using System.Drawing;
using System.Runtime.InteropServices;
        [DllImport("User32.dll")]
        public static extern IntPtr GetDC(IntPtr hwnd);
        [DllImport("User32.dll")]
        public static extern void ReleaseDC(IntPtr hwnd, IntPtr dc); 

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


                SolidBrush b = new SolidBrush(Color.White);
                g.FillRectangle(b, new Rectangle(0, 0, 1920, 1080));

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