how to determine if a delegate is 'empty'

[esdee]

controlButton.Click += new System.EventHandler(this.controlButton_StopClick);

how can i check if Click has not already been wired with a method in order to avoid multiple invoking of controlButton_StopClick ?

------------------------

http://esdeeblog.blogspot.com/

 

[B00gyeMan]

You need to override the Button class and do the following:

1) Add a private boolean variable to that class in order to determine that the delegate was already set or not:

private bool bAlreadyset = false;

2) Re-define the event handler like this:

public event EventHandler Click
{
    add
    {
        lock (this)
        {
            if (!bAlreadyset)
            {
                this.Click += value;
                bAlreadyset = true;
            }
            else
            {
                //throw an exception if you need to
            }
        }
    }
    remove
    {
        lock (this)
        {
            this.Click -= value;
            bAlreadyset = false;
        }
    }
}

This will ensure that the invocation list of this multi-cast delegate contains only one item, the first one that you set.

 

[chiph]

In your method that calls the listeners to your event, you'd do something like this:

// The delegate (method signature) that event handlers must follow
public delegate void ChangedEventDelegate(object sender, EventArgs e);

// The event that people will subscribe to
public event ChangedEventDelegate ChangedEvent;

protected void FireChangedEvent(MyEventArgs e)
{
  if (this.ChangedEvent != null)
  {
    foreach(ChangedEventDelegate d in this.ChangedEvent.GetInvocationList())
    {
       try
       {
         d(this, e);
       }
       catch
       {
          // Do nothing, as the subscribers are responsible
          // for handling exceptions in their handlers.  We
          // just have this try..catch here as a safety net
          // in case someone forgets.  It will allow us to
          // proceed to the next invocation target.

          // But we might want to log it anyway, so we can
          // smack the developer who forgot their try block
       }
    }
}

Chip H.

____________________________________________________________________
If you want to get the best response to a question, please read FAQ222-2244 first

 

[esdee]

thank you, guys!
im surprised that overiding the class is the only way though

by the way - chip: how does this ensure that the handler is called only once? from what i see what you've posted calls each registered handler as many times as it's been added

------------------------

http://esdeeblog.blogspot.com/

 

[B00gyeMan]

Actually, I misunderstood what you needed. The thing you need is to override the System.Windows.Forms.Button ( yup... ) class and add to that class a variable like this:

ArrayList arInvocationList = new ArrayList();

Then, using the new keyword, override the Click event like this:

      public [b]new[/b] event EventHandler Click
      {
         add
         {
            lock (this)
            {
               if ( !arInvocationList.Contains( value))
               {
                 this.Click += value;
               }
               else
               {
                  //throw an exception if you need to
               }
            }
         }
         remove
         {
            lock (this)
            {
               this.Click -= value;
               arInvocationList.Remove( value);
            }
         }
      }

This will ensure that a delegate isn't registered twice. We need the arInvocationList member because having add and remove accessors for the Click event means that we cannot access it inside these accessors (infinite loop).

 

[esdee]

cool
yes - the new implementation is much more precise,
thank you !


------------------------

http://esdeeblog.blogspot.com/

 

[chiph]

by the way - chip: how does this ensure that the handler is called only once? from what i see what you've posted calls each registered handler as many times as it's been added

No it doesn't. If a developer registeres for a callback twice, that's another situation where a smack is called-for.

Chip H.


____________________________________________________________________
If you want to get the best response to a question, please read FAQ222-2244 first