Make WPF window draggable, no matter what element is clicked

private void Window_MouseDown(object sender, MouseButtonEventArgs e)
{
if (e.ChangedButton == MouseButton.Left)
    this.DragMove();
}

Is throwing an exception in some cases (i.e. if on the window you also have a clickable image that when clicked opens a message box. When you exit from message box you will get error) It is safer to use

private void Window_MouseDown(object sender, MouseButtonEventArgs e)
{
if (Mouse.LeftButton == MouseButtonState.Pressed)
            this.DragMove();
}

So you are sure that left button is pressed at that moment.

if the wpf form needs to be draggable no matter where it was clicked the easy work around is using a delegate to trigger the DragMove() method on either the windows onload event or the grid load event

private void Grid_Loaded(object sender, RoutedEventArgs 
{
      this.MouseDown += delegate{DragMove();};
}

Sure, apply the following MouseDown event of your Window

private void Window_MouseDown(object sender, MouseButtonEventArgs e)
{
    if (e.ChangedButton == MouseButton.Left)
        this.DragMove();
}

This will allow users to drag the Window when they click/drag on any control, EXCEPT for controls which eat the MouseDown event (e.Handled = true)

You can use PreviewMouseDown instead of MouseDown, but the drag event eats the Click event, so your window stops responding to left-mouse click events. If you REALLY wanted to be able to click and drag the form from any control, you could probably use PreviewMouseDown, start a timer to begin the drag operation, and cancel the operation if the MouseUp event fires within X milliseconds.