Call unmanaged Code from C# - returning a struct with arrays

Question

[EDIT] I changed the source as suggested by Stephen Martin (highlighted in bold). And added the C++ source code as well.

I'd like to call an unmanaged function in a self-written C++ dll. This library reads the machine's shared memory for status information of a third party software. Since there are a couple of values, I'd like to return the values in a struct. However, within the struct there are char [] (Arrays of char with a fixed size). I now try to receive that struct from the dll call like this:

[StructLayout(LayoutKind.Sequential)]
public struct SYSTEM_OUTPUT
{
    UInt16 ReadyForConnect;        

    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
    String VersionStr;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 1024)]
    String NameOfFile;    
    // actually more of those
}

public partial class Form1 : Form
{
    public SYSTEM_OUTPUT output;

    [DllImport("testeshm.dll", EntryPoint="getStatus")]
    public extern static int getStatus(out SYSTEM_OUTPUT output);

    public Form1()
    {
        InitializeComponent();           

    }

    private void ReadSharedMem_Click(object sender, EventArgs e)
    {
        try
        {
            label1.Text = getStatus(out output).ToString();
        }
        catch (AccessViolationException ave)
        {
            label1.Text = ave.Message;
        }
    }
}

I will post code from the c++ dll as well, I'm sure there's more to hunt down. The original struct STATUS_DATA has an array of four instances of the struct SYSTEM_CHARACTERISTICS and within that struct there are char[]s, that are not being filled (yet), resulting in a bad pointer. That's why I'm trying to extract a subset of the first SYSTEM_CHARACTERISTICS item in STATUS_DATA.

#include <windows.h>
#include <stdio.h>
#include <conio.h>
#include <tchar.h>
#include <iostream>
#if defined(_MSC_VER)
#include <windows.h>
#define DLL extern "C" __declspec(dllexport)
#else
#define DLL
#endif

using namespace std;

enum { SYSID_LEN = 1024, VERS_LEN = 128, SCENE_LEN = 1024 };
enum { MAX_ENGINES = 4 };

struct SYSTEM_CHARACTERISTICS
{
    unsigned short  ReadyForConnect;
    char            VizVersionStr[VERS_LEN];
    char            NameOfFile[SCENE_LEN];

    char            Unimplemented[SCENE_LEN]; // not implemented yet, resulting to bad pointer, which I want to exclude (reason to have SYSTEM_OUTPUT)
};

struct SYSTEM_OUTPUT
{
    unsigned short  ReadyForConnect;        
    char            VizVersionStr[VERS_LEN];
    char            NameOfFile[SCENE_LEN];
};

struct STATUS_DATA
{
    SYSTEM_CHARACTERISTICS engine[MAX_ENGINES];
};


TCHAR szName[]=TEXT("E_STATUS");


DLL int getStatus(SYSTEM_OUTPUT* output)
{
    HANDLE hMapFile;
    STATUS_DATA* pBuf;

    hMapFile = OpenFileMapping(
        FILE_MAP_READ,          // read access
        FALSE,                  // do not inherit the name
        szName);                // name of mapping object 

    if (hMapFile == NULL) 
    { 
        _tprintf(TEXT("Could not open file mapping object (%d).\n"), 
            GetLastError());
        return -2;

    } 

    pBuf = (STATUS_DATA*) MapViewOfFile(hMapFile, FILE_MAP_READ, 0, 0, 0);                                          

    if (pBuf == NULL) 
    { 
        _tprintf(TEXT("Could not map view of file (%d).\n"), 
            GetLastError()); 

        CloseHandle(hMapFile);  
        return -1;

    }

    output->ReadyForConnect = pBuf->engine[0].ReadyForConnect;              
    memcpy(output->VizVersionStr, pBuf->engine[0].VizVersionStr, sizeof(pBuf->engine[0].VizVersionStr));
    memcpy(output->NameOfFile, pBuf->engine[0].NameOfFile, sizeof(pBuf->engine[0].NameOfFile));

    CloseHandle(hMapFile);
    UnmapViewOfFile(pBuf);  

    return 0;
}

Now I'm getting an empty output struct and the return value ist not 0 as intended. It is rather a changing number with seven digits, which leaves me puzzled... Have I messed up in the dll? If I make the unmanaged code executable and debug it, I can see, that output is being filled with the appropriate values.

Answer 1

When returning information in a struct the standard method is to pass a pointer to a struct as a parameter of the method. The method fills in the struct members and then returns a status code (or boolean) of some kind. So you probably want to change your C++ method to take a SYSTEM_OUTPUT* and return 0 for success or some error code:

public partial class Form1 : Form
{
    public SYSTEM_OUTPUT output;

    [DllImport("testeshm.dll", EntryPoint="getStatus")]
    public extern static int getStatus(out SYSTEM_OUTPUT output);

    public Form1()
    {
        InitializeComponent();           
    }

    private void ReadSharedMem_Click(object sender, EventArgs e)
    {
        try
        {
            if(getStatus(out output) != 0)
            {
                //Do something about error.
            }
        }
        catch (AccessViolationException ave)
        {
            label1.Text = ave.Message;
        }
    }
}

Answer 2

1. Make sure your ReadyForConnect field is not filled up to 4 bytes. In my project it turned out all short int (2 bytes) fields were filled with dummy bytes to 4 bytes in unmanaged DLL. If that's the issue, you should marshall the struct this way:

    [StructLayout(LayoutKind.Sequential)] 
    public struct SYSTEM_OUTPUT 
    {     
       [MarshalAs(UnmanagedType.I2)] 
       UInt16 ReadyForConnect;
       [MarshalAs(UnmanagedType.ByValArray, ArraySubType=UnmanagedType.I1, SizeConst=2)]
       byte[] aligment;          // 2 byte aligment up to 4 bytes margin
       [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]    
       String VersionStr;    
       [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 1024)]    
       String NameOfFile;        // ...
    }

2. If the strings are ANSI null terminated strings you can annotate them as:

  [MarshalAs(UnmanagedType.LPStr)]                   public String VersionStr;