C# – How to add an event handler for events from button in C# from source view (aspx)

The C# compiler's default implementation of adding an event handler calls Delegate.Combine, while removing an event handler calls Delegate.Remove:

Fire = (MyDelegate) Delegate.Remove(Fire, new MyDelegate(Program.OnFire));

The Framework's implementation of Delegate.Remove doesn't look at the MyDelegate object itself, but at the method the delegate refers to (Program.OnFire). Thus, it's perfectly safe to create a new MyDelegate object when unsubscribing an existing event handler. Because of this, the C# compiler allows you to use a shorthand syntax (that generates exactly the same code behind the scenes) when adding/removing event handlers: you can omit the new MyDelegate part:

Fire += OnFire;
Fire -= OnFire;

When the last delegate is removed from the event handler, Delegate.Remove returns null. As you have found out, it's essential to check the event against null before raising it:

MyDelegate handler = Fire;
if (handler != null)
    handler("Hello 3");

It's assigned to a temporary local variable to defend against a possible race condition with unsubscribing event handlers on other threads. (See my blog post for details on the thread safety of assigning the event handler to a local variable.) Another way to defend against this problem is to create an empty delegate that is always subscribed; while this uses a little more memory, the event handler can never be null (and the code can be simpler):

public static event MyDelegate Fire = delegate { };

You can automatically create a handler method by going to your page's OnLoad or Page_Load method, and adding a handler for the event. For example, for this Label:

<asp:label ID="MyLabel" runat="server" />

You would do this:

protected void OnLoad(object sender, EventArgs e)
{
     MyLabel.PreRender += 
}

At this point IntelliSense should kick in and offer to generate an event handler for you. If you hit TAB a couple of times, you should have a new MyLabel_PreRender method.

Good luck!