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?

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