How to unregister and register an awaitable Event handler?

Question

In my code I need to unregister and register an event handler, This works perfect:

_favoritsImageView.Click -= _favoritsImageView_Click(this, new CustomeClickEventArgs(item));
_favoritsImageView.Click += _favoritsImageView_Click(this, new CustomeClickEventArgs(item));
void _favoritsImageView_Click(object sender, CustomeClickEventArgs e)
{
       // handles the event
}

But for an awaitable event handler i have to use this syntax:

_favoritsImageView.Click -= async (s, e) => 
{ await _favoritsImageView_ClickAsync(s, new CustomeClickEventArgs(item)); };

_favoritsImageView.Click += async (s, e) => 
{ await _favoritsImageView_ClickAsync(s, new CustomeClickEventArgs(item)); };

async Task _favoritsImageView_ClickAsync(object sender, CustomeClickEventArgs e)
{
       // does async task
}

This does not work. Because anonymous methods do not have the same reference. So the first line does not un-register the already registered handler. And eventually the second line adds an extra event handlers to the click.

Which syntax do I need to use to add and remove an async event handler?

Thanks for any suggestion.

Answer 1

The synchronous code looks something like this:

private void FavoritsImageView_Click(object sender, CustomeClickEventArgs args)
{
    // your synchronous code here
}

…

_favoritsImageView.Click += FavoritsImageView_Click;
_favoritsImageView.Click -= FavoritsImageView_Click;

The async version looks almost the same, you only need to add async to the method:

private async void FavoritsImageView_Click(object sender, CustomeClickEventArgs args)
{
    // your asynchronous code here
}

…

_favoritsImageView.Click += FavoritsImageView_Click;
_favoritsImageView.Click -= FavoritsImageView_Click;

Note that an event handler is pretty much the only place where you should use async void. In most other situations, a synchronous void method should be converted to async Task method.

Answer 2

Which syntax do I need to use to add and remove an async event handler?

The same syntax as you'd need to use with regular event handlers. You need to save the delegate somewhere so you can later de-register it:

private EventHandler eventHandler = 
                            new EventHandler(async (s, e) => await FooAsync(s, e));

public async void SomeOtherEventHandler()
{
    var m = new M();
    m.X += eventHandler;
    m.OnFoo();
    m.X -= eventHandler;
    m.OnFoo();
}

public async Task FooAsync(object sender, EventArgs e)
{
    await Task.Delay(1000);
    Debug.WriteLine("Yay event handler");
}

public class M
{
    public event EventHandler X;
    public void OnX()
    {
        // If you're using C#-6, then X?.Invoke(null, EventArgs.Empty);

        var localX = X;
        if (localX != null)
            localX(null, EventArgs.Empty);
    }
}

Edit:

@svick suggests perhaps another solution, to simply make the method async void and register it directly, which is definitely shorter:

public async void SomeOtherEventHandler()
{
    var m = new M();
    m.X += FooAsync;
    m.OnFoo();
    m.X -= FooAsync;
    m.OnFoo();
}

public async void FooAsync(object sender, EventArgs e)
{
    await Task.Delay(1000);
    Debug.WriteLine("Yay event handler");
}