c# - Single-shot event subscription

Question

I'm fairly convinced that this isn't possible, but I'm going to ask nonetheless.

In order to make a single-shot subscription to events, I frequently find myself using this (self-invented) pattern:

EventHandler handler=null;
handler = (sender, e) =>
{
    SomeEvent -= handler;
    Initialize();
};
SomeEvent += handler;

It's quite a lot of boiler-plate, and it also makes Resharper whinge about modified closures. Is there a way of turning this pattern into an extension method or similar? A better way of doing it?

Ideally, I'd like something like:

SomeEvent.OneShot(handler)

Answer 1

It's not very easy to refactor to an extension method, because the only way you can refer to an event in C# is by subscribing (+=) to or unsubscribing (-=) from it (unless it's declared in the current class).

You could use the same approach as in Reactive Extensions: Observable.FromEvent takes two delegates to subscribe to the event an unsubscribe from it. So you could do something like that:

public static class EventHelper
{
    public static void SubscribeOneShot(
        Action<EventHandler> subscribe,
        Action<EventHandler> unsubscribe,
        EventHandler handler)
    {
        EventHandler actualHandler = null;
        actualHandler = (sender, e) =>
        {
            unsubscribe(actualHandler);
            handler(sender, e);
        };
        subscribe(actualHandler);
    }
}

...

Foo f = new Foo();
EventHelper.SubscribeOneShot(
    handler => f.Bar += handler,
    handler => f.Bar -= handler,
    (sender, e) => { /* whatever */ });