How to Repaint When Drawing Outside the Form Area

I am writing an application that needs to draw outside of it's main window area. I already have to code to actually do the drawing:

[DllImport("User32.dll")]
public static exte IntPtr GetDC(IntPtr hwnd);
[DllImport("User32.dll")]
public static exte void ReleaseDC(IntPtr hwnd, IntPtr dc);

IntPtr desktopPtr = GetDC(IntPtr.Zero);
Graphics g = Graphics.FromHdc(desktopPtr);

g.DrawLine(Pens.White, 0, 0, Screen.FromControl(this).WorkingArea.Width, Screen.FromControl(this).WorkingArea.Height);

g.Dispose();
ReleaseDC(IntPtr.Zero, desktopPtr);

However the on paint event is an unsuitable place to put the code because it's not called when something outside of the form is redrawn. So my question is, where could this code be placed so it is called whenever part of the screen is redrawn?