在c＃中取消订阅事件的正确方法是什么？

Question

I have a model class with an event which I subscribe from other classes. I want to subscribe and un-subscribe in each class correctly.

• First I want to guarantee that in MyClass I unsibscribe only once even that code is in few methods.

• Second there are other classes except MyClass witch use OnMyEvent so I don't want to unintentionally unsibscribe from the event in the class.

    MyClass(IModel model) { _model = model; _model.OnMyEvent +=EventHandle; } Close() { _model.OnMyEvent -=EventHandle; } Disconnect() { //I want to check if OnMyEvent has already unsibscribed //Moreover OnMyEvent is used in other classes and //I don't want to mess up with it here _model.OnMyEvent -=EventHandle; } 

Answer 1

If you only subscribe once, it doesn't matter how many times you unsubscribe - unsubscribing when you don't have a subscription is a no-op. Equally, the entire point of the event API is that you can't accidentally unsubscribe other subscriptions (either other types, or other instances of the same type).

As such, the code as shown should be fine, although it might be worth moving the two calls to a single method that handles this. That might be overkill, though.

Also, if your type is IDisposable , make sure it gets called in that code-path too (presumably by calling Close() ).

Answer 2

You can safely unsubscribe the same handler from an event multiple times. Additional checking is not required and would be contraproductive.

Answer 3

If you want to guarantee you only unsubscribe once, you can use the GetInvocationList method:

if (_model.OnMyEvent != null && _model.GetInvocationList().Contains(EventHandle))
{
    _model.OnMyEvent -= EventHandle
}

But as mentioned by the others, you can unsubscribe multiple times. If this really isn't a problem, keep it that way. The solution I propose is just code-noise. Simply unsubscribing in one line is much neater, and easier to read when your class starts to grow.

Answer 4

You can also control subscriptions and unsubsriptions with this declaration. But you also have to iterate through dictionary and call manually subscribed delegates.

    private Dictionary<string, EventHandler> TestEvents { get; }

    public event EventHandler TestEvent
    {
        add
        {
            string name = value.GetType().FullName;
            if (!TestEvents.ContainsKey(name))
            {
                TestEvents.Add(name, value);
            }
        }
        remove
        {
            string name = value.GetType().FullName;
            if (TestEvents.ContainsKey(name))
            {
                TestEvents.Remove(name);
            }
        }
    }