Unable to find an entry point when calling C++ dll in C#

Question

I am trying to learn P/Invoke, so I created a simple dll in C++

KingFucs.h:

namespace KingFuncs
{
    class KingFuncs
    {
    public:
        static __declspec(dllexport) int GiveMeNumber(int i);
    };
}

KingFuns.cpp:

#include "KingFuncs.h"
#include <stdexcept>

using namespace std;

namespace KingFuncs
{
    int KingFuncs::GiveMeNumber(int i)
    {
        return i;
    }
}

So it does compile, then I copied this dll into my WPF's debug folder, with code:

[DllImport("KingFuncDll.dll", EntryPoint = "GiveMeNumber", SetLastError = true, CharSet = CharSet.Ansi, ExactSpelling = true, CallingConvention = CallingConvention.StdCall)]
        public static extern int GiveMeNumber(
              int i
              );

And calling it in button click:

private void Button_Click(object sender, RoutedEventArgs e)
{
    int num = GiveMeNumber(123);
}

But it gives me exception:

Unable to find an entry point named 'GiveMeNumber' in DLL 'KingFuncDll.dll'.

Really.... what have I done wrong... It obviously able to find the DLL, otherwise would be another exception. But my method name is exactly the same.... I can't think of other reason.

Solution

You need to use extern "C" when you export your function so that you suppress C++ name mangling. And you also should not try to p/invoke to members of a class. Use free functions instead:

extern "C" {
    __declspec(dllexport) int GiveMeNumber(int i)
    {
        return i;
    }
}

On the managed side your DllImport attribute is all wrong. Don't use SetLastError which is for Win32 APIs only. Don't bother setting CharSet if there are not text parameters. No need for ExactSpelling. And the calling convention is presumably Cdecl.

[DllImport("KingFuncDll.dll", CallingConvention=CallingConvention.Cdecl)]
public static extern int GiveMeNumber(int i);

Comments

[King Chan]

Thanks, it worked. But it is not possible to call a function of a class? (This is my first time using extern "C", I thought I always need to define function within class in C++)

[David Heffernan]

You do not need to define a function within a class in C++. Old fashioned C functions are just fine.