why do the custom written events NEED to have a client??

[michal kreslik]

Hello, I have just found the custom-written event in C# needs to have at least one client registered to consume the event or else the application will crash. Why this is so? This means that if I unregister all the client methods from the event during the time the application is running and then the event occurs, it will crash. Or am I missing something? Evidence that this is so - test bench program:

using System;

namespace EventsTest
{
public delegate void FirstEventHandler();
public delegate void SecondEventHandler();

public class EventsClass 
{ 
    public event FirstEventHandler FirstEvent; 
    public event SecondEventHandler SecondEvent; 

    private void FireOffFirstEvent() 
    { 
        FirstEvent(); 
    } 

    private void FireOffSecondEvent() 
    { 
        SecondEvent(); 
    } 

    public void FireOffAllEvents() 
    { 
        FireOffFirstEvent(); 
        FireOffSecondEvent(); 
    } 
} 

public class EntryClass 
{ 
    static void Main() 
    { 
        EventsClass MyEvents = new EventsClass(); 
        string MyChoice; 

        do 
        { 
            Console.Write("Do you wish to register the event clients to the events? (y/n): "); 
            MyChoice = Console.ReadLine(); 

        } while (MyChoice != "y" && MyChoice != "n"); 

        switch (MyChoice) 
        { 
            case "y": 

                MyEvents.FirstEvent += new FirstEventHandler(FirstEventClient); 
                Console.WriteLine("First event registered."); 

                MyEvents.SecondEvent += new SecondEventHandler(SecondEventClient); 
                Console.WriteLine("Second event registered."); 

                break; 

            case "n": 

                Console.WriteLine("No events registered."); 

                break; 
        } 

        Console.WriteLine("Firing off all events.."); 

        MyEvents.FireOffAllEvents(); 

        Console.WriteLine("Press enter to continue.."); 
        Console.ReadLine(); 
    } 

    static void FirstEventClient() 
    { 
        Console.WriteLine("First event processed successfully."); 
    } 

    static void SecondEventClient() 
    { 
        Console.WriteLine("Second event processed successfully."); 
    } 
} 

}

[Qhalis]

From the code, it looks like you should be getting an unhandled null reference exception in FireOffFirstEvent() when you do not register the events. This is correct. When you create the instance of EventsClass, FirstEvent is null. If you do not register the events it is never set. You call FireOffAllEvents() which calls FireOffFirstEvent() which calls FirstEvent() - which is null One way to solve this is place a check around the delegate invocation, e.g. private void FireOffFirstEvent() { if (null != FirstEvent) { FirstEvent(); } } Q.

[michal kreslik]

OK. But then why there's not the same problem with not registering all the built-in events that come with, say, a windows form? Does that mean that VisualStudio doesn't even declare these events until I use them and register them? Michal

[mav northwind]

It's exactly the same with the "built-in" events. Every example for events I know and every place in the BCL where events are invoked first checks if there's an event handler attached. It has something to do with how the list of event listeners is managed.

Regards, mav -- Black holes are the places where god divided by 0...