C＃事件，如何提高它们？

Question

I'm trying to learn raising and handling events in C#.

Here's my simple example:

///////////////  creating a new BALL object ///    
ballClass ball = new ballClass();

void button1_Click(object sender, EventArgs e)
{
    // this should make the BALL object raise an event
    ball.onHit();
    label1.Text = "EVENT SEND";
}

// when event is fired, label text should change
void BallInPlayEvent(object sender, EventArgs e)
{
    label2.Text = "EVENT FOUND!";
}

and the ball class:

class ballClass
{
    public event EventHandler BallInPlay;

    public void onHit()
    {
        this.BallInPlay ///// ??? how should i raise this event?
    }
}

In the call I can't really understand, how do I raise the event? Any ideas?... :)

Thanks!

Answer 1

public void onHit()
{
   if(BallInPlay != null)
     BallInPlay(this, new EventArgs());
}

Answer 2

It's simple, just call the delegate directly, as in

BallInPlay(this, EventArgs.Empty);

The only thing you need to be careful about is a race condition, where BallInPlay is null or set to null before you call it.

The pattern to deal with that is

var bip = BallInPlay;
if (bip != null)
    bip(this, EventArgs.Empty);

Answer 3

Usually, to avoid testing the handler every time (as Ed Guiness showed), it is (IMHO) easier to simply initialize the event with an empty delegate, and then call it without checking:

class Ball
{        
    // add an empty delegate to our handler
    public event EventHandler BallInPlay = delegate { };

    protected virtual void OnHit(EventArgs e)
    {
        // no need to check, we have at least one
        // dummy delegate in the invocation list
        BallInPlay (this, e);
    }
}

Actually (although Jon Skeet will probably tell you this before I finish writing this post :), the only truly thread safe event call is the one discussed by in Jon Skeet's article .

Another important thing you should note is that classes shouldn't really fire events which reside in other classes (ie OnHit method should not be public ). If a class has a public event (like your BallClass ), it alone should be the one to fire this event.