How to ensure an event is only subscribed to once

Question

I would like to ensure that I only subscribe once in a particular class for an event on an instance.

For example I would like to be able to do the following:

Answer 1

U can use Postsharper to write one attribute just once and use it on normal Events. Reuse the code. Code sample is given below.

[Serializable]
public class PreventEventHookedTwiceAttribute: EventInterceptionAspect
{
    private readonly object _lockObject = new object();
    readonly List _delegates = new List();

    public override void OnAddHandler(EventInterceptionArgs args)
    {
        lock(_lockObject)
        {
            if(!_delegates.Contains(args.Handler))
            {
                _delegates.Add(args.Handler);
                args.ProceedAddHandler();
            }
        }
    }

    public override void OnRemoveHandler(EventInterceptionArgs args)
    {
        lock(_lockObject)
        {
            if(_delegates.Contains(args.Handler))
            {
                _delegates.Remove(args.Handler);
                args.ProceedRemoveHandler();
            }
        }
    }
}

Just use it like this.

[PreventEventHookedTwice]
public static event Action GoodEvent;

For details look at Implement Postsharp EventInterceptionAspect to prevent an event Handler hooked twice