Unauthorized access exception when value set for property using Secondary Thread - Silverlight

Thinking about threading in Silverlight 4, we choose between the two options

1. Background worker
2. Managing thread manually

Most of the time-consuming operations will be handled using the Background worker without blocking the UI. It ultimately executes in a separate thread. The main functions and events to know in Background worker are DoWork, RunWorkerCompleted and RunWorkerAsync. As the name specifies we need to add the event handler for DoWork event for doing the background operation. Add the handler for RunWorkerCompleted to receive the operation completed notification. RunWorkerAsync method is to start the operation. The main thing to note is that we should not access directly access the UI objects from the DoWork event.

Also there are times where we need to manage the threads manually. Since Silverlight runs in a Single threaded apartment (STA) application model and the default is the UI thread, accessing the UI objects from other thread leads to cross thread exception. Knowing this fact I worked with a secondary thread and updated a property in my ViewModel. In that scenario I got an exception and digging into the inner exception found out the cross thread exception. For understanding I've created a sample scenario that raises exception.

Snippet 1