Question

I’m developing an application which will display information derived from the EDID blocks (monitor model, ID, S/N, etc.) on a dialog on the corresponding monitor.

This code works for finding the EDID information for displays. It extracts the EDID information by enumerating the DISPLAY keys under HKLM\SYSTEM\CurrentControlSet\Enum\DISPLAY\[Monitor]\[PnPID]\Device Parameters\EDID.

Update: The above code is relying on “side effects” of PnP use of the registry. I am now using the SetupAPI to enumerate monitors, which correctly handles monitors being attached/removed (unlike the code from the link above.)

I am trying to correlate each Screen in Windows.Forms.Screen.AllScreens[] (\\.\DISPLAY1, \\.\DISPLAY2, etc.) with the entries returned from the above registry inspection.

Note: In the code block below, DisplayDetails.GetMonitorDetails() has now been replaced with more robust registry enumeration code using the SetupAPI, but the data returned is the same.

e.g.

private void Form1_Load(object sender, EventArgs e)
{
    Console.WriteLine("Polling displays on {0}:", System.Environment.MachineName);
    int i = 0;
    foreach ( DisplayDetails dd in DisplayDetails.GetMonitorDetails())
    {
        Console.WriteLine( "Info: Model: {0}, MonitorID: {1}, PnPID: {2}, Serial#:{3}", dd.Model, dd.MonitorID, dd.PnPID, dd.SerialNumber );
        Console.WriteLine( "Does this correlate to Screen: {0}?", Screen.AllScreens[i++].DeviceName );
    }
}

Output:

Info: Model: DELL P2411H, MonitorID: DELA06E, PnPID: 5&2e2fefea&0&UID1078018, Serial#:F8NDP0C…PU

Does this correlate to Screen: \\.\DISPLAY1?

Info: Model: DELL P2411H, MonitorID: DELA06E, PnPID: 5&2e2fefea&0&UID1078019, Serial#:F8NDP0C…AU

Does this correlate to Screen: \\.\DISPLAY2?

---

Answer: NO

In testing, I’ve found these don’t reliably correlate (I have a system in which the first display enumerated is \\.\DISPLAY2).

My Question:
Is there a way to reliably get the EDID information for a given Forms.Screen? I can get the EDID block, but have found no path to correlate this up to the UI top-level Form. Prompting the user is undesirable, as in my use case the two (or more) monitors will likely be the same model and resolution, and only differ by a few digits in the S/N.

I’ve looked for paths following the Forms.Screen API, Win32 EnumDisplay, other registry GUIDs (PnP and driver-related), but haven’t found any promising paths.

I have also investigated the WMI Win32_DesktopMonitor API (Windows 7), however it doesn’t appear to have any more information that would help me correlate it to the Windows.Forms.Screen.AllScreens[] entries.

I suspect if there is a way to do this, it’s through the SetupAPI, however I haven’t found it yet.

Answer 1

A method to resolve the GDI to SetupAPI is available in the EnumDisplayDevices API. If you pass in the EDD_GET_DEVICE_INTERFACE_NAME in for dwFlags, the monitor enumeration will return DeviceID information of the form:

Monitor 0 info:
DeviceName: \\.\DISPLAY1
MonitorInfo: Dell P2411H(Digital)
DeviceID: \\?\DISPLAY#DELA06E#5&2e2fefea&0&UID1078018#{e6f07b5f-ee97-4a90-b076-3
3f57bf4eaa7}
Monitor 1 info:
DeviceName: \\.\DISPLAY2
MonitorInfo: Dell P2411H(Digital)
DeviceID: \\?\DISPLAY#DELA06E#5&2e2fefea&0&UID1078019#{e6f07b5f-ee97-4a90-b076-3
3f57bf4eaa7}

The DeviceID fields now match the results from the didd.DevicePath, as retrieved in the C# fragment below:

    Guid MonitorGUID = new Guid(Win32.GUID_DEVINTERFACE_MONITOR);

    // We start at the "root" of the device tree and look for all
    // devices that match the interface GUID of a monitor
    IntPtr h = Win32.SetupDiGetClassDevs(ref MonitorGUID, IntPtr.Zero, IntPtr.Zero, (uint)(Win32.DIGCF_PRESENT | Win32.DIGCF_DEVICEINTERFACE));
    if (h.ToInt64() != Win32.INVALID_HANDLE_VALUE)
    {
        bool Success = true;
        uint i = 0;
        while (Success)
        {
            // create a Device Interface Data structure
            Win32.SP_DEVICE_INTERFACE_DATA dia = new Win32.SP_DEVICE_INTERFACE_DATA();
            dia.cbSize = (uint)Marshal.SizeOf(dia);

            // start the enumeration 
            Success = Win32.SetupDiEnumDeviceInterfaces(h, IntPtr.Zero, ref MonitorGUID, i, ref dia);
            if (Success)
            {
                // build a DevInfo Data structure
                Win32.SP_DEVINFO_DATA da = new Win32.SP_DEVINFO_DATA();
                da.cbSize = (uint)Marshal.SizeOf(da);

                // build a Device Interface Detail Data structure
                Win32.SP_DEVICE_INTERFACE_DETAIL_DATA didd = new Win32.SP_DEVICE_INTERFACE_DETAIL_DATA();
                didd.cbSize = (uint)(4 + Marshal.SystemDefaultCharSize); // trust me :)

                // now we can get some more detailed information
                uint nRequiredSize = 0;
                uint nBytes = Win32.BUFFER_SIZE;
                if (Win32.SetupDiGetDeviceInterfaceDetail(h, ref dia, ref didd, nBytes, out nRequiredSize, ref da))
                {
                    // Now we get the InstanceID
                    IntPtr ptrInstanceBuf = Marshal.AllocHGlobal((int)nBytes);
                    Win32.CM_Get_Device_ID(da.DevInst, ptrInstanceBuf, (int)nBytes, 0);
                    string InstanceID = Marshal.PtrToStringAuto(ptrInstanceBuf);
                    Console.WriteLine("InstanceID: {0}", InstanceID );
                    Marshal.FreeHGlobal(ptrInstanceBuf);
                   
                    Console.WriteLine("DevicePath: {0}", didd.DevicePath );
                }
                i++;
            }
        }
    }
    Win32.SetupDiDestroyDeviceInfoList(h);
}

Sample Output:

InstanceID: DISPLAY\DELA06E\5&2E2FEFEA&0&UID1078018
DevicePath: \\?\display#dela06e#5&2e2fefea&0&uid1078018#{e6f07b5f-ee97-4a90-b076-33f57bf4eaa7}

The DeviceName from the original EnumDisplayDevices matches the Forms.Screen.DeviceName property.

With these two pieces of information, it is now possible to read the EDID block during the SetupDIEnumDeviceInterface traversal using a fragment like the below:

private static byte[] GetMonitorEDID(IntPtr pDevInfoSet, SP_DEVINFO_DATA deviceInfoData)
{
    IntPtr hDeviceRegistryKey = SetupDiOpenDevRegKey(pDevInfoSet, ref deviceInfoData,
        DICS_FLAG_GLOBAL, 0, DIREG_DEV, KEY_QUERY_VALUE);
    if (hDeviceRegistryKey == IntPtr.Zero)
    {
        throw new Exception("Failed to open a registry key for device-specific configuration information");
    }

    IntPtr ptrBuff = Marshal.AllocHGlobal((int)256);
    try
    {
        RegistryValueKind lpRegKeyType = RegistryValueKind.Binary;
        int length = 256;
        uint result = RegQueryValueEx(hDeviceRegistryKey, "EDID", 0, ref lpRegKeyType, ptrBuff, ref length);
        if (result != 0)
        {
            throw new Exception("Can not read registry value EDID for device " + deviceInfoData.ClassGuid);
        }
    }
    finally
    {
        RegCloseKey(hDeviceRegistryKey);
    }
    byte[] edidBlock = new byte[256];
    Marshal.Copy(ptrBuff, edidBlock, 0, 256);
    Marshal.FreeHGlobal(ptrBuff);
    return edidBlock;
}

Which, finally, can be parsed for the VESA descriptor blocks, as shown in the DisplayDetails.GetMonitorDetails() method in this code.