从DLL文件调用消息框功能时出现奇怪的字符？

Question

I see a problem when call function from dll file which compiled in C++, dll file with functions code as follow:

#include "stdafx.h"
#include <windows.h>
#include <stdio.h>
#include <string.h>

char* str;
int _stdcall inputt(char* str_)
{
    str=str_;
    return 1;
}

int _stdcall Shooow()
{
    MessageBoxA(NULL,str,"Message...",MB_OK);
    return 0;
}

In this, I export two function inputt() and Shooow(). Dll file named "TestCCCC.dll". Then I call them in C# code as follow:

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

namespace muWrapper
{
    public partial class WndSample : Form
    {
        [DllImport("TestCCCC.dll")]
        private static extern int inputt(string FuncName);
        [DllImport("TestCCCC.dll")]
        private static extern int Shooow();

      public WndSample()
      {
        InitializeComponent();
      }

      private void button1_Click(object sender, EventArgs e)
      {
          int ret = inputt("aaaaaa");
          ret = Shooow();
      }
    }
}

When I run it, click the button with the first time, it show a message box with strange characters, not "aaaaaa" !!!? Continue to click with the second time, it show truly with "aaaaaa", and continue...and show truly and truly....

Tell me what was happen in this problem? How to code two function inputt() and Shooow() to show truly at the first time? Thanks.

Answer 1

inputt is passed a pointer to a temporary string. You can't just save the pointer, you'll need to save a copy of the whole string. Easiest to do that with std::string

#include "stdafx.h"
#include <windows.h>
#include <string>

static std::string str;
int _stdcall inputt(const char* str_)
{
    str=str_;
    return 1;
}

int _stdcall Shooow()
{
    MessageBoxA(NULL,str.c_str(),"Message...",MB_OK);
    return 0;
}

C# natively supports Unicode strings. If you want to handle arbitrary strings, you will need to change the DLLImport line to:

    [DllImport("TestCCCC.dll", CharSet = Unicode)]
    private static extern int inputt(string FuncName);

and then change the C++ to:

#include "stdafx.h"
#include <windows.h>
#include <string>

static std::wstring str;  // wide string
int _stdcall inputt(const wchar_t* str_) // wide char pointer.
{
    str=str_;
    return 1;
}

int _stdcall Shooow()
{
    MessageBoxW(NULL,str.c_str(),L"Message...",MB_OK);  // Use W version and long title.
    return 0;
}