C# unmanaged DLL Export / Import in C++

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I have following Code in C#: (I used R.Giesecke Dllexport Template to create a unmanaged Dll with C#... it creates me a ".lib" and ".dll" file with (managed) C# )

C#

//UnmanagedExports.cs
using System;
using System.Collections.Generic;
using System.Text;
using RGiesecke.DllExport;

namespace AddDll
{
   class MyAddDll
   {
      [DllExport("Add", CallingConvention = System.Runtime.InteropServices.CallingConvention.StdCall)]
      public static double Add(double a, double b)
      {
         return a + b;
      }
   }
}

The C# Dll Code runs well without errors and creats me a AddDll.lib and AddDll.dll file.

I would like to import this Dll in C++:

C++

//CallAdd.h

#pragma once

#define DllImport __declspec(dllimport) 

namespace AddDll
{
    class MyAddDll
    {
    public:
        static DllImport double Add(double a, double b);
    };
}

C++

// CallAdd.cpp

#include "stdafx.h"
#include"CallAdd.h"
#include <iostream>



using namespace std;

int main()
{
    double a = 4.5;
    double b = 5.5;

    cout << "a + b = " << AddDll::MyAddDll::Add(a, b) << endl;

    system("pause");

    return 0;
}

I've got following errors:

Warning 1 Platform is AnyCpu, creating binaries for each CPU platform in a separate subfolder...<br />
<br />
    Error 2 error LNK2019: unresolved external symbol "__declspec(dllimport) public: static double __cdecl AddDll::MyAddDll::Add(double,double)" (__imp_?Add@MyAddDll@AddDll@@SANNN@Z) referenced in function _main<br />
<br />
    Error 3 error LNK1120: 1 unresolved externals

In Visual Studio I did following under my C++ Console Application Properties:

- In Common Properties ‐> Framework and References. Add AddDll Project reference.
- In Configuration Properties ‐> Linker ‐> Input. Additional Dependencies AddDll.lib
- In Configuration Properties ‐> Linker ‐> General. Additional Library Directories, add
directory where AddDll.lib and AddDll.dll are located

Can anyone help me to eliminate the errors?

Thx in advance!!

Posted 1-Jul-14 1:16am

Member 10854414

Updated 19-Jun-22 4:21am

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Daniele Rota Nodari 23-Jul-14 2:44am

Hi there.

Have you resolved this issue?

6 solutions

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Martin Fisher2 · Answer 1 · 2022-06-20T00:46:00

Solution 10

There must be a match between the configuration of the managed code and the DLL, so you should create different versions of the DLL for Release/x86 (32 bit), Debug/x86, Release/x86 and Release/x64

Posted 20-Jun-22 0:46am

Martin Fisher2

Updated 20-Jun-22 1:34am

Shawn-USA · Answer 2 · 2016-09-27T08:02:00

Hello,

This question is 2 years old, but the answer is never old. I am answering the question with absolute correct answer. Other developers may benefit from my answer. It was posted three time because the web site stated it failed to post 2 times by mistake.

The problem relies on how you import the function in your native C++ code. You can not do the way you are doing now. If you use the following C++ code to import your function, the the name of the export function must be mangled, you must come up with exact mangled name as Visual C++ does. I have removed my article link, the article is well-written though.

C++

//CallAdd.h

#pragma once

#define DllImport __declspec(dllimport) 

namespace AddDll
{
    class MyAddDll
    {
    public:
        static DllImport double Add(double a, double b);
    };
}

Insead of using namespace and class, you should simply use extern "C" statement like the one shown below.

C++

extern "C"
{
    double Add(double a, double b);
}

Since the default calling convention in C++ is __cdecl, you will also need to change the calling convention in the C# code, remember that name of a __stdcall of a export function needs to be mangled as well, in this case, it is Add@16, you don't want to do mangled name, so, please just use __cdecl, it is much simpler.

C#

//UnmanagedExports.cs
using System;
using System.Collections.Generic;
using System.Text;
using RGiesecke.DllExport;
 
namespace AddDll
{
   class MyAddDll
   {
      [DllExport("Add", CallingConvention = System.Runtime.InteropServices.CallingConvention.Cdecl)]
      public static double Add(double a, double b)
      {
         return a + b;
      }
   }
}

Daniele Rota Nodari · Answer 3 · 2014-07-16T20:53:00

Hi.

The error you reported states the linker is looking for the function export with __cdecl calling convention but you exported from C# as StdCall.

Have you tried either changing the CallingConvention within your C# code or adding __stdcall keyword within the C import instruction?

Please see CallingConvention Enumeration[^], Linker Tools Error LNK2019[^] and __stdcall[^].

Regards,
Daniele.

David Knechtges · Answer 4 · 2014-07-01T05:48:00

Solution 3

You could also just compile your C# code as normal (.NET) code, and compile your C++ code as C++/CLI. That can use both .NET and unmanaged code in the same files. If you need separation between .NET and the unmanaged C++ code, you could also write a wrapper DLL in C++/CLI to interface between the 2.

Posted 1-Jul-14 5:48am

David Knechtges

Comments

Member 10854414 1-Jul-14 12:39pm

Hey David,
thx for your answer. The problem is, I cannot find an existing wrapper for arrays. I need a template to do this...

Praveen Kumar Katiyar · Answer 5 · 2014-07-01T02:59:00

Solution 2

have you checked this

Consume Managed C# DLL from Unmanaged C++ EXE - Cannot start the CLR host[^]

-praveen

Posted 1-Jul-14 2:59am

Praveen Kumar Katiyar

Comments

Member 10854414 1-Jul-14 12:39pm

Hey Pravveen,
the warning is away.

KarstenK · Answer 6 · 2014-07-01T02:48:00

Solution 1

you need to build the C#-dll for the same CPU as your C++ project. :-O

Posted 1-Jul-14 2:48am

KarstenK

Comments

Member 10854414 1-Jul-14 9:07am

Thx! The warning is away. But the errors are still there :(

C# unmanaged DLL Export / Import in C++

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