How to effectively draw on desktop in C#?

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I posted two solutions for a similar requirement here

Basically you have two options.

1- Get a graphics object for the desktop and start drawing to it. The problem is if you need to start clearing what you have previously drawn etc.

Point pt = Cursor.Position; // Get the mouse cursor in screen coordinates 

using (Graphics g = Graphics.FromHwnd(IntPtr.Zero)) 
{         
  g.DrawEllipse(Pens.Black, pt.X - 10, pt.Y - 10, 20, 20); 
}

2- The second option that I provide in the link above is to create a transparent top-most window and do all your drawing in that window. This basically provides a transparent overlay for the desktop which you can draw on. One possible downside to this, as I mention in the original answer, is that other windows which are also top-most and are created after your app starts will obscure your top most window. This can be solved if it is a problem though.

For option 2, making the form transparent is as simple as using a transparency key, this allows mouse clicks etc. to fall through to the underlying desktop.

BackColor = Color.LightGreen; 
TransparencyKey = Color.LightGreen;

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