Unhandled Exceptions in Backgroundworker

Unhandled exceptions in BackgroundWorker

What you're describing is not the defined behavior of BackgroundWorker. You're doing something wrong, I suspect.

Here's a little sample that proves BackgroundWorker eats exceptions in DoWork, and makes them available to you in RunWorkerCompleted:

var worker = new BackgroundWorker();
worker.DoWork += (sender, e) => 
    { 
        throw new InvalidOperationException("oh shiznit!"); 
    };
worker.RunWorkerCompleted += (sender, e) =>
    {
        if(e.Error != null)
        {
            MessageBox.Show("There was an error! " + e.Error.ToString());
        }
    };
worker.RunWorkerAsync();

My psychic debugging skills are revealing your problem to me: You are accessing e.Result in your RunWorkerCompleted handler -- if there's an e.Error, you must handle it without accessing e.Result. For example, the following code is bad, bad, bad, and will throw an exception at runtime:

var worker = new BackgroundWorker();
worker.DoWork += (sender, e) => 
    { 
        throw new InvalidOperationException("oh shiznit!"); 
    };
worker.RunWorkerCompleted += (sender, e) =>
    {
        // OH NOOOOOOOES! Runtime exception, you can't access e.Result if there's an
        // error. You can check for errors using e.Error.
        var result = e.Result; 
    };
worker.RunWorkerAsync();

Here's a proper implementation of the RunWorkerCompleted event handler:

private void RunWorkerCompletedHandler(object sender, RunWorkerCompletedEventArgs e)
{
    if (e.Error == null)
    {
       DoSomethingWith(e.Result); // Access e.Result only if no error occurred.
    }
}

VOILA, you won't receive runtime exceptions.

Unhandled exceptions in BackgroundWorker

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

Background worker exception handling

I suggest you to create some business specific exception, which describes operation which you were doing in background. And throw this exception with original exception as inner exception:

private void bgWorker_RunWorkerCompleted(
    object sender, RunWorkerCompletedEventArgs e)
{
    if (e.Error != null)
        throw new BusinessSpecificException("Operation failed", e.Error);
    // ...
}

Thus original exception with its stack trace will be available, and you'll have more descriptive exception thrown.

Note - if you don't want to create new exception class, you can use existing ApplicationException or Exception. But its not that informative and if you are going to catch it somewhere, then you'll not be able to catch this particular exception only

BackgroundWorker exception handling

That is the correct pattern for BackgroundWorker.

I suspect the problem is that your Main method is exiting before the BW has completed.

RunWorkerAsync will return immediately and if you are not waiting in Main, then your process will end, perhaps even before the BW has started, never mind completed.

Try adding a Console.ReadLine at the end of your Main method.

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Out of interest:

BW behaves differently in a Console app and a Windows app. If you use a WinForms or WPF app, there will be a derived SynchronizationContext on your UI thread and BW will marshal RunWorkerCompleted back to the UI thread and run it there. That's one of the main benefits of BW.

In a Console App, the default SynchronizationContext is used and this marshals RunWorkerCompleted onto a thread pool thread. This means you can block the Main thread and the completed handler will still run.

Unhandled exceptions and background workers

The other threads will carry on. Only the thread used by the BackgroundWorker will crash.

An unhandled exception will be thrown. Since there is no exception handling in the event handler, it will get caught by the CLR as a last resort. This will not influence other threads.

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