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    <p>Whenever you create an event handler for your form or controls it runs on the UI thread. The UI thread in a win forms app. is often also called the main application thread. This is the thread running the Application.Run() message loop.</p> <p>I find it concerning that you say you "naturally decided to create a thread". Introducing extra threads is almost guaranteed to increase complexity and make your application harder to debug, as you have immediately discovered.</p> <p>You have said you are inexperienced in non-web applications, so why introduce the complexity of threading on top of the learning curve of Windows Forms? Try a bit harder to get it working without using threads, then consider threads later on if it is still absolutely necessary.</p> <p>In Windows forms you can often avoid threads by using a Timer and/or Application.DoEvents(). I am not saying don't use threads, just that I have seen many applications where it doesn't even make sense to allow the user to continue working while a process has not complete. </p> <p>If you do want to stick with threads, I would also strongly recommend using the <a href="http://msdn.microsoft.com/en-us/library/system.componentmodel.backgroundworker.aspx" rel="nofollow noreferrer">BackgroundWorker</a> class. It nicely encapsulates a worker thread and provides good safe communication such as ProgressChanged and RunWorkerCompleted events.</p> <p>Looking at your code I would also suggest doing some reading on accessing UI controls from worker threads. In short <em>don't do it</em>, unless using Control.Invoke() or Control.BeginInvoke(). This <a href="https://stackoverflow.com/questions/229554/whats-the-difference-between-invoke-and-begininvoke">previous StackOverflow question</a> would be a good start.</p> <p>[UPDATE]</p> <p>There are a number of places where your code is setting or checking properties of a UI control <em>directly from the worker thread</em>. To avoid exceptions and undefined behaviour you must always access UI controls <em>from the thread that created them</em>, ie. the UI/Main thread. Using Invoke() or BeginInvoke() allow you to do this safely.</p>
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    1. COI'm of mixed feeling about this answer. I like the explanation of what the UI thread is, but asking why a thread is introduced seems to take a step backwards. The short answer is that Windows requires it to keep the GUI responsive, as it uses the GUI thread to do things like window paints/redraws, operating the message pump, etc... See what Jon Skeet says here: http://stackoverflow.com/questions/1035439/database-access-in-gui-thread-bad-isnt-it/1035451#1035451
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    2. CO@R Bemrose, introducing threads to an application where they don't add some real benefit or there may be a viable alternatives is a BIG no-no. When you combine this with inexperience in Windows Forms this is a recipe for problems. Remember KISS and "the simplest thing that could work"?
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    3. CO@R Bemrose, in that answer Jon says about a non-responsive GUI: "In some cases, particularly for "quick and dirty" tools, that's actually the right choice". The real point is that if you use threads, your debugging, testing and maintenance complexity automatically increases.
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