Question

I am trying to use DLLImport for using Win32 dll method in C#.

Win32 dll C++
// .h file

#ifdef IMPORTDLL_EXPORTS
#define IMPORTDLL_API __declspec(dllexport)
#else
#define IMPORTDLL_API __declspec(dllimport)
#endif

// This class is exported from the ImportDLL.dll
class IMPORTDLL_API CImportDLL {
public:
    CImportDLL(void);
    // TODO: add your methods here.
    int Add(int a , int b);
};

extern IMPORTDLL_API int nImportDLL;

IMPORTDLL_API int fnImportDLL(void);
IMPORTDLL_API int fnMultiply(int a,int b);

// .cpp file

// ImportDLL.cpp : Defines the exported functions for the DLL application.
//

#include "stdafx.h"
#include "ImportDLL.h"


// This is an example of an exported variable
IMPORTDLL_API int nImportDLL=0;

// This is an example of an exported function.
IMPORTDLL_API int fnImportDLL(void)
{
    return 42;
}

IMPORTDLL_API int fnMultiply(int a , int b)
{
    return (a*b);
}

Once i build this i get ImportDLL.dll

Now i create Windows Application and add this dll in debug folder and try to use this method using DLLImport

[DllImport("ImportDLL.dll")]
 public static extern int fnMultiply(int a, int b);

And I try to call this in C#
int a = fnMultiply(5, 6); // This line gives error Unable to find an entry point

Can any body tell what i am missing?
Thanks.

Answer 1

If you are exporting a C function from a native DLL, you may want to use the __stdcall calling convention (which is equivalent to WINAPI, i.e. the calling convention used by most Win32 API C-interface functions, and which is the default for .NET P/Invoke):

extern "C" MYDLL_API int __stdcall fnMultiply(int a, int b)
{
    return a*b;
}

// Note: update also the .h DLL public header file with __stdcall.

In addition, if you want to avoid name mangling, you may want to export using .DEF files.
e.g. Add a .DEF file to your native DLL project, and edit its content something like this:

LIBRARY MYDLL
EXPORTS
  fnMultiply @1
  ...

(You can use the command line tool DUMPBIN/EXPORTS, or a GUI tool like Dependency Walker, to check the actual name with which the function is exported from the DLL.)

Then you can use P/Invoke like this from C#:

[DllImport("MyDLL.dll")]
public static extern int fnMultiply(int a, int b);