BackgroundWorker and Clipboard

You could marshal this to the UI thread to make it work:

else{    [...]    this.Dispatcher.BeginInvoke(new Action(() => Clipboard.SetText(splitpermutation[i])));    [...]}

The BackgroundWorker runs on the .NET thread pool. Thread pool threads run in the COM multi-threaded apartment. To use the clipboard, you must be running in a single-threaded apartment. You could create your own thread and set it to run in an STA, but it would probably be best to use Control.Invoke (or BeginInvoke) to get back onto a user-interface thread (which must be an STA thread).

The exception you're getting is because you're trying to do something on the UI thread from outside the UI thread (a BackgroundWorker, as the name implies, does something in the background, and for that it needs to create a separate thread).

While the answer posted by Reed (that is, by using Dispatcher.BeginInvoke) is one way to do avoid this exception, I'm wondering WHY you are trying to send something to the clipboard from the main work method in the first place...

The BackgroundWorker exposes events like ProgressChanged (which you can call periodically from your work method) or RunWorkerCompleted (which will fire when the main work method finishes).

Using Clipboard.SetText in either of these events should not cause the exception you're seeing, and this seems to be the preferable way of doing things on the UI thread when working with the BackgroundWorker.