"Extend my Windows desktop onto this monitor" programmatically

Question

I would like to be able to set "Extend my Windows desktop onto this monitor" via code. A PowerShell script would be ideal. WMI seems the way forward but I have zero knowledge in WMI.

Answer 1

Windows 7, 8 and 10 are supposed to come with a small program that does exactly this: displayswitch.exe. This page lists the following parameters:

displayswitch.exe /internal Disconnect projector (same as "Show only on 1" from the Display Properties dialog) displayswitch.exe /clone        Duplicate screen displayswitch.exe /extend    Extend screen displayswitch.exe /external Projector only (disconnect local) (same as "Show only on 2" from the Display Properties dialog) 

For a one-click solution to the problem posed, simply create a *.bat-file containing the single line

call displayswitch.exe /extend 

and save it to your desktop.

[I tested this on Windows 8.1, and it has been confirmed to work on Windows 10.]

Answer 2

I've made a cleaner version that does not use sendkeys.

public class DisplayHelper
{
    [DllImport("user32.dll")]
    static extern DISP_CHANGE ChangeDisplaySettings(uint lpDevMode, uint dwflags);
    [DllImport("user32.dll")]
    static extern bool EnumDisplayDevices(string lpDevice, uint iDevNum, ref DISPLAY_DEVICE lpDisplayDevice, uint dwFlags);

    enum DISP_CHANGE : int
    {
        Successful = 0,
        Restart = 1,
        Failed = -1,
        BadMode = -2,
        NotUpdated = -3,
        BadFlags = -4,
        BadParam = -5,
        BadDualView = -1
    }

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
    struct DISPLAY_DEVICE
    {
        [MarshalAs(UnmanagedType.U4)]
        public int cb;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)]
        public string DeviceName;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
        public string DeviceString;
        [MarshalAs(UnmanagedType.U4)]
        public DisplayDeviceStateFlags StateFlags;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
        public string DeviceID;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
        public string DeviceKey;
    }

    [Flags()]
    enum DisplayDeviceStateFlags : int
    {
        /// <summary>The device is part of the desktop.</summary>
        AttachedToDesktop = 0x1,
        MultiDriver = 0x2,
        /// <summary>The device is part of the desktop.</summary>
        PrimaryDevice = 0x4,
        /// <summary>Represents a pseudo device used to mirror application drawing for remoting or other purposes.</summary>
        MirroringDriver = 0x8,
        /// <summary>The device is VGA compatible.</summary>
        VGACompatible = 0x16,
        /// <summary>The device is removable; it cannot be the primary display.</summary>
        Removable = 0x20,
        /// <summary>The device has more display modes than its output devices support.</summary>
        ModesPruned = 0x8000000,
        Remote = 0x4000000,
        Disconnect = 0x2000000
    }

    public static void EnableSecondaryDisplay()
    {
        var secondaryIndex = 1;
        var secondary = GetDisplayDevice(secondaryIndex);
        var id = secondary.DeviceKey.Split('\\')[7];

        using (var key = Registry.CurrentConfig.OpenSubKey(string.Format(@"System\CurrentControlSet\Control\VIDEO\{0}", id), true))
        {
            using (var subkey = key.CreateSubKey("000" + secondaryIndex))
            {
                subkey.SetValue("Attach.ToDesktop", 1, RegistryValueKind.DWord);
                subkey.SetValue("Attach.RelativeX", 1024, RegistryValueKind.DWord);
                subkey.SetValue("DefaultSettings.XResolution", 1024, RegistryValueKind.DWord);
                subkey.SetValue("DefaultSettings.YResolution", 768, RegistryValueKind.DWord);
                subkey.SetValue("DefaultSettings.BitsPerPel", 32, RegistryValueKind.DWord);
            }
        }

        ChangeDisplaySettings(0, 0);
    }

    private static DISPLAY_DEVICE GetDisplayDevice(int id)
    {
        var d = new DISPLAY_DEVICE();
        d.cb = Marshal.SizeOf(d);
        if (!EnumDisplayDevices(null, (uint)id, ref d, 0))
            throw new NotSupportedException("Could not find a monitor with id " + id);
        return d;
    }
}

I have only tested this on a newly installed computer.