在C＃中绑定C ++ DLL时的EntryPointNotFoundException

Question

I try to bind a simple c++ dll shown in http://msdn.microsoft.com/en-us/library/ms235636.aspx in my c# console app, but I get a EntryPointNotFoundException for Add within dll at runtime. My test class is

namespace BindingCppDllExample
{
    public class BindingDllClass
    {
        [DllImport("MathFuncsDll.dll")]
        public static extern double Add(double a, double b);
    }

    public class Program
    {
        public static void Main(string[] args)
        {
            double a = 2.3;
            double b = 3.8;
            double c = BindingDllClass.Add(a, b);

            Console.WriteLine(string.Format("{0} + {1} = {2}", a, b, c));
        }
    }
}

What is not correct?

Answer 1

You could try declaring the functions outside of a class and also exporting them with extern "C" :

Header:

// MathFuncsDll.h
namespace MathFuncs
{
    // Returns a + b
    extern "C" __declspec(dllexport) double Add(double a, double b);

    // Returns a - b
    extern "C" __declspec(dllexport) double Subtract(double a, double b);

    // Returns a * b
    extern "C" __declspec(dllexport) double Multiply(double a, double b);

    // Returns a / b
    // Throws DivideByZeroException if b is 0
    extern "C" __declspec(dllexport) double Divide(double a, double b);
}

Implementation:

// MyMathFuncs.cpp
#include "MathFuncsDll.h"
#include <stdexcept>

using namespace std;

namespace MathFuncs
{
    double Add(double a, double b)
    {
        return a + b;
    }

    double Subtract(double a, double b)
    {
        return a - b;
    }

    double Multiply(double a, double b)
    {
        return a * b;
    }

    double Divide(double a, double b)
    {
        if (b == 0)
        {
            throw new invalid_argument("b cannot be zero!");
        }

        return a / b;
    }
}

Calling code:

namespace BindingCppDllExample
{
    public class BindingDllClass
    {
        [DllImport("MathFuncsDll.dll")]
        public static extern double Add(double a, double b);
    }

    public class Program
    {
        public static void Main(string[] args)
        {
            double a = 2.3;
            double b = 3.8;
            double c = BindingDllClass.Add(a, b);

            Console.WriteLine(string.Format("{0} + {1} = {2}", a, b, c));
        }
    }
}

Answer 2

In cases such as this, you can download Dependency Walker , load your DLL into it and look at the Export Functions list. You can also use DumpBin for this.

By default, function exported from a C++ or C DLL are using Name Decoration (also called Name Mangling ).

As said on MSDN :

A decorated name for a C++ function contains the following information:

• The function name.
• The class that the function is a member of, if it is a member function. This may include the class that encloses the function's class, and so on.
• The namespace the function belongs to (if it is part of a namespace).
• The types of the function's parameters.
• The calling convention.
• The return type of the function.

So decorated name for your Add function, for example, will look like Add@MyMathFuncs@MathFuncs@@SANNN@Z .

But it is possible to force the C++ compiler to expose undecorated names for C++ functions by enclosing the function, and any function prototypes, within an extern "C" {…} block, as Darin Dimitrov suggested.

Although if you're going to use a third-party DLL (so you can't modify it) or you just don't want to expose decorated names for some reason, you can specify the function's name explicitly:

[DllImport("MathFuncsDll.dll", EntryPoint = "Add@MyMathFuncs@MathFuncs@@SANNN@Z")]
public static extern double Add(double a, double b);