dll import (Unable to find an entry point named "methodname"

Question

I am trying to import a c++ dll into my C# windows project. But i am getting an error "Unable to find an entry point named 'Add' in DLL 'Test.dll'"

My c++ source code:

#pragma once
 
using namespace System;
 
namespace Test {
 
    public ref class Class1
    {
    public: int Add(int i, int j)
        {
            return(i + j);
        }
    };
}

My C# windows source code:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;
using Test;// my c++ dll refernce

 
namespace WindowsApplication3
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
 
        [DllImport("Test.dll")]
        public static extern int Add(int a, int b);
        private void button1_Click(object sender, EventArgs e)
        {
            int p = Add(1, 2);
        }
    }
}

I placed the c++ dll in c# application output path i e debug folder.

Please help to find me the error i made in the c++ or c# code.

Vulpes · Answer

As you've removed the 'ref' keyword from Class1, it's no longer a managed class but an unmanaged class. Consequently, the C# code can't see its members.

sarin s · Answer

Hello Vulpes, Thank you very much. Now i am able to get the methods with class instance. If i remove ref keyword then can i get Add method by DllImport attribute ?

Vulpes · Answer

It's technically possible to call the Add function using DllImport but it's not straightforward. The main problem is that it's not a 'stand-alone' function but a class member. You would therefore need to add two 'stand-alone' functions to the dll: 1 .To create an object of that class and return a pointer to where it's stored on the unmanaged heap; and 2. To destroy it when you're finished. The Add method would then need to be called using the ThisCall convention and passed the object pointer as the first argument. All 3 functions would also need to be specifically exported using __declspec (dllexport). Unfortunately, as it's a C++ dll, the exported names would be mangled so you'd need to look them up with a tool such as dumpbin or Dependency Walker before you could call them from C#. An alternative would be to use the ordinal numbers of the functions though you'd still need these tools to look them up. In practice, few developers can be bothered with all this and instead create a managed wrapper for the unmanaged class which can then be used directly from C# without the need for DllImport.

sarin s · Answer

Hello Vulpes, Thank for helping me. I tried the same procedure what ever you explained. My C++ code i rewrite it as #pragma once using namespace System; namespace Test { public class Class1 { public:int Add(int i, int j) { return(i + j); } }; } In C# application i gave reference to C++ dll and added namespace in the source code. When i instantiate Class1 cls = new Class1(); cls. ----> i am not getting Add() method

Vulpes · Answer

As it's a C++/CLI .NET dll, you don't need to use DllImport which is only for importing unmanaged functions. Instead, you need to add a reference to the dll to your C# project in the usual way, namely: right click on References in Solution Explorer, click on Add Reference from the dropdown menu, select the Browse tab, navigate to your dll and OK it. You can then do: namespace WindowsApplication3 { public partial class Form1 : Form { public Form1() { InitializeComponent(); } private void button1_Click(object sender, EventArgs e) { Class1 c1 = new Class1(); int p = c1.Add(1, 2); } } }