Making C++ DLL for C#

Question

I have made a very simple Dll like this:

extern \"C\"
{
  __declspec(dllexport) int Try(int v)
  {
    return 10 + v;
  }
}

Then I want to use

Answer 1

The error message you've got contains a good advice indeed:

Check that the calling convention and parameters of the PInvoke signature match the target unmanaged signature.

You should have the same calling convention specified on both sides (C++ dll and C# assembly). In C++ you can specify it by prepending function declaration with one of __cdecl, __stdcall, etc.

extern "C"
{
  __declspec(dllexport) int __stdcall Try(int v)
  {
    return 10 + v;
  }
}

On the C# side you specify it with DllImport attribute, the default one is CallingConvention.StdCall which corresponds to __stdcall in C++, so, it looks like you have a __cdecl on the C++ side. To fix the issue either use __stdcall in your DLL as shown above, or use CDecl in C# like this:

class Program
{
    [DllImport("TestLib.dll", CallingConvention=CallingConvention.Cdecl)]
    public static extern int Try(int v);

    static void Main(string[] args)
    {
        Console.WriteLine("Wynik: " + Try(20));
        Console.ReadLine();
    }
}

Answer 2

The default calling convention in C and C++ is __cdecl; the default calling convention used by .NET P/Invoke is __stdcall -- you need to reconcile these two.

• Either make your native function __stdcall, as Hans suggested:

  __declspec(dllexport) int __stdcall Try(int v)
  

• Or make your managed P/Invoke signature use __cdecl:

  [DllImport("TestLib.dll", CallingConvention = CallingConvention.Cdecl)]
  public static extern int Try(int v);