C# pattern to prevent an event handler hooked twice [duplicate]

Answer 1

In silverlight you need to say e.Handled = true; in the event code.

void image_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
    e.Handled = true; //this fixes the double event fire problem.
    string name = (e.OriginalSource as Image).Tag.ToString();
    DoSomething(name);
}

Please tick me if this helps.

Answer 2

Microsoft's Reactive Extensions (Rx) framework can also be used to do "subscribe only once".

Given a mouse event foo.Clicked, here's how to subscribe and receive only a single invocation:

Observable.FromEvent<MouseEventArgs>(foo, nameof(foo.Clicked))
    .Take(1)
    .Subscribe(MyHandler);

...

private void MyHandler(IEvent<MouseEventArgs> eventInfo)
{
   // This will be called just once!
   var sender = eventInfo.Sender;
   var args = eventInfo.EventArgs;
}

In addition to providing "subscribe once" functionality, the RX approach offers the ability to compose events together or filter events. It's quite nifty.

Answer 3

I've tested each solution and the best one (considering performance) is:

private EventHandler _foo;
public event EventHandler Foo {

    add {
        _foo -= value;
        _foo += value;
    }
    remove {
        _foo -= value;
    }
}

No Linq using required. No need to check for null before cancelling a subscription (see MS EventHandler for details). No need to remember to do the unsubscription everywhere.

Answer 4

You need to implement the add and remove accessors on the event, and then check the target list of the delegate, or store the targets in a list.

In the add method, you can use the Delegate.GetInvocationList method to obtain a list of the targets already added to the delegate.

Since delegates are defined to compare equal if they're linked to the same method on the same target object, you could probably run through that list and compare, and if you find none that compares equal, you add the new one.

Here's sample code, compile as console application:

using System;
using System.Linq;

namespace DemoApp
{
    public class TestClass
    {
        private EventHandler _Test;

        public event EventHandler Test
        {
            add
            {
                if (_Test == null || !_Test.GetInvocationList().Contains(value))
                    _Test += value;
            }

            remove
            {
                _Test -= value;
            }
        }

        public void OnTest()
        {
            if (_Test != null)
                _Test(this, EventArgs.Empty);
        }
    }

    class Program
    {
        static void Main()
        {
            TestClass tc = new TestClass();
            tc.Test += tc_Test;
            tc.Test += tc_Test;
            tc.OnTest();
            Console.In.ReadLine();
        }

        static void tc_Test(object sender, EventArgs e)
        {
            Console.Out.WriteLine("tc_Test called");
        }
    }
}

Output:

tc_Test called

(ie. only once)

Answer 5

Explicitly implement the event and check the invocation list. You'll also need to check for null:

using System.Linq; // Required for the .Contains call below:

...

private EventHandler foo;
public event EventHandler Foo
{
    add
    {
        if (foo == null || !foo.GetInvocationList().Contains(value))
        {
            foo += value;
        }
    }
    remove
    {
        foo -= value;
    }
}

Using the code above, if a caller subscribes to the event multiple times, it will simply be ignored.

Answer 6

have your singleton object check it's list of who it notifies and only call once if duplicated. Alternatively if possible reject event attachment request.