WPF - Cannot change a GUI property inside the OnChanged method (fired from FileSystemWatcher)

Question

I want to change a GUI property in the OnChanged method... (in actuality im trying to set an image source.. but used a button here for simplicity). This is called everytime

Answer 1

I'm guessing that the FileSystemWatcher is calling your event handler on another thread. Inside your event handler, use your application's Dispatcher to marshal it back to the UI thread:

private void OnChanged(object source, FileSystemEventArgs e) {
    Application.Current.Dispatcher.BeginInvoke(new Action(() => DoSomethingOnUiThread()));
}

private void DoSomethingOnUiThread() {
    Test_Button.Width = 500;
}

Answer 2

The problem is that this event is raised on a background thread. You need to marshal the call back to the UI thread:

// do stuff here                    
Console.WriteLine("top");
this.Dispatcher.BeginInvoke(new Action( () =>
{
    // This runs on the UI thread
    BlueBan1_Image.Source = GUI.GetChampImageSource(JSONFile.Get("blue ban 1"), "avatar");
    Test_Button.Width = 500;
}));
Console.WriteLine("bottom");