How to create a DLL library in C and then use it with C#

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Introduction

After spending some time trying to implement this simple task, I started to search similar code examples over the Internet. I was really very surprised when found that all examples were slightly different from what I needed. Finally, I realized that there is no (at least spending 30 min in the net) easy-to-use example, that�s why I decided to write this article.

Assuming that you already know what a DLL is, let's begin with creating a simple one.

1. Start Visual Studio .NET.
2. Go to File->New->Project.
3. Select Visual C++ Project, and from the �Templates�, select �Win32 Project�.
4. Give the name to your project. This will be the name of your final DLL (in my case: TestLib).
5. Press OK.
6. Select DLL from �Application Type� (�Application Settings� tab).
7. Check �Empty Project� (we need to create our project from scratch, right?), and press Finish.

OK, now we should attach an empty source file to our blank project.

1. Start Solution Explorer (if it�s not displayed).
2. Right click to the �Source Files�, Add->Add New Item then select �C++ File� and give the name to it.
3. Press �Open�.

In the opened window, enter the following code:

#include <stdio.h>


extern "C"
{
  __declspec(dllexport) void DisplayHelloFromDLL()
  {
    printf ("Hello from DLL !\n");
  }
}

Please note that __declspec(dllexport) is an obligatory prefix which makes DLL functions available from an external application.

extern �C� (with brackets) is also very important, it shows that all code within brackets is available from �outside�. Although code will compile even without this statement, during runtime, you�ll get a very unpleasant error. So, do not forget to include it.

Build this application and your DLL will be ready-to-use.

Now it�s time to create an application which will use our DLL, as the main title explains. We will create this type of application using Microsoft�s C#.

Creating a simple C# application:

1. Start Visual Studio .NET.
2. Go to File->New->Project.
3. Select Visual C# Project. From the �Templates�, you can either select �Console Application�, or an �Empty Project� just like it was described above.
4. Give the name to your application.
5. Press OK.

Into the specified class, insert the following two lines:

[DllImport("TestLib.dll")]

public static extern void DisplayHelloFromDLL ();

In C#, keyword extern indicates that the method is implemented externally.

Your code should look something like this:

using System;
using System.Runtime.InteropServices;     // DLL support


class HelloWorld
{
    [DllImport("TestLib.dll")]
    public static extern void DisplayHelloFromDLL ();

    static void Main ()
    {
        Console.WriteLine ("This is C# program");
        DisplayHelloFromDLL ();
    }
}

Please, note that System.Runtime.InteropServices is required for operations with the DLL.

According to MSDN:

�The System.Runtime.InteropServices namespace provides a collection of classes useful for accessing COM objects, and native APIs from .NET�

OK, now build this application, and then copy the previously built DLL into the Debug/Release directory of the current application. The DLL should be in the same directory as your main application.

Now start an application. If everything you�ve made was correct, you should get something like this:

That�s all, really simple isn�t it?