Creating a modal error box after Exception thrown from another Thread.

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C#

MVVM

multi-threading

Hi everyone,

I've written an application in C# that uses WPF (utilising the MVVM design pattern) and LINQ to CRM to retrieve data from a database.

I have a problem with threading it seems...

This application uses the BackgroundWorker to run a database query. Once it retrieves this information it manipulates it in memory and creates a list of 'Structs'. Whilst it is processing this data I update the UI at regular intervals and show a loading animation.

So far everything works fine.

When this application is deployed however, there is a possibility that this operation could fail and in this case I want to capture this error and let my program fail gracefully. I handle this possibility by encapsulating the data processing in a 'try-catch' block. If an error occurs such as a database time out, I clear all temporary variables, including the data sources that my UI controls are bound to, hide my loading animation and return to the initial screen. The program is then in the state it should be when first initialised.

To alert the user to this event I show a MessageBox which tells them what has happened.

Here lies the problem..

This MessageBox is not modal, it still allows the user to interact with the main window. This could cause an issue, as a user could try to restart this operation before they have selected OK, to acknowledge the error, clearing the temporary variables and resetting the loading animation.

This might seem trivial but as this is an end-user application it needs to be polished.

What I think the issue is...

The MessageBox is being created on a thread other then the STA thread (which I understand to be the main thread of execution, correct me if I'm wrong) therefore, you are able to minimise it and still interact with the other window as it is not blocking the execution of the main thread.

Things I have already tried...

The try catch block is inside the background.DoWork += delegate {...} block, so instead of creating the message box inside the catch block of that delegate, I throw a custom event, naively thinking that outside the scope of that delegate it would return to the STA thread and create a thread blocking modal. No dice.

In this event handler I create a new thread and set it's apartment state to STA.
myThread.SetApartmentState(ApartmentState.STA); This was a bad idea and didn't work but it was a very long shot.

Here is the basic structure of my code :

BackgroundWorker background = new BackgroundWorker();
								background.RunWorkerCompleted += new 

WorkerFailed += new workerFailedEvent(GetDD_Command_WorkerFailed);	
			
			background.DoWork += delegate
			{
			try
			{
			 // Do very long operation
			}
			catch (Exception e)
			{	
				// Something went wrong, call event						
				WorkerFailed(e, new EventArgs()); 
			}	
		};
		background.RunWorkerAsync();	
	}

	void myProgram_Command_WorkerFailed(Exception ex, EventArgs e)
	{						
		Exception exception = (Exception)ex; 
		// Creates Message box
		System.Windows.Forms.MessageBox.Show
			(
			exception.InnerException == null ? exception.Message.ToString() : exception.InnerException.ToString(),
			"Error",
			MessageBoxButtons.OK,
			MessageBoxIcon.Error
			);
			// Also clears cache and clears temp variables and hides loading animation
		}

Thanks for any help or advice anyone gives! :)

Larry