Graphics currentGraphics = Graphics.FromHwnd(new WindowInteropHelper(mainWindow).Handle);
double dpixRatio = currentGraphics.DpiX/96;

using (ManagementClass mc = new ManagementClass("Win32_DesktopMonitor"))
{
    using (ManagementObjectCollection moc = mc.GetInstances())
    {

        int PixelsPerXLogicalInch = 0; // dpi for x
        int PixelsPerYLogicalInch = 0; // dpi for y

        foreach (ManagementObject each in moc)
        {
            PixelsPerXLogicalInch = int.Parse((each.Properties["PixelsPerXLogicalInch"].Value.ToString()));
            PixelsPerYLogicalInch = int.Parse((each.Properties["PixelsPerYLogicalInch"].Value.ToString()));
        }
    }
}