+= operator with Events

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public void Bar()
{
    Foo foo = new Foo();
    **foo.MyEvent += foo_MyEvent;**
    foo.FireEvent();        
}

void foo_MyEvent(object sender, EventArgs e)
{
    ((Foo)sender).MyEvent -= foo_MyEvent;
}

Hey I'm a bit unfamiliar with events, could someone tell me what the += operator does with events?

events c#
6 Answers

One of the great features of delegates is that you can combine them together. This is called multicasting. You can use the + or += operator to add another method to the invocation list of an existing delegate instance. Similarly, you can also remove a method from an invocation list by using the decrement assignment operator (- or -=). This feature forms the base for events in C#. Below is a multicast delegate example.

class Program
{
    static void Hello(string s)
    {
        Console.WriteLine("  Hello, {0}!", s);
    }

    static void Goodbye(string s)
    {
        Console.WriteLine("  Goodbye, {0}!", s);
    }

    delegate void Del(string s);

    static void Main()
    {
        Del a, b, c, d;

        // Create the delegate object a that references 
        // the method Hello:
        a = Hello;

        // Create the delegate object b that references 
        // the method Goodbye:
        b = Goodbye;

        // The two delegates, a and b, are composed to form c: 
        c = a + b;

        // Remove a from the composed delegate, leaving d, 
        // which calls only the method Goodbye:
        d = c - a;

        Console.WriteLine("Invoking delegate a:");
        a("A");
        Console.WriteLine("Invoking delegate b:");
        b("B");
        Console.WriteLine("Invoking delegate c:");
        c("C");
        Console.WriteLine("Invoking delegate d:");
        d("D");


        /* Output:
        Invoking delegate a:
          Hello, A!
        Invoking delegate b:
          Goodbye, B!
        Invoking delegate c:
          Hello, C!
          Goodbye, C!
        Invoking delegate d:
          Goodbye, D!
        */

        Console.ReadLine();
    }
}

All this is possible because delegates inherit from the System.MulticastDelegate class that in turn inherits from System.Delegate. Because of this, you can use the members that are defined in those base classes on your delegates. You can learn more about this in the article Delegates and Events in C# .NET

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