How to call a C++ function with a struct pointer parameter from C#?

Question

Okay one more function that it's not yet working. I am basically calling some C++ functions from C# by using P/Invoke. The problematic function does query a show laser device for some device related information, such as minimal and maximal scan rates and maximal points per second.

The problematic function is:

int GetDeviceInfo(DWORD deviceIndex, DeviceInfo* pDeviceInfo);

Here's the C++ header file that I was given. That's a link to the very brief C++ SDK description. I don't have the sources to rebuild the DLL file and I also don't have the *.pdb file (the manufacturer can not supply it):

#pragma once

#ifdef STCL_DEVICES_DLL
#define STCL_DEVICES_EXPORT extern "C" _declspec(dllexport) 
#else
#define STCL_DEVICES_EXPORT extern "C" _declspec(dllimport)
#endif

enum SD_ERR
{
    SD_ERR_OK = 0,
    SD_ERR_FAIL,
    SD_ERR_DLL_NOT_OPEN,
    SD_ERR_INVALID_DEVICE,  //device with such index doesn't exist
    SD_ERR_FRAME_NOT_SENT,
};

#pragma pack (1)
struct LaserPoint
{
    WORD x;
    WORD y;
    byte colors[6];
};

struct DeviceInfo
{
    DWORD maxScanrate;
    DWORD minScanrate;
    DWORD maxNumOfPoints;
    char type[32];
};

//////////////////////////////////////////////////////////////////////////
///Must be called when starting to use
//////////////////////////////////////////////////////////////////////////
STCL_DEVICES_EXPORT int OpenDll();

//////////////////////////////////////////////////////////////////////////
///All devices will be closed and all resources deleted
//////////////////////////////////////////////////////////////////////////
STCL_DEVICES_EXPORT void CloseDll();

//////////////////////////////////////////////////////////////////////////
///Search for .NET devices (Moncha.NET now)
///Must be called after OpenDll, but before CreateDeviceList!
///In pNumOfFoundDevs can return number of found devices (optional)
//////////////////////////////////////////////////////////////////////////
STCL_DEVICES_EXPORT int SearchForNETDevices(DWORD* pNumOfFoundDevs);

//////////////////////////////////////////////////////////////////////////
///Creates new list of devices - previous devices will be closed
///pDeviceCount returns device count
//////////////////////////////////////////////////////////////////////////
STCL_DEVICES_EXPORT int CreateDeviceList(DWORD* pDeviceCount);

//////////////////////////////////////////////////////////////////////////
///Returns unique device name
///deviceIndex is zero based device index
//////////////////////////////////////////////////////////////////////////
STCL_DEVICES_EXPORT int GetDeviceIdentifier(DWORD deviceIndex, WCHAR** ppDeviceName);

//////////////////////////////////////////////////////////////////////////
///Send frame to device, frame is in following format:
///WORD x
///WORD y
///byte colors[6]
///so it's 10B point (=> dataSize must be numOfPoints * 10)
///scanrate is in Points Per Second (pps)
//////////////////////////////////////////////////////////////////////////
STCL_DEVICES_EXPORT int SendFrame(DWORD deviceIndex, byte* pData, DWORD numOfPoints, DWORD scanrate);

//////////////////////////////////////////////////////////////////////////
///Returns true in pCanSend if device is ready to send next frame
//////////////////////////////////////////////////////////////////////////
STCL_DEVICES_EXPORT int CanSendNextFrame(DWORD deviceIndex, bool* pCanSend);

//////////////////////////////////////////////////////////////////////////
///Send DMX if device supports it - pDMX must be (!!!) 512B long
//////////////////////////////////////////////////////////////////////////
STCL_DEVICES_EXPORT int SendDMX(DWORD deviceIndex, byte* pDMX);

//////////////////////////////////////////////////////////////////////////
///Send blank point to position x, y
//////////////////////////////////////////////////////////////////////////
STCL_DEVICES_EXPORT int SendBlank(DWORD deviceIndex, WORD x, WORD y);

//////////////////////////////////////////////////////////////////////////
///Get device info
//////////////////////////////////////////////////////////////////////////
STCL_DEVICES_EXPORT int GetDeviceInfo(DWORD deviceIndex, DeviceInfo* pDeviceInfo);

This is the complete C# test code I am currently using. All the functions work fine, except for GetDeviceInfo(...):

using System;
using System.Threading;
using System.Runtime.InteropServices;

namespace MonchaTestSDK {

    public class Program {

        [DllImport("..\\..\\dll\\StclDevices.dll", CallingConvention = CallingConvention.Cdecl)]                                    // OK
        public static extern int OpenDll();
        [DllImport("..\\..\\dll\\StclDevices.dll", CallingConvention = CallingConvention.Cdecl)]                                    // OK
        public static extern void CloseDll();
        [DllImport("..\\..\\dll\\StclDevices.dll", CallingConvention = CallingConvention.Cdecl)]                                    // OK
        public static extern int SearchForNETDevices(ref UInt32 pNumOfFoundDevs);
        [DllImport("..\\..\\dll\\StclDevices.dll", CallingConvention = CallingConvention.Cdecl)]                                    // OK
        public static extern int CreateDeviceList(ref UInt32 pDeviceCount);
        [DllImport("..\\..\\dll\\StclDevices.dll", CallingConvention = CallingConvention.Cdecl)]                                    // OK
        public static extern int GetDeviceIdentifier(UInt32 deviceIndex, out IntPtr ppDeviceName);
        [DllImport("..\\..\\dll\\StclDevices.dll", CallingConvention = CallingConvention.Cdecl)]                                    // OK
        public static extern int SendFrame(UInt32 deviceIndex, LaserPoint[] pData, UInt32 numOfPoints, UInt32 scanrate);
        [DllImport("..\\..\\dll\\StclDevices.dll", CallingConvention = CallingConvention.Cdecl)]                                    // OK
        public static extern int CanSendNextFrame(UInt32 deviceIndex, ref bool pCanSend);
        [DllImport("..\\..\\dll\\StclDevices.dll", CallingConvention = CallingConvention.Cdecl)]                                    // OK
        public static extern int SendBlank(UInt32 deviceIndex, UInt16 x, UInt16 y);
        [DllImport("..\\..\\dll\\StclDevices.dll", CallingConvention = CallingConvention.Cdecl)]                                    // FAILS
        public static extern int GetDeviceInfo(UInt32 deviceIndex, ref DeviceInfo pDeviceInfo);

        [StructLayout(LayoutKind.Sequential, Pack=1)]
        public struct LaserPoint {
            public UInt16 x;
            public UInt16 y;
            [MarshalAs(UnmanagedType.ByValArray, SizeConst = 6)]
            public byte[] colors;
        }

        [StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi)]
        public struct DeviceInfo {
            public UInt32 maxScanrate;
            public UInt32 minScanrate;
            public UInt32 maxNumOfPoints;
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)]
            public string deviceType;
        }

        public static void Main(string[] args) {
            Console.WriteLine("Moncha SDK\n");

            OpenDll();
            Console.WriteLine("StclDevices.dll is open.");

            UInt32 deviceCount1 = 0;
            int r1 = SearchForNETDevices(ref deviceCount1);
            Console.WriteLine("SearchForNETDevices() [" + r1+"]: "+deviceCount1);

            UInt32 deviceCount2 = 0;
            int r2 = CreateDeviceList(ref deviceCount2);
            Console.WriteLine("CreateDeviceList() ["+r2+"]: "+deviceCount2);

            IntPtr pString;
            int r3 = GetDeviceIdentifier(0, out pString);
            string devname = Marshal.PtrToStringUni(pString);
            Console.WriteLine("GetDeviceIdentifier() ["+r3+"]: "+devname);

            DeviceInfo pDevInfo = new DeviceInfo();
            pDevInfo.type = "";
            int r4 = GetDeviceInfo(0, ref pDevInfo);
            Console.WriteLine("GetDeviceInfo() ["+r4+"]: ");
            Console.WriteLine("  - min: "+pDevInfo.minScanrate);
            Console.WriteLine("  - max: " + pDevInfo.maxScanrate);
            Console.WriteLine("  - points: " + pDevInfo.maxNumOfPoints);
            Console.WriteLine("  - type: " + pDevInfo.deviceType);

            Thread.Sleep(5000);
            CloseDll();
        }

    }
}

On line 73 line 64 (cp. screenshot):

int r4 = GetDeviceInfo(0, ref pDevInfo); 

I receive the following error:

An unhandled exception of type 'System.NullReferenceException' occured in MonchaTestSDK.exe
Additional information: Object reference not set to an instance of an object

This is the stack trace (can't provide better stack trace without the DLL's *.pdb file I guess):

MonchaTestSDK.exe!MonchaTestSDK.Program.Main(string[] args) Line 73 + 0xa bytes C# mscoreei.dll!73a8d91b()
[Frames below may be incorrect and/or missing, no symbols loaded for mscoreei.dll]
mscoree.dll!73cae879()
mscoree.dll!73cb4df8()
kernel32.dll!74a08654()
ntdll.dll!77354b17()
ntdll.dll!77354ae7()

Some disassembly:

            int r4 = GetDeviceInfo(0, ref pDevInfo);
05210749  int         3  
0521074A  push        ebp  
0521074B  cwde  
0521074C  xor         ecx,ecx  
0521074E  call        0521011C  
05210753  int         3  
05210754  test        dword ptr [eax-1],edx  
05210757  ?? ?? 
05210758  dec         dword ptr [ebx-0AF7Bh]  
0521075E  dec         dword ptr [ecx-6F466BBBh]

Any idea what I am doing wrong here?

---

Update 1: Suggested debug options:

As suggested in the comments, I tried to enable native/unmanaged code debugging:

1. Debug > Windows > Exceptions Settings > "Win32 Exceptions" checkbox ticked

2. Project > Properties > Debug tab > "Enable unmanaged code debugging" checkbox ticked

I still don't get any meaningful exception stack. The manufacturer can't supply me the DLL's *.pdb file.

Here's an image showing the debugger when stopped at the problematic line (debug settings are also shown):

---

Update 2: Minimal Required Code (cp. comment of mpromonet)

This is the minimal required code to be able to call GetDeviceInfo(...):

public static void Main(string[] args) {
    OpenDll();
    UInt32 deviceCount = 0;
    CreateDeviceList(ref deviceCount);
    DeviceInfo pDevInfo = new DeviceInfo();
    GetDeviceInfo(0, ref pDevInfo);            // error occurs on this line
    CloseDll();
}

This leads to the exact same error as before:

An unhandled exception of type 'System.NullReferenceException' occured in MonchaTestSDK.exe
Additional information: Object reference not set to an instance of an object

Removing the call GetDeviceInfo(0, ref pDevInfo); from the code above allows the program to exit without any error.

---

Update 3: Removing char[] deviceType from DeviceInfo struct completely

I removed char[] deviceType from the struct defintion:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
public struct DeviceInfo {
    public UInt32 maxScanrate;
    public UInt32 minScanrate;
    public UInt32 maxNumOfPoints;
    //[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)]
    //public string deviceType;
}

When I run my C# test code now, I successfully receive maxScanrate, minScanrate and maxNumOfPoints back from the C++ DLL. Here's the corresponding console output:

GetDeviceInfo() [0]:
   - min: 1000
   - max: 40000
   - points: 3000

Finally ending in the following error message:

Exception thrown at 0x67623A68 (clr.dll) in MonchaTestSDK.exe: 0xC0000005: Access violation reading location 0x00000000.

---

Final Update

I finally got an updated DLL from the manufacturer. There was indeed a bug within the SDK that caused the stack to get corrupted. So basically the following solution now works fine without any issues:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
public struct DeviceInfo {

    public UInt32 maxScanrate;
    public UInt32 minScanrate;
    public UInt32 maxNumOfPoints;

    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)]
    public string deviceType;

}

private void queryDeviceProperties(UInt32 index) {
    HwDeviceInfo pDevInfo = new HwDeviceInfo();
    int code = GetDeviceInfo(index, ref pDevInfo);
    if(code==0) {
        Console.WriteLine(pDevInfo.minScanrate);
        Console.WriteLine(pDevInfo.maxScanrate);
        Console.WriteLine(pDevInfo.maxNumOfPoints);
        Console.WriteLine(pDevInfo.type);
    } else {
        Console.WriteLine("Error Code: "+code);
    }
}

Thank you all for the great support!

Answer 1

[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)] declares that the field is stored a char[32] array as in the header, i.e. space for 31 characters and a null terminator.

Marshalling this to a string shouldn't be a problem, nothing that the dll writes to the array should be able to cause a NullReferenceException.

I can compile a stub dll that loads fine using your C# code and can send back ANSI strings, with addition of typedef byte... and a stub method body e.g.:

int GetDeviceInfo(DWORD deviceIndex, DeviceInfo* pDeviceInfo)
{
    std::string testString = "test string thats quite loooooooong"; 
    pDeviceInfo->maxScanrate = 1234;
    pDeviceInfo->minScanrate = 12345;
    pDeviceInfo->maxNumOfPoints = 100 + deviceIndex;
    sprintf_s(pDeviceInfo->type, "%.31s", testString.c_str());
    return 0;
}

This works for me with VS2017 C++ and .Net 4.6.1.

What happens if you change the C# declaration to this:

    [StructLayout(LayoutKind.Sequential, Pack = 1)]
    public struct DeviceInfo
    {
        public UInt32 maxScanrate;
        public UInt32 minScanrate;
        public UInt32 maxNumOfPoints;
        public UInt64 deviceTypePart1;
        public UInt64 deviceTypePart2;
        public UInt64 deviceTypePart3;
        public UInt64 deviceTypePart4;

        public string GetDeviceType()
        {
            if (Marshal.SizeOf(this) != 44) throw new InvalidOperationException();
            List<byte> bytes = new List<byte>();
            bytes.AddRange(BitConverter.GetBytes(deviceTypePart1));
            bytes.AddRange(BitConverter.GetBytes(deviceTypePart2));
            bytes.AddRange(BitConverter.GetBytes(deviceTypePart3));
            bytes.AddRange(BitConverter.GetBytes(deviceTypePart4));
            return Encoding.GetEncoding(1252).GetString(bytes.ToArray());
        }
    }

[Edit]

I've no idea why hand cranking the marshaling fixes this - be sure to 'load test' in case there are heap/stack corruption bugs still lurking.

In your old code, does Marshal.SizeOf return something other than 44?