Hi all

I'm currently using ModuleCommunication to get a shopping cart module to update its list of items when the button is clicked on the details page.  I can get this to work in Firefox but in IE i get the error: Object reference not set to an instance of an object.  This refers to the method ModuleCommunication(this, args).

Strange that this only happens in one browser.  Any help would be much appreciated.

Si

Hi Si,

When raising events in C#, you are required to ensure that the event is not null before raising it.  The basic pattern* is this:

if(ModuleCommunication != null)

      ModuleCommunication(this, args);

The only way you could be generating that error on that line would be if ModuleCommunication were null, so this would appear to be the source of your problem. 

Once that is corrected, you will need to figure out why your receiver is not subscribing to the communication event when you are not logged in as an administrator.  Probably the most common cause of this would be the receiving module not being displayed on the page due to permissions.  Are you able to see both modules -- on the same page -- when not logged in as admin?

Hope this gets you started!

Brandon

* There is actually an interesting race condition in this pattern such that you risk failure under some multithreading situations.  This is an ongoing  weakness in C# and the solutions (even the local variable version) are neither elegant nor satisfactory.  Google for more information; the above pattern is fine for most situations.

Some more info on the problem:

I have a search results page and a details page.  On both there is a button to add an item to the shopping cart.  It does this by using IMC.  So when 'Add to cart' is pressed on either the results page or details page it runs ModuleCommunication(this,args).  Here is where it gets strange.

If you are logged in as admin both the buttons work and send the correct data to the shopping cart module. When loged in as a regular user or not logged in at all only the results page button works. The details page just throws a NullReferenceException. 

Other info:

• The results page creates a session which holds our search results and item information.
• The details page loads this session variable to access this info
• The ModuleCommunicationEventArgs object just holds a string
• ShoppingCart module also loads the session variable and another session holding the cart contents.
• All coded in C#

Hopefully this can shed some light on the problem.

Si