You've been tinkering with threading. Drag+Drop doesn't like threads. It will only work on threads that are properly initialized with Thread.SetApartmentState() to select STA and that pump a message loop with Application.Run() or Form.ShowDialog().
1、加[STAThread]
2、Thread t = new Thread(new ThreadStart(A));
t.SetApartmentState(ApartmentState.STA);
t.Start( );
并不能加[STAThread]
有时加[STAThread]这个都会出现问题，不知道是不是Application.Run()上没有做好，但用2方法很少出问题。