c# - BackgroundWorker completes before DoWork

Question

I'm using a background worker to handle the loading of a file to stop my ui from freezing however it seems that the RunWorkerCompleted is finishing before my DoWork event has completed (Causes errors when exiting dialog)... is there anything I'm doing wrong? Am I better off doing this over a task?

public static <T> LoadDesign(string xmlPath)
{
    PleaseWait pw = new PleaseWait(xmlPath);
    pw.ShowDialog();
    return pw.design;
}


private PleaseWait(string xmlFile)
{
   InitializeComponent();
   bw = new BackgroundWorker();
   bw.WorkerSupportsCancellation = true;
   bw.DoWork += (s, e) =>
   {
      design = (Cast)DllCall containing XmlSerializer.Deserialize(...,xmlFile);
   };
   bw.RunWorkerCompleted += (s, e) => {
   //Exit please wait dialog
      this.Close(); 
   };
   if (!bw.IsBusy)
       bw.RunWorkerAsync();
}

I believe the issue may be down to the fact that my background worker is calling a dll and not waiting for the response. I've tried to add checks such as while(design == null) to no avail..

Edit2 The error is NRE as the design hasn't been loaded, I can easily fix this but would rather get the threading working instead.

Answer 1

There are lots of little mistakes. Given that we are probably not looking at the real code and that we don't have a debugger with a Call Stack window to see where it actually crashes, any one of them might be a factor.

• Testing bw.IsBusy and not starting the worker when it is true is a grave mistake. It can't ever be busy in the code as posted but if it actually is possible for it to be true then you've got a nasty bug in your code. Since you actually did subscribe the events on a busy worker. Now the RunWorkerCompleted event handler will run twice.

• Using the Close() method to close a dialog is not correct. A dialog should be closed by assigning its DialogResult property. Not the gravest kind of mistake but wrong nonetheless.

• There's a race in the code, the worker can complete before the dialog is ever displayed. A dialog can only be closed when its native window was created. In other words, the IsHandleCreated must be true. You must interlock this to ensure this can never happen. Subscribe the dialog's Load event to get the worker started.

• You blindly assume that the worker will finish the job and produce a result. That won't be the case when its DoWork method died from an exception. Which is caught by BackgroundWorker and passed to the RunWorkerCompleted event handler as the e.Error property. You must check this property and do something reasonable if it isn't null.

Judging from the comments, I'd guess at the latter bullet being the cause. You debug this by using Debug + Exceptions, tick the Thrown checkbox for CLR exceptions. The debugger will now stop when the exception is thrown, allowing you to find out what went wrong.