Marshal a C structure into C#

Question

I'm trying to call a function in C that has the following signature from C#

typedef struct _wfs_result 
{
   ULONG RequestID;
   USHORT hService;
   TIMESTAMP tsTimestamp;
   LONG hResult;
   union {
      DWORD dwCommandCode;
      DWORD dwEventID;
   } u;
   LPVOID lpBuffer;
} WFSRESULT, *LPWFSRESULT;

LONG WFSGetInfo(USHORT hService, DWORD dwCategory, LPVOID lpQueryDetails, DWORD dwTimeOut, LPWFSRESULT *lppResult)

typedef struct _wfs_pin_status
{
   WORD fwDevice;
   WORD fwEncStat;
   LPSTR lpszExtra;
   DWORD dwGuidLights[WFS_PIN_GUIDLIGHTS_SIZE];
   WORD fwAutoBeepMode;
   DWORD dwCertificateState;
   WORD wDevicePosition;
   USHORT usPowerSaveRecoveryTime;
} WFSPINSTATUS, *LPWFSPINSTATUS;

and my C# code look like this:

[DllImport("msxfs")]
public static extern int WFSGetInfo(ushort hService, uint dwCategory, IntPtr lpQueryDetails, uint dwTimeOut, ref WFSRESULT lppResult);

[StructLayout(LayoutKind.Explicit)]
public struct WFSRESULT
{
   [FieldOffset(0)]
   public uint RequestID;
   [FieldOffset(4)]
   public ushort hService;
   [FieldOffset(6)]
   public SYSTEMTIME tsTimestamp;
   [FieldOffset(22)]
   public int hResult;
   [FieldOffset(26)]
   public uint dwCommandCode;
   [FieldOffset(26)]
   public uint dwEventID;
   [FieldOffset(30)]
   public IntPtr lpBuffer; //It should be pointer to a structure that contain more information
}

[StructLayout(LayoutKind.Sequential)]
public struct SYSTEMTIME
{
   public ushort wYear;
   public ushort wMonth;
   public ushort wDayOfWeek;
   public ushort wDay;
   public ushort wHour;
   public ushort wMinute;
   public ushort wSecond;
   public ushort wMilliseconds;
}

[StructLayout(LayoutKind.Sequential)]
public struct WFSPINSTATUS
{
   public ushort fwDevice;
   public ushort fwEncStat;
   public string lpszExtra;
   [MarshalAs(UnmanagedType.ByValArray, SizeConst = 32)]
   public uint[] dwGuidLights;
   public ushort fwAutoBeepMode;
   public uint dwCertificateState;
   public ushort wDevicePosition;
   public ushort usPowerSaveRecoveryTime;
}

private void WfsGetInfo()
{
   WFSRESULT wfsRESULT = new WFSRESULT();

   int hResult = WFSGetInfo(_lphService, InfoCommands.WFS_INF_PIN_CAPABILITIES, IntPtr.Zero, WFS_INDEFINITE_WAIT, ref wfsRESULT);

   WFSPINSTATUS pinStatus = Marshal.PtrToStructure<WFSPINSTATUS>(wfsRESULT.lpBuffer);
}

My problem that whenever I call WFSGetInfo function it will be succeeded (hResult == 0), but only RequestID in wfsRESULT will be filled and all other values will be 0 (default value), and when I try to convert lpBuffer to WFSPINSTATUS following exception occurred. System.NullReferenceException: 'Object reference not set to an instance of an object.'

I do not think the problem is in the called dll msxfs because It is a standard windows dll.

I try a lot of solution and techniques (for example I try to set SYSTEMTIME layout to Explicit) but with same result; I will not add my trials to shorten the code.

I done a lot of search regarding marshaling structure and union in C, and following site was very helpful Marshaling with C#

I'm not sure if this information is helpful but I'm building an CEN/XFS integration with EPP device.