About DllImport in C#

Question

Possible Duplicate:
Visual C++ managed app: Unable to find an entry point named ‘Add’

I'm learning to use platform invoke in C#.I follow the tutorial on msdn:

It works perfectly with C++ to use the MathFuncsDll.dll.

When I use:

[DllImport("MathFuncsDll.dll")]

in my C# code, a dll not found exception raised. Then I change it to:

[DllImport("C:\\...\\MathFuncsDll.dll")]

then a entry point not found exception appears.

How can I fix these problems?

For clarification, here is my code: The C++ dll:

header file:

//MathFuncsDll.h

#ifdef MATHFUNCSDLL_EXPORTS
#define MATHFUNCSDLL_API __declspec(dllexport) 
#else
#define MATHFUNCSDLL_API __declspec(dllimport) 
#endif

namespace MathFuncs
{
    class MyMathFuncs
    {
    public:
    // Returns a + b
    static __declspec(dllexport) double Add(double a, double b);

    // Returns a - b
    static __declspec(dllexport) double Subtract(double a, double b);

    // Returns a * b
    static __declspec(dllexport) double Multiply(double a, double b);

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

The .cpp file:

// MathFuncsDll.cpp
// compile with: /EHsc /LD

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

using namespace std;

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

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

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

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

here is the C# app call the functions:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Runtime.InteropServices;

namespace CSharpConsole
{
    class Program
    {
        //[DllImport("MathDll.dll")]

        [DllImport(@"C:\Users\...\Debug\MathDll.dll")]
        public static extern double Add(double a, double b);
        static void Main(string[] args)
        {
            double a = 6.2;
            int b = 3;

            Console.WriteLine(Add(a,b));
        }
    }
}

I create a completely new solution for this, the first DllIport line still doesn't work.And the second line give entry point exception.

You can also take a look at other questions that may contain information about what you're looking for: http://stackoverflow.com/questions/7276389/confused-over-dll-entry-points-entry-point-not-found-exception — IvanL, Nov 21 '12 at 11:10
@weismat You'll be able to call static C++ methods using p/invoke. There'll be mangling. — David Heffernan, Nov 21 '12 at 11:33

David Heffernan · Accepted Answer · 2012-11-21T14:34:25.240

2

When you use

[DllImport("MathFuncsDll.dll")]

the system will look for the DLL in the DLL search path. The first place searched is the directory which contains your executable file. That's the best place to put your DLL.

If you see "entry point not found" then that means simply that the entry point was not found.

The article you link to does not use p/invoke to link to the C++ DLL. That article describes how to link to a native DLL from managed C++. So you appear to be doing something different from that since you are using C# and p/invoke. Unless you supply precise details of what you are doing, for example the code, it's hard to prescribe a remedy.

OK, now you have added some code. That helps a lot.

You should not use static methods for p/invoke exports. Remove the MyMathFuncs and have plain old C-style functions:

extern "C" 
{

    double __declspec(dllexport) Add(double a, double b)
    {
         return a + b;
    }
}

No need for a header file that declares those functions. Just a single .cpp file should get it done. The use of extern "C" stops your names from being mangled by the C++ compiler.

If you need to inspect your DLL to see what it exports, use Dependency Walker.

On the C# side use:

[DllImport(@"...", CallingConvention=CallingConvention.Cdecl)]
public static extern double Add(double a, double b);

edited Nov 21 '12 at 14:34

answered Nov 21 '12 at 11:09

David Heffernan

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I have a project written C++ and I need to "integrate" that to a C# project.I search in google and found that the C++ project should be compile into .dll and call its functions in C#.I did exactly like the article, then created a C# project call those functions (in different solution) and add the dll project into that solution. – Husky Nov 21 '12 at 13:03
OK, but I don't see how that relates to the question. – David Heffernan Nov 21 '12 at 13:04
I just start learning C# so I decided to begin with a small project first (like the article) before moving into the bigger one.Now i'm stuck in that small project :( – Husky Nov 21 '12 at 13:24
I use the info in that article to create a C++ dll, and use pInvoke to call functions from that dll.But I still fail to do it. – Husky Nov 21 '12 at 14:11
Thanks a lot, it works perfectly now :).But the "big" project I mentioned above need to run on different computer, is there any way to import the dll file without using the static directory ? I'm a newbie, how to accept an answer ? – Husky Nov 21 '12 at 14:50
Put the DLL in the same directory as the exe. I say that in first paragraph. – David Heffernan Nov 21 '12 at 15:29

score 0 · Answer 2 · edited May 23 '17 at 12:10

0

Are you absolutely sure that the specified .dll exists in the same folder as your running executable? Because that's the first place that .NET will search for your .dll. If if doesn't find it there, it will search the GAC. But I doubt you have placed it there.

Also it might be a permissions issue (though quite unlikely), see this question for more information: DllImport generates System.DllNotFoundException

edited May 23 '17 at 12:10

Community

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1

answered Nov 21 '12 at 11:12

pyrocumulus

9,072
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I do exactly like what the article said; then I wrote a new project in C# just call the functions in MathFuncs.However, that project is in a different solutions,maybe that's why the dll is not found. – Husky Nov 21 '12 at 12:53

About DllImport in C#

2 Answers2