Fun with delegates and events

[janneman]

Hi everybody,

I am currently working on a C# project, but I am kind of stuck. If anybody can give me a hand with this I would really appreciate it.

We are trying to build an application with a user GUI. When I click button1 I want to call another method to do some work and when it is done, it should callback via a delegate. Unfortunately it is giving me the error “Object reference not set to an instance of an object.”. This should not be too complicated, but somehow I am not able to resolve this myself.

This is my code:

=== BEGIN GUI CODE ===

public delegate void ButtonsServerTreeViewDelegate(object sender);
private void button1_Click(object sender, EventArgs e)
{    // Setup listener, for updating the GUI...
    DiscTreeView DiscTreeViewObject = new DiscTreeView();
    DiscTreeViewObject.ButtonsServerTreeView += new ButtonsServerTreeViewDelegate(CallbackButtonsServerTreeView);
 
  //  DiscTreeViewObject.LoadServerTreeView();
    PathSelectionTreeView.LoadServerTreeView();}
 
public void CallbackButtonsServerTreeView(object sender)
{    label31.Text = "HI";}

=== END GUI CODE ===

==== BEGIN OTHER CLASS CODE ====

public event ButtonsServerTreeViewDelegate ButtonsServerTreeView;
public void LoadServerTreeView()
{   ButtonsServerTreeView(this);}

==== END OTHER CLASS CODE ====

Note that the LoadServerTreeView is called with PathSelectionTreeView and not with the DiscTreeViewObject (commented out), since PathSelectionTreeView contains my TreeView.

Is there anyway to perform the callback, if I call my LoadServerTreeView via PathSelectionTreeView ?

Again, any help is greatly appreciated.

Warm Regards,

Jan

[balabaster]

At what line are you getting that exception? It seems that you are referencing a property or method of an object that you haven't instantiated or is currently set as null or nothing...

[janneman]

Now I know that the exception occurs because ButtonsServerTreeView is null. I know how to prevent the event from being send, but is there some kind of way I can send events from the LoadServerTreeView ?

[vekipeki]

Actually, you MUST check if your event is non-null before invoking it - the usual way is to do it like this:

      public event ButtonsServerTreeViewDelegate ButtonsServerTreeView;
      public void LoadServerTreeView()
      {
             ButtonsServerTreeViewDelegate handler = ButtonsServerTreeView;
             if (handler != null)
                 handler(this);
      }

The whole point of using events is that your event-firing class should never know or care who and when will subscribe to any of its events (e.g. a button control has many events but in most cases you will only handle the MouseClick event).

Also note that the event handler in code snippet above is first referenced by a temporary handler variable, to avoid a race condition when another thread can unsubscribe from your event just after you have checked that it is not null. This way your still have a valid reference to a handler which gets executed anyway.

Another solution would be to create an empty delegate instance in your delegate chain just to assure that your event is never null:

      public event ButtonsServerTreeViewDelegate ButtonsServerTreeView = delegate { };
      public void LoadServerTreeView()
      {
             ButtonsServerTreeView(this);
      }

The latter solution gives a slight overhead when executing it, but makes your code slightly cleaner. And it's easier to just go through your existing code and add " = delegate {}; " to all your events :o).