将C ++ DLL包装一秒以加载到C＃中

Question

I'm trying to expose a function from a DLL (we'll call foo.dll) to use in C#. However, I don't have access to the source so I thought I might be able to write a second DLL (bar.dll) which includes the first, reimplements the functions by returning the results from foo.dll. I do have the .lib and .h files so I'm hoping I can write a little tool to walk through the headers and generate my 'wrapper' dlls (is that the right term?)

I've set up a very simple test for this, implementing foo.dll myself. I'm very new to C++ so please be gentle.

foo.h

#pragma once

double Add(double a, double b);

foo.cpp

#include <stdio.h>
#include "foo.h"

double Add(double a, double b)
{
    return a + b;
}

I build this and reference the .lib into my project for bar, along with copying in foo.h.

bar.h

#pragma once

extern "C"
{
    namespace foo {
        #include "foo.h"
    }

    __declspec(dllexport) double Add(double a, double b);
}

bar.cpp

#include <stdio.h>
#include "bar.h"

__declspec(dllexport) double Add(double a, double b)
{
    return foo::Add(a, b);
}

So far so simple. I'm then trying to use DllImport to bring this into C# thusly:

using System;
using System.Runtime.InteropServices;

namespace ConsoleApplication1
{
    class Program
    {
        [DllImport("bar.dll", CallingConvention = CallingConvention.StdCall)]
        private static extern double Add(double a, double b);

        static void Main(string[] args)
        {
            Console.WriteLine("PRE");
            Console.WriteLine(Add(1.5, 8.5));
            Console.WriteLine("POST");
            Console.ReadKey();
        }
    }
}

However, when I run this I just get "PRE" output and the program just waits. I've troubleshooted through error messages up to this stage, but I'm not really sure how to work out what's happening in this situation.

If I reimplement bar.cpp to return a + b the C# application runs as expected, outputting: PRE 10.0 POST

I've had a read through a good many posts on the subject, some suggest adding __stdcall before the name when declaring my variable, but that gives me a StackOverflow error.

It's my first SO post (though of course I've been lurking for years) so feel free to let me know if I can do anything to improve the question.

Answer 1

Since you're creating bar.dll anyway , use C++/CLI to wrap foo.dll ; if your P/Invokes become non-trivial (as is likely dealing with legacy C++ code), you'll be glad you went in this direction.

namespace BarAssembly
{
   public ref struct Bar abstract sealed // "abstract sealed" == C#'s "static"
   {
      static double Add(double a, double b) {
         return foo::Add(a, b);
      }
   }
}

Your C# code is now simply

    static void Main(string[] args)
    {
        Console.WriteLine("PRE");
        Console.WriteLine(BarAssembly.Bar.Add(1.5, 8.5));
        Console.WriteLine("POST");
        Console.ReadKey();
    }

No need for [DllImport] although you will have to add a reference to the bar.dll assembly.

Answer 2

It could be the calling convention, but newer versions of .NET are actually pretty good about throwing an exception for most CC violations. I'd recommend sticking some printf's in your C++ wrapper to see if you can figure out where it hangs.

Option B - there's actually a wrapper generator called SWIG which does exactly what you're doing for dozens of different languages. It works quite well.