Question

What is the best practice to share memory of a struct from a C# program to a C++ win32 DLL?

I’ve used structs in managed shared memory using Boost between two C++ programs and it worked great. I’m lost on the best way to accomplish this between where the struct gets populated in the C# program and the C++ DLL that is an SNMP subagent.

Here’s the C++ DLL:

//====================           Code Excerpt from the main cpp file              ====================== 
#include "stdafx.h" 
//=================           Here we are setting up the shared memory area   =====================
#pragma data_seg (".SHAREDMEMORY")
    struct sharedData {
     int sharedA;
     int sharedB;
    };
    static sharedData A;

#pragma data_seg() 
#pragma comment(linker,"/SECTION:.SHAREDMEMORY,RWS")

BOOL APIENTRY DllMain( HANDLE hModule, DWORD ul_reason_for_call, LPVOID lpReserved ) 
{ 
     return TRUE; 
} 
//=============================================================================================
//====================== Here we are writing wrappers to the shared memory area ===========================
//=You must declare it as an Extern "C" to prevent name mangling. This is absolutely necessary in order to import it into c#  =
//=============================================================================================
extern "C" __declspec(dllexport) sharedData __stdcall getMyData() 
{
    A.sharedA = 1237;
    A.sharedB = 31337;
     //return gshared_nTest; 
    return A;
} 
extern "C" __declspec(dllexport) void __stdcall setMyData( sharedData buff ) 
{ 
     A = buff; 
}

Here’s the calling C# function:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace sharedMemTestCS
{
    public partial class frmSharedMemTestCS : Form
    {
    struct sharedData {
     int sharedA;
     int sharedB;
    };
    static sharedData A;

    //==============  here we are importing the methods from the win32 dll into the c# console application =================
    [DllImport(@"C:\Documents and Settings\My Documents\Visual Studio 2010\Projects\sharedMemTestCPP\Debug\sharedMemTestCPP.dll")]  
    public static extern sharedData getMyData();  
    [DllImport(@"C:\Documents and Settings\My Documents\Visual Studio 2010\Projects\sharedMemTestCPP\Debug\sharedMemTestCPP.dll")]  
    public static extern void setMyData(int data);

        public frmSharedMemTestCS()
        {
            InitializeComponent();
            //==============  here i am incrementing the value =================

            //== i use a message box so that i can have multiple console applications running at once and it will pause at the messagebox (if i don't click ok)
            //== i do this so i can see the values changing in the shared memory.
            //MessageBox.Show( getMyData().ToString() );
            getMyData();
            //txtBoxA.Text = (getMyData().ToString() );
        }

        private void btnAdd_Click(object sender, EventArgs e)
        {
            //setMyData( getMyData() + 100 );
            //txtBoxA.Text = (getMyData().ToString() );
        }
    }
}

The error message I get is:

Error 1   Inconsistent accessibility: return type

‘sharedMemTestCS.frmSharedMemTestCS.sharedData’ is less accessible
than method
‘sharedMemTestCS.frmSharedMemTestCS.getMyData()’ c:\documents and
settings\mconrad\my documents\visual studio
2010\Projects\sharedMemTestCS\sharedMemTestCS\Form1.cs 23 37 sharedMemTestCS

Answer 1

The best practice for sharing memory would be to use the MemoryMappedFile class in C# and CreateFileMapping/MapViewOfFile in C++.