One shot events using Lambda in C#

Question

I find myself doing this sort of thing quite often:-

 EventHandler eh = null;  //can\'t assign lambda directly since it uses eh
 eh = (s, args) =>
 {
     //s         


        

Answer 1

You can use reflection:

public static class Listener {

  public static void ListenOnce(this object eventSource, string eventName, EventHandler handler) {
    var eventInfo = eventSource.GetType().GetEvent(eventName);
    EventHandler internalHandler = null;
    internalHandler = (src, args) => {
      eventInfo.RemoveEventHandler(eventSource, internalHandler);
      handler(src, args);
    };
    eventInfo.AddEventHandler(eventSource, internalHandler);
  }

  public static void ListenOnce(this object eventSource, string eventName, EventHandler handler) where TEventArgs : EventArgs {
    var eventInfo = eventSource.GetType().GetEvent(eventName);
    EventHandler internalHandler = null;
    internalHandler = (src, args) => {
      eventInfo.RemoveEventHandler(eventSource, internalHandler);
      handler(src, args);
    };
    eventInfo.AddEventHandler(eventSource, internalHandler);
  }

}

Use it like so:

variableOfSomeType.ListenOnce("SomeEvent", 
  (s, args) => Console.WriteLine("I should print only once!"));

variableOfSomeType.ListenOnce("SomeOtherEvent", 
  (s, args) => Console.WriteLine("I should print only once!"));