DpiTest.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;

namespace DpiTest
{
    class Program
    {
        public enum MONITOR_DPI_TYPE
        {
            MDT_EFFECTIVE_DPI = 0,
            MDT_ANGULAR_DPI = 1,
            MDT_RAW_DPI = 2,
            MDT_DEFAULT = MDT_EFFECTIVE_DPI
        }

        [DllImport("shcore.dll")]
        public static extern uint GetDpiForMonitor(IntPtr hMonitor, MONITOR_DPI_TYPE type, out int dpiX, out int dpiY);

        public const int MONITOR_DEFAULTTONULL = 0;
        public const int MONITOR_DEFAULTTOPRIMARY = 1;
        public const int MONITOR_DEFAULTTONEAREST = 2;

        [DllImport("user32.dll", ExactSpelling = true)]
        public static extern IntPtr MonitorFromWindow(IntPtr handle, int flags);

        [DllImport("user32.dll", SetLastError = false)]
        public static extern IntPtr GetDesktopWindow();

        [DllImport("user32.dll", SetLastError = true)]
        public static extern bool SetProcessDPIAware();


        static void Main(string[] args)
        {
            SetProcessDPIAware();

            IntPtr hMonitor = MonitorFromWindow(GetDesktopWindow(), MONITOR_DEFAULTTOPRIMARY);

            int dpi, unused;
            GetDpiForMonitor(hMonitor, MONITOR_DPI_TYPE.MDT_RAW_DPI, out dpi, out unused);
            Console.WriteLine("Raw DPI: {0}", dpi);
        }
    }
}