:51:07Z

I think you are confusing events with delegates. Events are effectively delegates, but you cannot use the 'add' and 'remove' accessors with delegates - however the += and -= works the same with both.

obj.OnMyEvent += (Action)(() => Console.WriteLine("DO something!"));

This is basically adding a target to the invocation list of a delegate, so that delegate must have a similar type (in this case a parameterless Action delegate).

The suggested implementation is below:

private Action actiondelegate = (Action)(() => {});

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

Note that you need an empty Action in your Action delegate - this is because if it is null the TryGetMember and TrySetMember wont work correctly.

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