DpiHelper.cs

using System;
using System.Runtime.InteropServices;

public static class SHCore
{
    public enum PROCESS_DPI_AWARENESS : int
    {
        UNAWARE = 0,
        SYSTEM_AWARE = 1,
        PER_MONITOR_AWARE = 2
    }

    [DllImport("SHCore.dll", ExactSpelling = true)]
    public static extern int SetProcessDpiAwareness(PROCESS_DPI_AWARENESS value);
}

public static class User32
{
    [DllImport("user32.dll", ExactSpelling = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    public static extern bool SetProcessDPIAware();

    [DllImport("User32.dll", ExactSpelling = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    public static extern bool SetProcessDpiAwarenessContext(IntPtr value);

    [DllImport("User32.dll", ExactSpelling = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    public static extern bool IsValidDpiAwarenessContext(IntPtr value);

    public static IntPtr UNSPECIFIED_DPI_AWARENESS_CONTEXT = IntPtr.Zero;

    public static class DPI_AWARENESS_CONTEXT
    {
        public static readonly IntPtr UNAWARE = (IntPtr)(-1);
        public static readonly IntPtr SYSTEM_AWARE = (IntPtr)(-2);
        public static readonly IntPtr PER_MONITOR_AWARE = (IntPtr)(-3);
        public static readonly IntPtr PER_MONITOR_AWARE_V2 = (IntPtr)(-4);
        public static readonly IntPtr UNAWARE_GDISCALED = (IntPtr)(-5);
    }
}

public static class DpiHelper
{
    /// <summary>
    ///  Sets the DPI awareness. If not available on the current OS, it falls back to the next possible.
    /// </summary>
    /// <returns>true/false - If the process DPI awareness is successfully set, returns true. Otherwise false.</returns>
    public static bool SetApplicationDpiAwareness(HighDpiMode highDpiMode)
    {
        bool success = false;

        if (OperatingSystem.IsWindowsVersionAtLeast(10))
        {
            // SetProcessIntPtr needs Windows 10 RS2 and above
            IntPtr rs2AndAboveDpiFlag;
            switch (highDpiMode)
            {
                case HighDpiMode.SystemAware:
                    rs2AndAboveDpiFlag = User32.DPI_AWARENESS_CONTEXT.SYSTEM_AWARE;
                    break;
                case HighDpiMode.PerMonitor:
                    rs2AndAboveDpiFlag = User32.DPI_AWARENESS_CONTEXT.PER_MONITOR_AWARE;
                    break;
                case HighDpiMode.PerMonitorV2:
                    // Necessary for RS1, since this SetProcessIntPtr IS available here.
                    rs2AndAboveDpiFlag = User32.IsValidDpiAwarenessContext(User32.DPI_AWARENESS_CONTEXT.PER_MONITOR_AWARE_V2)
                        ? User32.DPI_AWARENESS_CONTEXT.PER_MONITOR_AWARE_V2
                        : User32.DPI_AWARENESS_CONTEXT.SYSTEM_AWARE;
                    break;
                case HighDpiMode.DpiUnawareGdiScaled:
                    // Let's make sure, we do not try to set a value which has been introduced in later Windows releases.
                    rs2AndAboveDpiFlag = User32.IsValidDpiAwarenessContext(User32.DPI_AWARENESS_CONTEXT.UNAWARE_GDISCALED)
                        ? User32.DPI_AWARENESS_CONTEXT.UNAWARE_GDISCALED
                        : User32.DPI_AWARENESS_CONTEXT.UNAWARE;
                    break;
                default:
                    rs2AndAboveDpiFlag = User32.DPI_AWARENESS_CONTEXT.UNAWARE;
                    break;
            }

            success = User32.SetProcessDpiAwarenessContext(rs2AndAboveDpiFlag);
        }
        else if (OperatingSystem.IsWindowsVersionAtLeast(8))
        {
            // 8.1 introduced SetProcessDpiAwareness
            SHCore.PROCESS_DPI_AWARENESS dpiFlag;
            switch (highDpiMode)
            {
                case HighDpiMode.DpiUnaware:
                case HighDpiMode.DpiUnawareGdiScaled:
                    dpiFlag = SHCore.PROCESS_DPI_AWARENESS.UNAWARE;
                    break;
                case HighDpiMode.SystemAware:
                    dpiFlag = SHCore.PROCESS_DPI_AWARENESS.SYSTEM_AWARE;
                    break;
                case HighDpiMode.PerMonitor:
                case HighDpiMode.PerMonitorV2:
                    dpiFlag = SHCore.PROCESS_DPI_AWARENESS.PER_MONITOR_AWARE;
                    break;
                default:
                    dpiFlag = SHCore.PROCESS_DPI_AWARENESS.SYSTEM_AWARE;
                    break;
            }

            success = SHCore.SetProcessDpiAwareness(dpiFlag) == 0;
        }
        else
        {
            // Vista or higher has SetProcessDPIAware
            SHCore.PROCESS_DPI_AWARENESS dpiFlag = (SHCore.PROCESS_DPI_AWARENESS)(-1);
            switch (highDpiMode)
            {
                case HighDpiMode.DpiUnaware:
                case HighDpiMode.DpiUnawareGdiScaled:
                    // We can return, there is nothing to set if we assume we're already in DpiUnaware.
                    return true;
                case HighDpiMode.SystemAware:
                case HighDpiMode.PerMonitor:
                case HighDpiMode.PerMonitorV2:
                    dpiFlag = SHCore.PROCESS_DPI_AWARENESS.SYSTEM_AWARE;
                    break;
            }

            if (dpiFlag == SHCore.PROCESS_DPI_AWARENESS.SYSTEM_AWARE)
            {
                success = User32.SetProcessDPIAware();
            }
        }

        return success;
    }
}