Passing delegate as method parameter

Question

I'm currently developing an EventManager class to ensure that no events are left wired to dead WCF duplex clients, and also to control prevent multiple wiring from the same client to the one event.

Now basically, I'm what stuck with is trying to pass the event delegate to a function that will control the assignment like this.

var handler = new SomeEventHandler(MyHandler);
Wire(myObject.SomeEventDelegate, handler);

To call this:

private void Wire(Delegate eventDelegate, Delegate handler)
{
    // Pre validate the subscription.
    eventDelegate = Delegate.Combine(eventDelegate, handler);
    // Post actions (storing subscribed event delegates in a list)
}

Update

The code for SomeEventDelegate wrapper is:

public Delegate SomeEventDelegate
{
    get { return SomeEvent; }
    set { SomeEvent = (SomeEventHandler) value; }
}

event SomeEventHandler SomeEvent;

Obviously the delegate is not being returned to the myObject.SomeEventDelegate And I cannot return the Delegate from the method because I need some validation after too. Do you have any idea on how to do this?

C# events generate a hidden delegate for you. To access this event, refer to: http://stackoverflow.com/questions/1129517/c-how-to-find-if-an-event-is-hooked-up/1129530#1129530. — Steve Guidi, Oct 14 '10 at 23:54

Cameron · Accepted Answer · 2010-10-14T23:58:42.927

6

Use the C# ref parameter modifier:

var handler = new SomeEventHandler(MyHandler);
Wire(ref myObject.SomeEventDelegate, handler);

private void Wire(ref Delegate eventDelegate, Delegate handler)
{
    // Pre validate the subscription.
    eventDelegate = Delegate.Combine(eventDelegate, handler);
    // Post actions (storing subscribed event handlers in a list)
}

Note also that there exists some nice syntactic sugar (as of C# 2.0) for assigning and combining delegates (see this article, for example):

Wire(ref myObject.SomeEventDelegate, MyHandler);

private void Wire(ref Delegate eventDelegate, Delegate handler)
{
    // Pre validate the subscription.
    eventDelegate += handler;
    // Post actions (storing subscribed event handlers in a list)
}

It has been pointed out to me that ref only works with fields, not properties. In the case of a property, an intermediary variable can be used:

var tempDelegate = myObject.SomeEventDelegate;
Wire(ref tempDelegate, MyHandler);
myObject.SomeEventDelegate = tempDelegate;

edited Oct 14 '10 at 23:58

answered Oct 14 '10 at 23:28

Cameron

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Note that it will only work if SomeeventDelegate is a field... Properties can't be passed as ref parameters – Thomas Levesque Oct 14 '10 at 23:41
hi there, the ref keyword doesn't work here because the myObject.SomeEventDelegate is just a wrapper property to make the event delegate accessible outside the its class. – Guilherme Duarte Oct 14 '10 at 23:43
@Thomas: Ah, good point. I had forgotten about that. In that case, an intermediary variable can be used. Editing... – Cameron Oct 14 '10 at 23:54
Ok, I believe it works now! I changed the event declaration following the Steve Guidi suggestion and after I could use the ref keyword. Thank you all for your help! – Guilherme Duarte Oct 15 '10 at 00:12

Passing delegate as method parameter

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