Use Windows API from C# to set primary monitor

Question

I\'m trying to use the Windows API to set the primary monitor. It doesn\'t seem to work - my screen just flicks and nothing happens.

    public const int DM_         


        

Answer 1

I ran into exactly the same problem, both from C# and after following the advice here to try it in C++. I eventually discovered that the thing the Microsoft documentation doesn't make clear is that the request to set the primary monitor will be ignored (but with the operation reported as successful!) unless you also set the position of the monitor to (0, 0) on the DEVMODE struct. Of course, this means that you also need to shift the positions of your other monitors so that they stay in the same place relative to the new primary monitor. Per the documentation (http://msdn.microsoft.com/en-us/library/windows/desktop/dd183413%28v=vs.85%29.aspx), call ChangeDisplaySettingsEx for each monitor with the CDS_NORESET flag and then make a final call with everything null.

The following code worked for me:

    public static void SetAsPrimaryMonitor(uint id)
    {
        var device = new DISPLAY_DEVICE();
        var deviceMode = new DEVMODE();
        device.cb = Marshal.SizeOf(device);

        NativeMethods.EnumDisplayDevices(null, id, ref device, 0);
        NativeMethods.EnumDisplaySettings(device.DeviceName, -1, ref deviceMode);
        var offsetx = deviceMode.dmPosition.x;
        var offsety = deviceMode.dmPosition.y;
        deviceMode.dmPosition.x = 0;
        deviceMode.dmPosition.y = 0;

        NativeMethods.ChangeDisplaySettingsEx(
            device.DeviceName, 
            ref deviceMode, 
            (IntPtr)null, 
            (ChangeDisplaySettingsFlags.CDS_SET_PRIMARY | ChangeDisplaySettingsFlags.CDS_UPDATEREGISTRY | ChangeDisplaySettingsFlags.CDS_NORESET), 
            IntPtr.Zero);

        device = new DISPLAY_DEVICE();
        device.cb = Marshal.SizeOf(device);

        // Update remaining devices
        for (uint otherid = 0; NativeMethods.EnumDisplayDevices(null, otherid, ref device, 0); otherid++)
        {
            if (device.StateFlags.HasFlag(DisplayDeviceStateFlags.AttachedToDesktop) && otherid != id)
            {
                device.cb = Marshal.SizeOf(device);
                var otherDeviceMode = new DEVMODE();

                NativeMethods.EnumDisplaySettings(device.DeviceName, -1, ref otherDeviceMode);

                otherDeviceMode.dmPosition.x -= offsetx;
                otherDeviceMode.dmPosition.y -= offsety;

                NativeMethods.ChangeDisplaySettingsEx(
                    device.DeviceName,
                    ref otherDeviceMode,
                    (IntPtr)null,
                    (ChangeDisplaySettingsFlags.CDS_UPDATEREGISTRY | ChangeDisplaySettingsFlags.CDS_NORESET),
                    IntPtr.Zero);

            }

            device.cb = Marshal.SizeOf(device);
        }

        // Apply settings
        NativeMethods.ChangeDisplaySettingsEx(null, IntPtr.Zero, (IntPtr)null, ChangeDisplaySettingsFlags.CDS_NONE, (IntPtr)null);
    }

Note that a signature for ChangeDisplaySettingsEx with a DEVMODE struct as the second parameter obviously won't allow you to pass in IntPtr.Zero. Create yourself two different signatures for the same extern call, i.e.

    [DllImport("user32.dll")]
    public static extern DISP_CHANGE ChangeDisplaySettingsEx(string lpszDeviceName, ref DEVMODE lpDevMode, IntPtr hwnd, ChangeDisplaySettingsFlags dwflags, IntPtr lParam);

    [DllImport("user32.dll")]
    public static extern DISP_CHANGE ChangeDisplaySettingsEx(string lpszDeviceName, IntPtr lpDevMode, IntPtr hwnd, ChangeDisplaySettingsFlags dwflags, IntPtr lParam);

Answer 2

I can't really help you with the winapi-stuff but if you are using a Nvidia card you may have a look at the NVcontrolPanel Api Documentation Then you could make the secondary output your primary using rundll32.exe NvCpl.dll,dtcfg primary 2 Hope that will help you.

Answer 3

Here is the full code based on ADBailey's solution:

public class MonitorChanger
{
    public static void SetAsPrimaryMonitor(uint id)
    {
        var device = new DISPLAY_DEVICE();
        var deviceMode = new DEVMODE();
        device.cb = Marshal.SizeOf(device);

        NativeMethods.EnumDisplayDevices(null, id, ref device, 0);
        NativeMethods.EnumDisplaySettings(device.DeviceName, -1, ref deviceMode);
        var offsetx = deviceMode.dmPosition.x;
        var offsety = deviceMode.dmPosition.y;
        deviceMode.dmPosition.x = 0;
        deviceMode.dmPosition.y = 0;

        NativeMethods.ChangeDisplaySettingsEx(
            device.DeviceName,
            ref deviceMode,
            (IntPtr)null,
            (ChangeDisplaySettingsFlags.CDS_SET_PRIMARY | ChangeDisplaySettingsFlags.CDS_UPDATEREGISTRY | ChangeDisplaySettingsFlags.CDS_NORESET),
            IntPtr.Zero);

        device = new DISPLAY_DEVICE();
        device.cb = Marshal.SizeOf(device);

        // Update remaining devices
        for (uint otherid = 0; NativeMethods.EnumDisplayDevices(null, otherid, ref device, 0); otherid++)
        {
            if (device.StateFlags.HasFlag(DisplayDeviceStateFlags.AttachedToDesktop) && otherid != id)
            {
                device.cb = Marshal.SizeOf(device);
                var otherDeviceMode = new DEVMODE();

                NativeMethods.EnumDisplaySettings(device.DeviceName, -1, ref otherDeviceMode);

                otherDeviceMode.dmPosition.x -= offsetx;
                otherDeviceMode.dmPosition.y -= offsety;

                NativeMethods.ChangeDisplaySettingsEx(
                    device.DeviceName,
                    ref otherDeviceMode,
                    (IntPtr)null,
                    (ChangeDisplaySettingsFlags.CDS_UPDATEREGISTRY | ChangeDisplaySettingsFlags.CDS_NORESET),
                    IntPtr.Zero);

            }

            device.cb = Marshal.SizeOf(device);
        }

        // Apply settings
        NativeMethods.ChangeDisplaySettingsEx(null, IntPtr.Zero, (IntPtr)null, ChangeDisplaySettingsFlags.CDS_NONE, (IntPtr)null);
    }
}

[StructLayout(LayoutKind.Explicit, CharSet = CharSet.Ansi)]
public struct DEVMODE
{
    public const int CCHDEVICENAME = 32;
    public const int CCHFORMNAME = 32;

    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = CCHDEVICENAME)]
    [System.Runtime.InteropServices.FieldOffset(0)]
    public string dmDeviceName;
    [System.Runtime.InteropServices.FieldOffset(32)]
    public Int16 dmSpecVersion;
    [System.Runtime.InteropServices.FieldOffset(34)]
    public Int16 dmDriverVersion;
    [System.Runtime.InteropServices.FieldOffset(36)]
    public Int16 dmSize;
    [System.Runtime.InteropServices.FieldOffset(38)]
    public Int16 dmDriverExtra;
    [System.Runtime.InteropServices.FieldOffset(40)]
    public UInt32 dmFields;

    [System.Runtime.InteropServices.FieldOffset(44)]
    Int16 dmOrientation;
    [System.Runtime.InteropServices.FieldOffset(46)]
    Int16 dmPaperSize;
    [System.Runtime.InteropServices.FieldOffset(48)]
    Int16 dmPaperLength;
    [System.Runtime.InteropServices.FieldOffset(50)]
    Int16 dmPaperWidth;
    [System.Runtime.InteropServices.FieldOffset(52)]
    Int16 dmScale;
    [System.Runtime.InteropServices.FieldOffset(54)]
    Int16 dmCopies;
    [System.Runtime.InteropServices.FieldOffset(56)]
    Int16 dmDefaultSource;
    [System.Runtime.InteropServices.FieldOffset(58)]
    Int16 dmPrintQuality;

    [System.Runtime.InteropServices.FieldOffset(44)]
    public POINTL dmPosition;
    [System.Runtime.InteropServices.FieldOffset(52)]
    public Int32 dmDisplayOrientation;
    [System.Runtime.InteropServices.FieldOffset(56)]
    public Int32 dmDisplayFixedOutput;

    [System.Runtime.InteropServices.FieldOffset(60)]
    public short dmColor; // See note below!
    [System.Runtime.InteropServices.FieldOffset(62)]
    public short dmDuplex; // See note below!
    [System.Runtime.InteropServices.FieldOffset(64)]
    public short dmYResolution;
    [System.Runtime.InteropServices.FieldOffset(66)]
    public short dmTTOption;
    [System.Runtime.InteropServices.FieldOffset(68)]
    public short dmCollate; // See note below!
    [System.Runtime.InteropServices.FieldOffset(72)]
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = CCHFORMNAME)]
    public string dmFormName;
    [System.Runtime.InteropServices.FieldOffset(102)]
    public Int16 dmLogPixels;
    [System.Runtime.InteropServices.FieldOffset(104)]
    public Int32 dmBitsPerPel;
    [System.Runtime.InteropServices.FieldOffset(108)]
    public Int32 dmPelsWidth;
    [System.Runtime.InteropServices.FieldOffset(112)]
    public Int32 dmPelsHeight;
    [System.Runtime.InteropServices.FieldOffset(116)]
    public Int32 dmDisplayFlags;
    [System.Runtime.InteropServices.FieldOffset(116)]
    public Int32 dmNup;
    [System.Runtime.InteropServices.FieldOffset(120)]
    public Int32 dmDisplayFrequency;
}

public enum DISP_CHANGE : int
{
    Successful = 0,
    Restart = 1,
    Failed = -1,
    BadMode = -2,
    NotUpdated = -3,
    BadFlags = -4,
    BadParam = -5,
    BadDualView = -6
}

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public 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()]
public 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 = 0x10,
    /// <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,
}

[Flags()]
public enum ChangeDisplaySettingsFlags : uint
{
    CDS_NONE = 0,
    CDS_UPDATEREGISTRY = 0x00000001,
    CDS_TEST = 0x00000002,
    CDS_FULLSCREEN = 0x00000004,
    CDS_GLOBAL = 0x00000008,
    CDS_SET_PRIMARY = 0x00000010,
    CDS_VIDEOPARAMETERS = 0x00000020,
    CDS_ENABLE_UNSAFE_MODES = 0x00000100,
    CDS_DISABLE_UNSAFE_MODES = 0x00000200,
    CDS_RESET = 0x40000000,
    CDS_RESET_EX = 0x20000000,
    CDS_NORESET = 0x10000000
}

public class NativeMethods
{
    [DllImport("user32.dll")]
    public static extern DISP_CHANGE ChangeDisplaySettingsEx(string lpszDeviceName, ref DEVMODE lpDevMode, IntPtr hwnd, ChangeDisplaySettingsFlags dwflags, IntPtr lParam);

    [DllImport("user32.dll")]
    // A signature for ChangeDisplaySettingsEx with a DEVMODE struct as the second parameter won't allow you to pass in IntPtr.Zero, so create an overload
    public static extern DISP_CHANGE ChangeDisplaySettingsEx(string lpszDeviceName, IntPtr lpDevMode, IntPtr hwnd, ChangeDisplaySettingsFlags dwflags, IntPtr lParam);

    [DllImport("user32.dll")]
    public static extern bool EnumDisplayDevices(string lpDevice, uint iDevNum, ref DISPLAY_DEVICE lpDisplayDevice, uint dwFlags);

    [DllImport("user32.dll")]
    public static extern bool EnumDisplaySettings(string deviceName, int modeNum, ref DEVMODE devMode);
}

[StructLayout(LayoutKind.Sequential)]
public struct POINTL
{
    public int x;
    public int y;
}

Answer 4

According to the documentation for ChangeDisplaySettingsEx, "the dmSize member must be initialized to the size, in bytes, of the DEVMODE structure." Furthermore, the EnumDisplaySettings documentation states, "Before calling EnumDisplaySettings, set the dmSize member to sizeof(DEVMODE), and set the dmDriverExtra member to indicate the size, in bytes, of the additional space available to receive private driver data". I don't see this happening in the code sample given in the question; that's one reason why it may be failing.

Additionally, you might have errors in the definitions of the DEVMODE and DISPLAY_DEVICE structs, which were not included in the question. Roger Lipscombe's suggestion to get it working from C/C++ first is an excellent way to rule out this type of problem.

Finally, check the return value from ChangeDisplaySettingsEx and see if that gives a clue as to why it might be failing.