How to write a "Preview"-type event that can cancel a main event?

Question

Hi,

Let's say I have an event. Let's call it DoStuff. Before DoStuff happens, I'm trying to use another event, PreviewDoStuff, to check if I need to prevent DoStuff from being called. The problem I'm facing with this is that I'm unable to get any kind of value back from the PreviewDoStuff, like a return code or anything due to how events are written. And event args aren't passed back to the caller, so I can't get the event args "handled" property. I'm sure there has to be a way to do it, but I'm not sure how. Any ideas?

Thanks!

Answer 1

Declare the event as EventHandler<CancelEventArgs>. Then the listener can set Cancel to true, and you can use that value to determine whether to raise the "real" event or not.

Mandatory code sample:

public event EventHandler<CancelEventArgs> PreviewDoStuff;
public event EventHandler DoStuff;

private void RaiseDoStuff()
{
    CancelEventArgs args = new CancelEventArgs();
    OnPreviewDoStuff(args);
    if (!args.Cancel)
    {
        OnDoStuff(EventArgs.Empty);
    }
}

protected void OnPreviewDoStuff(CancelEventArgs e)
{
    EventHandler<CancelEventArgs> previewDoStuff = PreviewDoStuff;
    if (previewDoStuff != null)
    {
        previewDoStuff(this, e);
    }
}

protected void OnDoStuff(EventArgs e)
{
    EventHandler doStuff = DoStuff;
    if (doStuff != null)
    {
        doStuff(this, e);
    }
}

For an example of this in real-life use, check the FormClosing event, which uses a FormClosingEventArgs class, which in turn inherits from CancelEventArgs.

Answer 2

Have your PreviewDoStuff set an internal flag which DoStuff checks when it fires. Even better, have the code that raises DoStuff check the flag before it raises the event.

Answer 3

Can you provide some of your code how you are writing DoSTuff and PreviewDoStuff?

Answer 4

I assume you mean you want to create your own event called DoStuff. I think it's possible to pass "ref" arguments:

public delegate void PreviewDoStuffFunc(ref bool Handled);
public event PreviewDoStuffFunc PreviewDoStuff;

But the standard way would be to use something like CancelEventArgs:

public event CancelEventArgs PreviewDoStuff;
public event EventArgs DoStuff;

Then after you fire the preview event, you check the Cancel:

var e = new CancelEventArgs();    
if (PreviewDoStuff != null)
    PreviewDoStuff(this, e);
if (!e.Cancel)
    DoStuff(this, EventArgs.Empty);