Sunstorm Blog

Exceptions in BackgroundWorker

To save 15 minutes of confusion.

If an exception is thrown in a BackgroundWorker’s DoWork handler, the backgroundworker will catch it and place the exception in RunWorkerCompleted’s e.Error.

If it seems like the Exception’s still being thrown with no apparent way to catch it, it’s because if e.Error is not null. When that’s the case, e.Result will throw that error (wrapped in a TargetInvocationException) when accessed. Do not access e.Result without checking for e.Error first.

private void worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
  EndCreate(e.Error, e.Result); // <--- THIS WILL CRASH IF e.Error IS NOT NULL
}
 
private void worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
  // THIS WILL NOT.
  if (e.Error == null) {
    EndCreate(e.Error, e.Result);
  }
  else {
    EndCreate(e.Error, null); // accessing e.Result throws the exception
  }
}