如何在ac＃类中公开事件？

Question

I am building a simple class to hold related methods. Part of this code includes synchronising to a database. The built in SyncOrchestrator class includes a SessionProgress event handler which I can wire up an event to.

What I would like to do is instance my class and then hook up an some code to this event so that I can display a progress bar (ill be using BGWorker).

So, my question is probably c# 101, but how do I expose this event through my class the correct way so that I can wire it up?

Thanks

Answer 1

I think you're looking for something like this:

(I also suggest you read the Events tutorial on MSDN .)

public class SyncOrchestrator
{
    // ...

    public event EventHandler<MyEventArgs> SessionProgress;

    protected virtual void OnSessionProgress(MyEventArgs e)
    {
        // Note the use of a temporary variable here to make the event raisin
        // thread-safe; may or may not be necessary in your case.
        var evt = this.SessionProgress;
        if (evt  != null)
            evt (this, e);
    }

    // ...
}

where the MyEventArgs type is derived from the EventArgs base type and contains your progress information.

You raise the event from within the class by calling OnSessionProgress(...) .

Register your event handler in any consumer class by doing:

// myMethodDelegate can just be the name of a method of appropiate signature,
// since C# 2.0 does auto-conversion to the delegate.
foo.SessionProgress += myMethodDelegate;

Similarly, use -= to unregister the event; often not explicitly required.

Answer 2

Like this:

public event EventHandlerDelegate EventName;

EventHandlerDelegate should obviously be the name of a delegate type that you expect people to provide to the event handler like so:

anObject.EventName += new EventHandlerDelegate(SomeMethod);

When calling the event, make sure you use this pattern:

var h = EventName;
if (h != null)
    h(...);

Otherwise you risk the event handler becoming null in between your test and actually calling the event.

Also, see the official documentation on MSDN .