How can I add a SWIG-generated C++ DLL reference to a C# project?

Question

I am using SWIG to generate a DLL that will expose C++ functionality to a C# project. At the moment I:

1. Define a SWIG interface file

   %module example
   %{
   /* Includes the header in the wrapper code */
   #include "../pointmatcher/PointMatcher.h"
   %}
   
   ...
   
   %include "../pointmatcher/PointMatcher.h"
   

2. Use SWIG to generate a .cxx wrapper

   swig.exe -c++ -csharp -outdir csharp example.i
   

3. Compile the .cxx wrapper with MSBUILD via CMake

   # create wrapper DLL
   add_library(example SHARED ${WRAP_CSHARP_FILE})
   target_link_libraries(example pointmatcher)
   install(TARGETS example
           ARCHIVE DESTINATION ${INSTALL_LIB_DIR}
           LIBRARY DESTINATION ${INSTALL_LIB_DIR}
           RUNTIME DESTINATION ${INSTALL_BIN_DIR})
   

I then have a DLL file (example.dll) which I can inspect via Dependency Walker, and confirm that methods are being exposed as follows:

However, when I try to add this MSVC DLL as a reference to a C# project I get the error "It is not a valid assembly or COM component".

Based on answers at How can I add a VC++ DLL as a reference in my C# Visual Studio project? I have confirmed that SWIG itself generates P/Invoke calls, and that tlbimp doesn't recognise the DLL either.

Answer 1

You don't add the C++ dll to your project in the same way you would a C# dll. Instead it is called through the PInvoke system.

SWIG will generate some C# code for you, the easiest way to access the dll is to include those files in your porject, which expose the dll functionality through some C# functions which you can call.

You can also use the dll through PInvoke yourself. You need to create a C# function that will act as a wrapper:

C++ Header:

#ifndef TESTLIB_H
#define TESTLIB_H

extern "C" {
    int myfunc(int a);
}

#endif

C++ Code:

int myfunc(int a)
{
    return a+1;
}

C# Code:

using System;
using System.Runtime.InteropServices;

class Libtest
{
    [DllImport ("function")]
    private static extern int myfunc(int a);

    public static void Main()
    {
        int val = 1;
        Console.WriteLine(myfunc(val));
    }
}

Output:

2

Location of the DLL

The compiled C++ dll needs to either be copied into the C# project bin directory, or if the path is known it can be added to the DllImport call:

[DllImport("path/to/dll/function.dll")]

To achieve this with swig use the -dllimport flag:

swig -cpp -csharp ... -dllimport "/path/to/dll/function.dll" ...

If you want to set the path dynamically (to allow for loading 32-bit or 64-bit versions dynamically selected at runtime) you can use the kernel32 function SetDllDirectory which is also loaded using DllImport.