New Snippets Site

[Leslie Sanford]

Delegates/events are immutable, so they have the same semantics as using the string class, for example. Consider:

using System;
using System.Diagnostics;

namespace Tester
{
class Class1
{
public delegate void MyEventHandler(object sender, EventArgs e);

    public static event MyEventHandler MyEvent;

    private static int invocationCount = 0;

    private static void HandleMyEvent(object sender, EventArgs e)
    {
        Console.WriteLine("Handling event...");
        invocationCount++;
    }
	
    \[STAThread\]
    static void Main(string\[\] args)
    {
        MyEventHandler handler0 = MyEvent;

        MyEvent += new MyEventHandler(HandleMyEvent);

        MyEventHandler handler1 = MyEvent;

        MyEvent += new MyEventHandler(HandleMyEvent);

        MyEventHandler handler2 = MyEvent;
        Debug.Assert(handler0 == null);
        Debug.Assert(handler1 != null);
        Debug.Assert(handler1.GetInvocationList().Length == 1);
        Debug.Assert(handler2 != null);
        Debug.Assert(handler2.GetInvocationList().Length == 2);

        handler1(null, EventArgs.Empty);
        handler2(null, EventArgs.Empty);

        Debug.Assert(invocationCount == 3);            
    }
}

}

With the output:

Handling event...
Handling event...
Handling event...

[Marc Clifton]

Leslie Sanford wrote:

Delegates/events are immutable, so they have the same semantics as using the string class, for example.

Ah! Thanks for the example. It's definitely not obvious from the symantics of the language. Marc VS2005 Tips & Tricks -- contributions welcome!