Why isn't DragDrop working?

I am trying to do something very simple, yet it's proving to be the most complicated thing I have ever done.

I am handling the DragDrop event. All I want to do is simply display a MessageBox when the DragDrop event is called - regardless of whether there is any data being dropped:

    private void dropper_DragDrop(object sender, DragEventArgs e)
    {
        /// TODO: Get the data and do something with it.

        // DEBUG ONLY:
        MessageBox.Show("Dropped.");
        dropper.BackColor = detector.BackColor;

    }

Doesn't work. Does not print to console. Does now show messagebox. Does not change background color.