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_ORIENTATION = 0x00000001;
public const int DM_PAPERSIZE = 0x00000002;
public const int DM_PAPERLENGTH = 0x00000004;
public const int DM_PAPERWIDTH = 0x00000008;
public const int DM_SCALE = 0x00000010;
public const int DM_POSITION = 0x00000020;
public const int DM_NUP = 0x00000040;
public const int DM_DISPLAYORIENTATION = 0x00000080;
public const int DM_COPIES = 0x00000100;
public const int DM_DEFAULTSOURCE = 0x00000200;
public const int DM_PRINTQUALITY = 0x00000400;
public const int DM_COLOR = 0x00000800;
public const int DM_DUPLEX = 0x00001000;
public const int DM_YRESOLUTION = 0x00002000;
public const int DM_TTOPTION = 0x00004000;
public const int DM_COLLATE = 0x00008000;
public const int DM_FORMNAME = 0x00010000;
public const int DM_LOGPIXELS = 0x00020000;
public const int DM_BITSPERPEL = 0x00040000;
public const int DM_PELSWIDTH = 0x00080000;
public const int DM_PELSHEIGHT = 0x00100000;
public const int DM_DISPLAYFLAGS = 0x00200000;
public const int DM_DISPLAYFREQUENCY = 0x00400000;
public const int DM_ICMMETHOD = 0x00800000;
public const int DM_ICMINTENT = 0x01000000;
public const int DM_MEDIATYPE = 0x02000000;
public const int DM_DITHERTYPE = 0x04000000;
public const int DM_PANNINGWIDTH = 0x08000000;
public const int DM_PANNINGHEIGHT = 0x10000000;
public const int DM_DISPLAYFIXEDOUTPUT = 0x20000000;

public const int ENUM_CURRENT_SETTINGS = -1;
public const int CDS_UPDATEREGISTRY = 0x01;
public const int CDS_TEST = 0x02;
public const int CDS_SET_PRIMARY = 0x00000010;

public const long DISP_CHANGE_SUCCESSFUL = 0;
public const long DISP_CHANGE_RESTART = 1;
public const long DISP_CHANGE_FAILED = -1;
public const long DISP_CHANGE_BADMODE = -2;
public const long DISP_CHANGE_NOTUPDATED = -3;
public const long DISP_CHANGE_BADFLAGS = -4;
public const long DISP_CHANGE_BADPARAM = -5;
public const long DISP_CHANGE_BADDUALVIEW = -6;

    public static void SetPrimary(Screen screen)
{
  DISPLAY_DEVICE d = new DISPLAY_DEVICE();
  DEVMODE dm = new DEVMODE();
  d.cb = Marshal.SizeOf(d);
  uint deviceID = 1;
  User_32.EnumDisplayDevices(null, deviceID, ref  d, 0); // 
  User_32.EnumDisplaySettings(d.DeviceName, 0, ref dm);
  dm.dmPelsWidth = 2560;
  dm.dmPelsHeight = 1600;
  dm.dmPositionX = screen.Bounds.Right;
  dm.dmFields = DM_POSITION | DM_PELSWIDTH | DM_PELSHEIGHT;
  User_32.ChangeDisplaySettingsEx(d.DeviceName, ref dm, IntPtr.Zero, CDS_SET_PRIMARY, IntPtr.Zero);
}

I call the method like this:

SetPrimary(Screen.AllScreens[1])

Any ideas?

Answer 1

I'd rule out P/Invoke problems by getting it to work from C++ first.

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

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.

Answer 4

Did you ever figure out how to make this work?

Bill