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DllImportËùÔÚµÄÃû×Ö¿Õ¼ä 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));
         }