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How to detect if additional monitor is connected and active

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Before you begin:
Nowadays having two monitors connected to a PC becomes more a common place rather than exception. This code sample explains how to detect all available monitors through enumerating display devices and their properties.


See also:
  • How to change display settings: screen resolution, screen refresh rate
  • How to adjust monitor brightness (Vista, monitor with DDC support)
  • using System;
    using System.Runtime.InteropServices;
    using System.Windows.Forms;
    
    namespace ConsoleApplication1
    {
        partial class Program
        {
            public const int ENUM_CURRENT_SETTINGS = -1;
    
            [StructLayout(LayoutKind.Sequential)]
            public struct DISPLAY_DEVICE
            {
                public int cb;
                [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)]
                public string DeviceName;
                [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
                public string DeviceString;
                public int StateFlags;
                [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
                public string DeviceID;
                [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
                public string DeviceKey;
    
                public DISPLAY_DEVICE(int flags)
                {
                    cb = 0;
                    StateFlags = flags;
                    DeviceName = new string((char)32, 32);
                    DeviceString = new string((char)32, 128);
                    DeviceID = new string((char)32, 128);
                    DeviceKey = new string((char)32, 128);
                    cb = Marshal.SizeOf(this);
                }
            }
    
            [StructLayout(LayoutKind.Sequential)]
            public struct DEVMODE
            {
                [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)]
                public string dmDeviceName;
                public short dmSpecVersion;
                public short dmDriverVersion;
                public short dmSize;
                public short dmDriverExtra;
                public int dmFields;
                public short dmOrientation;
                public short dmPaperSize;
                public short dmPaperLength;
                public short dmPaperWidth;
                public short dmScale;
                public short dmCopies;
                public short dmDefaultSource;
                public short dmPrintQuality;
                public short dmColor;
                public short dmDuplex;
                public short dmYResolution;
                public short dmTTOption;
                public short dmCollate;
                [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)]
                public string dmFormName;
                public short dmUnusedPadding;
                public short dmBitsPerPel;
                public int dmPelsWidth;
                public int dmPelsHeight;
                public int dmDisplayFlags;
                public int dmDisplayFrequency;
            }
    
            static void EnumMonitors()
            {
                foreach (Screen screen in Screen.AllScreens)
                {
                    Console.WriteLine("*** Managed output");
    
                    Console.WriteLine("Display Device: {0}\t\n" +
                        "\tBPP={1}\n\tWidth={2}\n\tHeight={3}\n\n",
                        screen.DeviceName, screen.BitsPerPixel,
                        screen.Bounds.Width, screen.Bounds.Height);
    
                    DISPLAY_DEVICE monitor =
                        new DISPLAY_DEVICE(0);
    
                    EnumDisplayDevices(screen.DeviceName, 0,
                        ref monitor, 0);
    
                    DEVMODE dm = new DEVMODE();
    
                    EnumDisplaySettings(screen.DeviceName,
                        ENUM_CURRENT_SETTINGS, ref dm);
    
                    Console.WriteLine("*** Unmanaged output");
    
                    Console.WriteLine("\tMonitor: {0}\n\t\t{1}\n\t\t{2}",
                        monitor.DeviceName, 
                        monitor.DeviceString, monitor.DeviceID);
    
                    Console.WriteLine(
                        "\t\t\tFreq.={0}\n" +
                        "\t\t\tBPP={1}\n" +
                        "\t\t\tWidth={2}\n" +
                        "\t\t\tHeight={3}\n",
                        dm.dmDisplayFrequency, dm.dmBitsPerPel,
                        dm.dmPelsWidth, dm.dmPelsHeight);
    
                }
                Console.WriteLine("Number of monitors: {0}", 
                    Screen.AllScreens.Length);
            }
    
            [DllImport("User32.dll")]
            private static extern bool EnumDisplayDevices(
                string lpDevice, int iDevNum,
                ref DISPLAY_DEVICE lpDisplayDevice, int dwFlags);
    
            [DllImport("User32.dll")]
            private static extern bool EnumDisplaySettings(
                string devName, int modeNum, ref DEVMODE devMode);
    
        }
    }
    

    User rating: 9/10 (1 votes)
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    4306 bytes  
    Created: 2008-12-09 14:50:37  
    Modified: 2014-01-30 13:58:32  
    Visits in 7 days: 136  
    Listed functions:
    EnumDisplayDevices
    EnumDisplaySettings
    GetMonitorInfo
    MonitorFromWindow
    Printer friendly API declarations
    My comment:
    In the cursor created by this code sample, each connected monitor is indicated by non-empty MonitorName value.

    For an active monitor (the desktop extends to such monitor), several other values are populated:

  • Screenwidth
  • Screenheight
  • Bitsperpixel
  • DsppositionX
  • DsppositionY

    From the screen dimensions and the display position it is possible to determine whether a given window is visible on a particular display.

    * * *
    Read Two monitors are better than one by Tony Northrup published on Microsoft website. Bet he might be busy writing a sequel "N+1 monitors better than N".

    * * *
    Some graphics cards and external units (Matrox) support connections to multiple monitors, three and more.
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