I updated my answer from 2015. Here is some utility code that uses the latest DPI features from Windows 10 (in particular, the GetDpiForWindow function, which is the only method that supports DPI_AWARENESS windows / applications / processes, etc.).), But reverts to older ones (dpi on monitor and desktop dpi), so it should still work with Windows 7.
It does not depend on WPF or Winforms, but only on Windows itself.
// note this class considers dpix = dpiy public static class DpiUtilities { // you should always use this one and it will fallback if necessary // https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-getdpiforwindow public static int GetDpiForWindow(IntPtr hwnd) { var h = LoadLibrary("user32.dll"); var ptr = GetProcAddress(h, "GetDpiForWindow"); // Windows 10 1607 if (ptr == IntPtr.Zero) return GetDpiForNearestMonitor(hwnd); return Marshal.GetDelegateForFunctionPointer<GetDpiForWindowFn>(ptr)(hwnd); } public static int GetDpiForNearestMonitor(IntPtr hwnd) => GetDpiForMonitor(GetNearestMonitorFromWindow(hwnd)); public static int GetDpiForNearestMonitor(int x, int y) => GetDpiForMonitor(GetNearestMonitorFromPoint(x, y)); public static int GetDpiForMonitor(IntPtr monitor, MonitorDpiType type = MonitorDpiType.Effective) { var h = LoadLibrary("shcore.dll"); var ptr = GetProcAddress(h, "GetDpiForMonitor"); // Windows 8.1 if (ptr == IntPtr.Zero) return GetDpiForDesktop(); int hr = Marshal.GetDelegateForFunctionPointer<GetDpiForMonitorFn>(ptr)(monitor, type, out int x, out int y); if (hr < 0) return GetDpiForDesktop(); return x; } public static int GetDpiForDesktop() { int hr = D2D1CreateFactory(D2D1_FACTORY_TYPE.D2D1_FACTORY_TYPE_SINGLE_THREADED, typeof(ID2D1Factory).GUID, IntPtr.Zero, out ID2D1Factory factory); if (hr < 0) return 96; // we really hit the ground, don't know what to do next! factory.GetDesktopDpi(out float x, out float y); // Windows 7 Marshal.ReleaseComObject(factory); return (int)x; } public static IntPtr GetDesktopMonitor() => GetNearestMonitorFromWindow(GetDesktopWindow()); public static IntPtr GetShellMonitor() => GetNearestMonitorFromWindow(GetShellWindow()); public static IntPtr GetNearestMonitorFromWindow(IntPtr hwnd) => MonitorFromWindow(hwnd, MONITOR_DEFAULTTONEAREST); public static IntPtr GetNearestMonitorFromPoint(int x, int y) => MonitorFromPoint(new POINT { x = x, y = y }, MONITOR_DEFAULTTONEAREST); private delegate int GetDpiForWindowFn(IntPtr hwnd); private delegate int GetDpiForMonitorFn(IntPtr hmonitor, MonitorDpiType dpiType, out int dpiX, out int dpiY); private const int MONITOR_DEFAULTTONEAREST = 2; [DllImport("kernel32", CharSet = CharSet.Auto, SetLastError = true)] private static extern IntPtr LoadLibrary(string lpLibFileName); [DllImport("kernel32", CharSet = CharSet.Ansi, SetLastError = true)] private static extern IntPtr GetProcAddress(IntPtr hModule, string lpProcName); [DllImport("user32")] private static extern IntPtr MonitorFromPoint(POINT pt, int flags); [DllImport("user32")] private static extern IntPtr MonitorFromWindow(IntPtr hwnd, int flags); [DllImport("user32")] private static extern IntPtr GetDesktopWindow(); [DllImport("user32")] private static extern IntPtr GetShellWindow(); [StructLayout(LayoutKind.Sequential)] private partial struct POINT { public int x; public int y; } [DllImport("d2d1")] private static extern int D2D1CreateFactory(D2D1_FACTORY_TYPE factoryType, [MarshalAs(UnmanagedType.LPStruct)] Guid riid, IntPtr pFactoryOptions, out ID2D1Factory ppIFactory); private enum D2D1_FACTORY_TYPE { D2D1_FACTORY_TYPE_SINGLE_THREADED = 0, D2D1_FACTORY_TYPE_MULTI_THREADED = 1, } [InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("06152247-6f50-465a-9245-118bfd3b6007")] private interface ID2D1Factory { int ReloadSystemMetrics(); [PreserveSig] void GetDesktopDpi(out float dpiX, out float dpiY); // the rest is not implemented as we don't need it } } public enum MonitorDpiType { Effective = 0, Angular = 1, Raw = 2, }