EnumDisplayDevices - passing null causes error

Question

The MSDN documentation for EnumDisplayDevices states that passing NULL as the first parameter to the function returns information about the display adaptor(s) on the machine (passing a string returns information about the device with that name).

Various C# examples that I have seen on-line pass null to the function as follows:

result = EnumDisplayDevices(null, devNum, ref dd, flags);

However, when I pass null as the first parameter, I get a System.AccessViolationException with the message "Attempted to read or write protected memory".

If I change null to any random non-null string (eg, "hello"), then the function call succeeds (I just don't get any device information because there isn't a device called "hello").

So how do I pass null as the first parameter to the EnumDisplayDevices function? (I need to be able to pass names in subsequent calls to the function)

Relevant snippets of my code follow:

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

    [StructLayout(LayoutKind.Sequential)]
    public struct DISPLAY_DEVICE
    {
        public int cb;
        public string DeviceName;
        public string DeviceString;
        public int StateFlags;
        public string DeviceID;
        public string DeviceKey;
    }

    #region Public Interface
    public ObservableCollection<DisplayDevice> LoadDisplayDevices()
    {
        ObservableCollection<DisplayDevice> displayDevices = new ObservableCollection<DisplayDevice>();

        uint devNum = 0;
        uint flags = 0;
        bool result = false;

        DISPLAY_DEVICE dd = new DISPLAY_DEVICE();
        dd.cb = (int)Marshal.SizeOf(dd);

        try
        {
            result = EnumDisplayDevices(null, devNum, ref dd, flags);

            ...

Answer 1

Original definition from MSDN:

typedef struct _DISPLAY_DEVICE {
  DWORD cb;
  TCHAR DeviceName[32];
  TCHAR DeviceString[128];
  DWORD StateFlags;
  TCHAR DeviceID[128];
  TCHAR DeviceKey[128];
} DISPLAY_DEVICE, *PDISPLAY_DEVICE;

All string fields are defined as fixed-size arrays. Your DISPLAY_DEVICE structure definition contains multiple string values without instructions how to marshal them, so they will be passed as pointers. You'll need to use MarshalAsAttribute to fix this:

[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;
}