//DllImport所在的名字空間
 using System.Runtime.InteropServices;
[DllImport("User32.dll")]
public extern static System.IntPtr GetDC(System.IntPtr hWnd); 
private void button19_Click(object sender, EventArgs e)
{
    System.IntPtr    DesktopHandle   =    GetDC(System.IntPtr.Zero);
    Graphics g = Graphics.FromHdc(DesktopHandle);
    g.DrawRectangle(new Pen(Color.Red),new Rectangle(10,10,100,100));
}