CrossThreadMessagingException WPF OpenFileDialog

I get this exception:

Microsoft.VisualStudio.Debugger.Runtime.CrossThreadMessagingException was unhandled HResult=-2146232832 Message=An exception 'Microsoft.VisualStudio.Debugger.Runtime.CrossThreadMessagingException' occurred Source=Microsoft.VisualStudio.Debugger.Runtime StackTrace: at Microsoft.VisualStudio.Debugger.Runtime.Main.ThrowCrossThreadMessageException(String formatString) InnerException:

When I use this code:

    public string ShowOpenFileDialog(string initialPath, string filter = null)
        var dialog = new OpenFileDialog()
            InitialDirectory = Directory.Exists(initialPath) ? Path.GetDirectoryName(initialPath) : String.Empty,
            FileName = File.Exists(initialPath) ? Path.GetFileName(initialPath) : String.Empty,
            Filter = filter

        if (dialog.ShowDialog() == true)
            return dialog.FileName;

        return String.Empty;

I am not explicitly using multithreading, so what is causing this? It happens frequently during debugging, but not all the time. I've changed the method signature about a million times, because I was passing in ref/out parameters and thought they might be the cause.

edit: I'm calling it from the UI Thread. Button in View -> Command in ViewModel -> ShowOpenFileDialog.


Although I'm not still sure why this is happening, I've figured out how to fix in my case. I have multiple projects in my solution, and I've configured two startup projects (client + server). When I start both projects with debugging, it appears that the debugger has trouble with that. So I just switch off debugging for the project that matters least at the moment, so there is only one project for the debugger to handle.

I have same problems (CrossThreadMessagingException) with multiple startup project in Visual Studio 2012.

This problem can be solved in VS 2012, if you assign one project as "Start without debugging".

What the interest, this problem is not apparent in the Visual Studio 2013.

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