Unhandled exceptions in BackgroundWorker

My WinForms app uses a number of BackgroundWorker objects to retrieve information from a database. I'm using BackgroundWorker because it allows the UI to remain unblocked during long-running database queries and it simplifies the threading model for me.

I'm getting occasional DatabaseExceptions in some of these background threads, and I have witnessed at least one of these exceptions in a worker thread while debugging. I'm fairly confident these exceptions are timeouts which I suppose its reasonable to expect from time to time.

My question is about what happens when an unhandled exception occurs in one of these background worker threads.

I don't think I can catch an exception in another thread, but can I expect my WorkerCompleted method to be executed? Is there any property or method of the BackgroundWorker I can interrogate for exceptions?

If the operation raises an exception that your code does not handle, the BackgroundWorker catches the exception and passes it into the RunWorkerCompleted event handler, where it is exposed as the Error property of System.ComponentModel.RunWorkerCompletedEventArgs. If you are running under the Visual Studio debugger, the debugger will break at the point in the DoWork event handler where the unhandled exception was raised.

http://msdn.microsoft.com/en-us/library/system.componentmodel.backgroundworker.dowork.aspx

I am fully using BackgroundWorker over a years and really know it in deep.

Just recently, My RunWorkerCompleted does not catch the e.Error when I simply Throw New Exception("Test") in DoWork. However Unhandled Exception raised. Catch in DoWork is not the best practice thus e.Error got no meaning.

When I try to create new Form with new BackgroundWorker, e.Error in RunWorkerCompleted handled successfully. There should be something wrong in my complicated BackgroundWorker.

After a few days googling and debugging, trying an error. I found this in my RunWorkerCompleted:

• Check for e.Error first, then e.Cancelled and lastly e.Result
• Do not get the e.Result if e.Cancelled = True.
• Do not get the e.Result if e.Error is not null (or Nothing) **

** This is where I miss. If you trying to use e.Result if e.Error is not null (or Nothing), Unhandled Exception will thrown.

UPDATE: In the e.Result get property .NET design it to check for e.Error first, if got error, then they will re-throw the same exception from DoWork. That is why we get Unhandled exception in RunWorkerCompleted but actually the exception is come from DoWork.

Here is the best practice to do in RunWorkerCompleted:

If e.Error IsNot Nothing Then
  ' Handle the error here
Else
  If e.Cancelled Then
    ' Tell user the process canceled here
  Else
    ' Tell user the process completed
    ' and you can use e.Result only here.
  End If
End If

If you want an object that accessible to all DoWork, ProgressChanged and RunWorkerCompleted, use like this:

Dim ThreadInfos as Dictionary(Of BackgroundWorker, YourObjectOrStruct)

You can easily access ThreadInfos(sender).Field anywhere you want.

By default it will be caught and stored by the BackgroundWorker. From MSDN:

If the operation raises an exception that your code does not handle, the BackgroundWorker catches the exception and passes it into the RunWorkerCompleted event handler, where it is exposed as the Error property of System.ComponentModel.RunWorkerCompletedEventArgs. If you are running under the Visual Studio debugger, the debugger will break at the point in the DoWork event handler where the unhandled exception was raised.

As it was already noted:

If the operation raises an exception that your code does not handle, the BackgroundWorker catches the exception and passes it into the RunWorkerCompleted event handler, where it is exposed as the Error property of System.ComponentModel.RunWorkerCompletedEventArgs.

This is important whenever you're interacting with the original thread. For example if you want the result of your exception to be written in some kind of a label on your form, that's when you mustn't catch the exception in the DoWork of the BackgroundWorker, but instead handle the e.Error from RunWorkerCompletedEventArgs.

If you analyze the BackgroundWorker code with reflector you can see its all handled pretty straightforward: Your DoWork gets executed in a try-catch block, and the exception is just passed to RunWorkerCompleted. Which is the reason why I disagree with the 'preferred' method of always catching all your exceptions in the DoWork event.

In short, to answer the original question:

Yes - you can count on your RunWorkerCompleted to always be fired.

Use e.Error from RunWorkerCompleted to check for exceptions in the other thread.

This will work only without the debugger attached, when run from Visual Studio, the debugger will catch the unhanded exception in DoWork method and will break the execution, however you may click continue and RunWorkerCompleted will be reached and you'll be able to read exception via the e.Error field.