Decorations on C++ DLL function declarations really needed?

Just a small common mistake . You are using long in C/C++ and long in VB. The problem is that long in C/C++ is 32 bit and long in VB (and other .NET languages like C#) is 64 bit.

Depending on your need you must do one of the following:

1) use Integer in VB if you need 32 bit values

2) use long long instead of long in C/C++ if you need 64 bit values

Your code does not compile.

geob.cpp

First

long _stdcall CGeob::summer(long num1, long num2) //Its a CGeob class member.
{
long sum;
printf("got there");
sum = num1 + num2;
return sum;
}

Second,

// This class is exported from the geob.dll
class GEOB_API CGeob {
public:
CGeob(void);
long sum;
long num1;
long num2;
//CGeob(void);

long _stdcall summer(long num1, long num2); //this member cannot be accessed from VB

// TODO: add your methods here.
};

GEOB_API long _stdcall summer(long, long); //Only this can be accessed.

Private Declare Function summer Lib "geob.dll" (ByVal num1 As Integer, ByVal num2 As Integer) As Integer

This is mine diffenition of summer function in VB.

So mine code is

Module Module1
Private Declare Function summer Lib "geob.dll" (ByVal num1 As Integer, ByVal num2 As Integer) As Integer

Sub Main()
Dim s As Integer
s = summer(1, 2)
Console.Write(": ")
Console.WriteLine(s)
Console.ReadLine()
End Sub

End Module

geob.h

// The following ifdef block is the standard way of creating macros which make exporting
// from a DLL simpler. All files within this DLL are compiled with the GEOB_EXPORTS
// symbol defined on the command line. this symbol should not be defined on any project
// that uses this DLL. This way any other project whose source files include this file see
// GEOB_API functions as being imported from a DLL, wheras this DLL sees symbols
// defined with this macro as being exported.
#ifdef GEOB_EXPORTS
#define GEOB_API __declspec(dllexport)
#else
#define GEOB_API __declspec(dllimport)
#endif

// This class is exported from the geob.dll
class GEOB_API CGeob {
public:
CGeob(void);
long sum;
long num1;
long num2;
//CGeob(void);

long _stdcall summer(long num1, long num2);

// TODO: add your methods here.
};

extern GEOB_API int nGeob;
extern GEOB_API long sum;
extern GEOB_API long num1;
extern GEOB_API long num2;

GEOB_API int fnGeob(void);

extern "C" GEOB_API long _stdcall summer(long, long);

geob.cpp

// geob.cpp : Defines the entry point for the DLL application.
//

#include "stdafx.h"
#include "stdio.h"
#include "geob.h"

BOOL APIENTRY _stdcall DllMain( HANDLE hModule,
DWORD ul_reason_for_call,
LPVOID lpReserved
)
{
switch (ul_reason_for_call)
{
case DLL_PROCESS_ATTACH:
case DLL_THREAD_ATTACH:
case DLL_THREAD_DETACH:
case DLL_PROCESS_DETACH:
break;
}
return TRUE;
}

// This is an example of an exported variable
GEOB_API int nGeob=0;
GEOB_API long sum;
GEOB_API long num1;
GEOB_API long num2;
// This is an example of an exported function.
GEOB_API int fnGeob(void)
{
return 42;
}

// This is the constructor of a class that has been exported.
// see geob.h for the class definition
CGeob::CGeob()
{
return;
}

long _stdcall /*CGeob::*/summer(long num1, long num2)
{
long sum;
printf("got there");
sum = num1 + num2;
return sum;
}

geob.def - requried, otherwise entry point not found

LIBRARY "geob"
EXPORTS
summer

Output:

got there: 3

Can you send:

IDE version,

error in VB.

Did you add a .def file

Show me options of C++ - compiler and C++ - linker command line.

Did you add a .def file to your C++ project

How do you compile your C++ project

Unfortuntly I cannot check it under XL2000,

But my code works under XL2003.

Private Declare Function summer Lib "c:\Documents and Settings\buz.HQ\Desktop\geob.dll" (ByVal num1 As Integer, ByVal num2 As Integer) As Integer

Sub Main()
    Dim s As Integer
    s = summer(1, 2)
    ActiveSheet.Range("A1").Value = s
End Sub

And cell A1 containes 3.