System.InvalidOperationException error when populating an array

Question

So I have a program that works as the UI for a console app. I tried to add a progress bar to it. When the user clicks on the Start button, it does this:

MainProgress.Value = 0;
MainProgress.Maximum = PackageNwCheckbox.IsChecked == true ? 4 : 3;
BackgroundWorker compilerWorker = new BackgroundWorker();
compilerWorker.WorkerReportsProgress = true;
compilerWorker.DoWork += StartCompiler;
compilerWorker.ProgressChanged += CompilerReport;
compilerWorker.RunWorkerAsync();

This is in order to update the progress bar when the GUI program works and feeds the console program. When the program starts populating the array called filemap like this:

filemap = Directory.GetFiles(ProjectLocation.Text + "\\www\\js\\", "*.js");
//The variable is an array of strings.

The app crashes and the error says

The call thread couldn't access the item because another thread has it.

Answer 1

In desktop applications with background threads, if you're attempting to update a view. You will need to invoke that update with the dispatcher. Somewhere in your CompilerReport method you want something like this:

MainProgress.Dispatcher.Invoke(() =>
{
    // Do update here
});

With more code, I could probably detail a better answer

Answer 2

You can't access ProjectLocation.Text from a background thread. You can only access members of a UI control on the thread on which it was originally created.

So if you want the current text of ProjectLocation in your DoWork event handler, you should use the dispatcher:

string location = Dispatcher.Invoke(() => ProjectLocation.Text);
filemap = Directory.GetFiles(location + "\\www\\js\\", "*.js");

Or pass it as an argument to the RunWorkerAsync method.