How to get scaling factor for each monitor, e.g. 1, 1.25, 1.5

Question

I know this has been asked before, but I've tried all the answers I've found and none of them seem to work for me. Answers seem to work on a single monitor, or require a window handle, or to be in a WPF application. I've a C# class library with no UI that is called from a different language all together.

I've been asked to determine the scaling factor, e.g. 1, 1.25, 1.5, etc. for each monitor attached to the current PC in a C# class library.

I also need to provide the resolution and colour depth for each monitor. The registry does hold the DpiValue, whatever that is, in Windows 10 under

Computer\HKEY_CURRENT_USER\Control Panel\Desktop\PerMonitorSettings

However I have no idea how to map those to a Screen in order to get the matching resolution returned in

System.Windows.Forms.Screen.AllScreens

So does anyone have a way of getting this information?

Answer 1

I believe I have finally (after a long time of searching) found an answer that works, it even works on my high DPI Surface Book 2 screen. I have tested it as much as I can, and so far it's always returned the correct value.

Here's how I did it, thanks to whoever posted the code fragments in the past where I gathered this from.

First you need a structure to call EnumDisplaySettings in user32.dll

    [StructLayout(LayoutKind.Sequential)]
    public struct DEVMODE
    {
        private const int CCHDEVICENAME = 0x20;
        private const int CCHFORMNAME = 0x20;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 0x20)]
        public string dmDeviceName;
        public short dmSpecVersion;
        public short dmDriverVersion;
        public short dmSize;
        public short dmDriverExtra;
        public int dmFields;
        public int dmPositionX;
        public int dmPositionY;
        public ScreenOrientation dmDisplayOrientation;
        public int dmDisplayFixedOutput;
        public short dmColor;
        public short dmDuplex;
        public short dmYResolution;
        public short dmTTOption;
        public short dmCollate;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 0x20)]
        public string dmFormName;
        public short dmLogPixels;
        public int dmBitsPerPel;
        public int dmPelsWidth;
        public int dmPelsHeight;
        public int dmDisplayFlags;
        public int dmDisplayFrequency;
        public int dmICMMethod;
        public int dmICMIntent;
        public int dmMediaType;
        public int dmDitherType;
        public int dmReserved1;
        public int dmReserved2;
        public int dmPanningWidth;
        public int dmPanningHeight;
    }

Then you need to declare the external function call

[DllImport("user32.dll")]
public static extern bool EnumDisplaySettings(string lpszDeviceName, int iModeNum, ref DEVMODE lpDevMode);

Then you need to use it to calculate the screen scaling

            Screen[] screenList = Screen.AllScreens;

            foreach (Screen screen in screenList)
            {
                DEVMODE dm = new DEVMODE();
                dm.dmSize = (short)Marshal.SizeOf(typeof(DEVMODE));
                EnumDisplaySettings(screen.DeviceName, -1, ref dm);

                var scalingFactor = Math.Round(Decimal.Divide(dm.dmPelsWidth, screen.Bounds.Width), 2);
            }

Hope others find this useful.