困惑的DLL入口点（入口点未找到异常）

Question

I'm trying to learn how to use DLL's in C#. I have a very simple DLL just to test the basics.

// MainForm.cs


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 DLL_Test
{
        public partial class Form1 : Form
        {
            [DllImport("TestDLL.dll",
                        EntryPoint="?Add@@YGHHH@Z",
                        ExactSpelling = true,
                        CallingConvention = CallingConvention.StdCall)]
            public static extern int Add(int a, int b);

            public Form1()
            {
                InitializeComponent();
            }

            private void button1_Click(object sender, EventArgs e)
            {
                int num;
                try
                {
                    num = Add(2, 3);
                    richTextBox1.AppendText(num.ToString() + "\n");
                }
                catch (DllNotFoundException ex)
                {
                    MessageBox.Show(ex.ToString());
                }
                catch (EntryPointNotFoundException ex)
                {
                    MessageBox.Show(ex.ToString());
                }
             }
         }
}

And the DLL code:

// TestDLL.cpp

__declspec(dllexport) int __stdcall Add(int a, int b) {
return(a + b);
}

dumpbin returns the following:

ordinal   hint   RVA        name
      1      0   00011005   ?Add@@YGHHH@Z = @ILT+0(?Add@@YGHHH@Z)

This (and other attempts listed below) have all returned the same exception:

System.EntryPointException: Unable to find entry point named "..."

So I am at a loss for how to solve this. Perhaps I do not understand how DllMain functions as the C# entry point for a DLL. TestDLL.dll works when I test it in a C++ application.

After searching for help, I've attempted the following changes:

// TestDLL.cpp

extern "C" __declspec(dllexport) int __stdcall Add(int a, int b) {
return(a + b);
}

Which results in this from dumpbin

ordinal   hint   RVA        name
      1      0   00011005   _Add@8 = @ILT+135(_Add@8)

Thus, I changed my C# code:

 // MainForm.cs

...

[DllImport("TestDLL.dll",
                        EntryPoint="_Add",
                        ExactSpelling = true,
                        CallingConvention = CallingConvention.StdCall)]
            public static extern int Add(int a, int b);

...

I've also tried __cdecl :

// TestDLL.cpp

extern "C" __declspec(dllexport) int __cdecl Add(int a, int b) {
return(a + b);
}

.

// MainForm.cs

...

[DllImport("TestDLL.dll",
                        EntryPoint="_Add",
                        ExactSpelling = true,
                        CallingConvention = CallingConvention.Cdecl)]
            public static extern int Add(int a, int b);

...

Perhaps I'm misunderstanding the calling conventions. Any help would be very appreciated. Thank you.

Answer 1

use

extern "C" __declspec(dllexport) int __stdcall Add(int a, int b) { ... }

and

[DllImport("TestDLL.dll", CallingConvention = CallingConvention.Stdcall)] 
public static extern int Add(int a, int b); 

extern "C" will prevent name mangling with params and return type such as ?Add@@YGHHH@Z . __stdcall will prepend an _ and add @8 : _Add@8 (where 8 is the total size of arguments). Note that it also affects the way parameters are pushed on the stack.

In your DLLImport statement, since you specify CallingConvention.StdCall , you don't need to specify the name mangling. Just give the regular name ( Add ) and .NET will take care of name mangling ( _Add@8 ).

Note that you must specify the CallingConvention or .NET wouldn't emit the correct code to push arguments on the stack

Answer 2

For future reference: I had a similar problem, solved creating an EMPTY C++ dll project. Probably the standard Visual Studio template causes some trouble.

Refer to this link: http://www.codeproject.com/Articles/9826/How-to-create-a-DLL-library-in-C-and-then-use-it-w

Answer 3

The following should work.

Unmanged:

extern "C" __declspec(dllexport) int Add(int a, int b) 
{
    return(a + b);
}

Managed:

class Program
{
    [DllImport("TestDLL.dll")]
    public static extern int Add(int a, int b);

    static void Main()
    {
        Console.WriteLine(Add(1, 2));
    }
}