+= operator with Events

问题:

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?

回答1:

+= subscribes to an event. The delegate or method on the right-hand side of the += will be added to an internal list that the event keeps track of, and when the owning class fires that event, all the delegates in the list will be called.

回答2:

The answer you have accepted is a nice simplified version of what += does, but it's not the full story.

The += operator calls the add method on the event. Similarly -= calls remove. This usually results in the delegate being added to the internal list of handlers which are called when the event is fired, but not always.

It is perfectly possible to define add to do something else. This example may help to demonstrate what happens when you call +=:

class Test
{
    public event EventHandler MyEvent
    {
        add
        {
            Console.WriteLine("add operation");
        }

        remove
        {
            Console.WriteLine("remove operation");
        }
    }       

    static void Main()
    {
        Test t = new Test();

        t.MyEvent += new EventHandler (t.DoNothing);
        t.MyEvent -= null;
    }

    void DoNothing (object sender, EventArgs e)
    {
    }
} 

Output:

add operation
remove operation

See Jon Skeet's article on events and delegates for more information.

回答3:

In this case method foo_MyEvent will fire whene that event is called (so you subscribe to the event in line 4)

回答4:

Event is just the immutable list of delegates (i.e. subscribes which will get called when something publishes/invokes that event). You could argue that we could have used List instead. If we went that way, someone could have tempered with our subscribers.

In above scenario where you use List, you could do:

lstDelegate = newDelegate

and you have wiped existing subscribers (lstDelegate only contains ur delegate callback now).

To stop that behaivour we have Event. When you use event, complier wont allow you do that, you are only allowed to add/remove your own delegate by using += and -=. That's how I try to differentiate it anyway. hope it helps.

回答5:

It add's handler to an event. It means that method on the right side of the operator will be invoked when the event will be risen.