Today I remembered the need to blog again. It sounds weird, but it isn’t … A while ago I ran into a failing multicast delegate and decided to blog it. Since I ran into one again today and having to look up my previous solution again the hard way, I discovered I should have done it immediately. Check the story below:

Multicast delegates are great. Today I had to implement a custom user control (insurance cost calculation) with several child controls (one for each prospect) which depend on each other (e.g. parent and children). I use a delegate to notify the parent control that one of it’s children selections has changed.

The original call:

public event EventHandler SelectionChanged;

/// <summary>
/// This method cares for correct event handling
/// </summary>
private void OnSelectionChanged()
{
  // The SelectionChanged event will be null if there are no subscribers.
  if (SelectionChanged != null) SelectionChanged(this, EventArgs.Empty);
}

For some dumb reason I ran into a NRE using the delegate. Guess what happened? Subsequent delegate subscribers did not get fired anymore. The reason is that a delegate that fails for some reason, fails entirely. After researching the problem again, I changed the code provided hereabove with the following piece of code:

public event EventHandler SelectionChanged;

/// <summary>
/// This method cares for correct event handling, including the protection against
/// failing individual event subscribers
/// </summary>
private void OnSelectionChanged()
{
  // The SelectionChanged event will be null if there are no subscribers.
  if (SelectionChanged != null)
  {
    try
    {
      Delegate[] list = SelectionChanged.GetInvocationList();
      foreach (Delegate del in list)
      {
        try
        {
          // Only the current subscriber will file, the rest of the delegate's
          // invocation will proceed as normal.
          EventHandler handler = (EventHandler) del;
          handler(this, EventArgs.Empty);
        }
        catch { }
      }
    }
    catch { }
  }
}