How to effectively draw on desktop in C#?

Question

I want to draw directly on the desktop in C#. From searching a bit, I ended up using a Graphics object from the Desktop HDC (null). Then, I painted normally using this Graph

Answer 1

To get an OnPaint for the desktop you would need to subclass the desktop window and use your drawing logic when it receives a WM_PAINT/WM_ERASEBKGND message.

As the thread you linked to says, you can only intercept messages sent to a window of an external process using a hook (SetWindowsHookEx from a DLL).

As mentioned a transparent window is another way to do it, or (depending on the update frequency) copying, drawing and setting a temporary wallpaper (as bginfo does).