Prevent next event handler being called

Question

I have two event handlers wired up to a button click in a Windows form like so:

this.BtnCreate.Click += new System.EventHandler(new RdlcCreator().FirstHandler);
         


        

Answer 1

System.ComponentModel namespace contains a CancelEventHandler delegate which is used for this purpose. One of the arguments it provides is a CancelEventArgs instance which contains a boolean Cancel property which can be set be any of the handlers to signal that execution of the invocation list should be stopped.

However, to attach it to a plain EventHandler delegate, you will need to create your own wrapper, something like:

public static class CancellableEventChain
{
    public static EventHandler CreateFrom(params CancelEventHandler[] chain)
    {
        return (sender, dummy) =>
        {
            var args = new CancelEventArgs(false);
            foreach (var handler in chain)
            {
                handler(sender, args);
                if (args.Cancel)
                    break;
            }
        };
    }
}

For your example, you would use it like this:

this.BtnCreate.Click += CancellableEventChain.CreateFrom(
    new RdlcCreator().FirstHandler,
    this.BtnCreate_Click
    /* ... */
);

Of course, you would need to capture the created chain handler in a field if you need to unsubscribe (detach) it later.