Pass char array value from C++ to C# app

Question

I have a huge C++ structure with a lot of validation code in C++, that I want to import in a C# project. I'm able to transfer all value except the CHAR* and CHAR[].

With a CHAR*, my string is full of chiness caracter BUT, if i go look in memory, my string is there, i can see "This is a test #1".

With a CHAR[x], I can see only the 1rst char, same in memory.

In the following test, i can extract the integer : value1 = data.Value1; and value 1 is 123, but the the CHAR.

Question: What i miss, why i can't get the value with a char array. Thank you

The C++ DLL

//This is the main DLL file.
#include "stdafx.h"
#include <windows.h>
#include <stdio.h>

extern "C"
{
  public struct Data
  {
    int Value1;
    char* pValue2;
    char Value3[1024];
  };

  typedef int( *FN_CCP_INVOKE_NEW_BOARD_OPTIMIZER) (struct Data* data);

  FN_CCP_INVOKE_NEW_BOARD_OPTIMIZER _pInvokeCallback;

  int __declspec(dllexport)  DLLTestCPlusPlus_Initialize()
  {
    return 0;
  }

  int __declspec(dllexport) DLLTestCPlusPlus_RegisterDllInvokeProcessCallback(void* fnInvokeCaller)
  {
    _pInvokeCallback = (FN_CCP_INVOKE_NEW_BOARD_OPTIMIZER)fnInvokeCaller;

    struct Data data;

    // INT
    data.Value1 = 123;

    // CHAR*
    data.pValue2 = new char[1024];
    sprintf(data.pValue2, "This is a test #1");

    // CHAR [1024]
    sprintf(data.Value3, "This is a test #2");

    if (_pInvokeCallback)
    {
      _pInvokeCallback(&data);
    }

    return 0;
  }
}

And here's the C# program that import the DLL.

using System;
using System.Runtime.InteropServices;

  public unsafe struct Data
  {
    public int Value1;
    public char* pValue2;
    public fixed char Value3[1024];
  }

  public static class Interop
  {
    public delegate Int32 Callback([MarshalAs(UnmanagedType.Struct)] Data data);

    [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    public static extern bool SetDllDirectory(string lpPathName);


    [DllImport("C:\\DATA\\CODE\\ApplicationTestCSharp\\x64\\Debug\\DLLTestCPlusPlus.dll", CallingConvention = CallingConvention.Cdecl)]
    public static extern Int32 DLLTestCPlusPlus_Initialize();

    [DllImport("C:\\DATA\\CODE\\ApplicationTestCSharp\\x64\\Debug\\DLLTestCPlusPlus.dll", CallingConvention = CallingConvention.Cdecl)]
    public static extern Int32 DLLTestCPlusPlus_RegisterDllInvokeProcessCallback([MarshalAs(UnmanagedType.FunctionPtr)] Callback handler);
  }

  public class MyTest
  {
    private Interop.Callback _callback = null;

    public MyTest()
    {
      int returnCode = 0;

      returnCode = Interop.DLLTestCPlusPlus_Initialize();

      _callback = new Interop.Callback(CallbackHandler);

      returnCode = Interop.DLLTestCPlusPlus_RegisterDllInvokeProcessCallback(_callback);
    }

    private Int32 CallbackHandler(Data data)
    {
      int value1 = 0;
      string value2 = "";
      string value3 = "";

      unsafe
      {
        // INT
        value1 = data.Value1;

        // CHAR* - MUST BE "This is a test #1"
        value2 = new string(data.pValue2);

        // CHAR [1024] - "This is a test #2"
        value3 = new string(data.Value3);
      }

      return 1;
    }
  }

  class Program
  {
    static void Main(string[] args)
    {
      MyTest myTest = new MyTest();

    }
  }

Hanzi suggests that you're passing ASCII text to something expecting UTF-16. P/Invoke lets you specify what type of text it is. — Qaz
– Qaz, Commented Sep 21, 2017 at 12:49
On the C# side, in your unsafe struct, use byte instead of char and convert to text using Encoding.Default.GetString. — David Heffernan
– David Heffernan, Commented Sep 21, 2017 at 13:01

montagneBleu · Accepted Answer · 2017-09-21 17:01:43Z

2

Ok i found it, i made that change in the structure declaration

   /*
      public char* pValue2;
      public fixed char Value3[1024];
      */
      [MarshalAs(UnmanagedType.LPStr)] public String pValue2;
      [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 1024)] public String pValue3;

And pull the data like that!

  // CHAR* - "This is a test #1";
  // value2 = new string(data.pValue2);
     value2 = data.pValue2

  // CHAR [1024] - "This is a test #2"
  //value3 = new string(data.Value3);
    value3 = data.pValue3;

answered Sep 21, 2017 at 17:01

montagneBleu

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