using (ManagementClass mc = new ManagementClass("Win32_DesktopMonitor"))

            {

                using (ManagementObjectCollection moc = mc.GetInstances())

                {

                    int PixelsPerXLogicalInch = ; // dpi for x

                    int PixelsPerYLogicalInch = ; // dpi for y

                    foreach (ManagementObject each in moc)

                    {

                        PixelsPerXLogicalInch = int.Parse((each.Properties["PixelsPerXLogicalInch"].Value.ToString()));

                        PixelsPerYLogicalInch = int.Parse((each.Properties["PixelsPerYLogicalInch"].Value.ToString()));

                    }

                    Console.WriteLine("PixelsPerXLogicalInch:" + PixelsPerXLogicalInch.ToString());

                    Console.WriteLine("PixelsPerYLogicalInch:" + PixelsPerYLogicalInch.ToString());

                    Console.Read();

                }

            }

using (Graphics graphics = Graphics.FromHwnd(IntPtr.Zero))

            {

                float dpiX = graphics.DpiX;

                float dpiY = graphics.DpiY;

            }