OOP Series - Part One - Building And Consuming A Class Library (DLL) Using C#

The first step in Object Oriented Programming is to learn how to create a DLL because most of the reusable components are written in the form of a DLL (Dynamic Link Library).

.NET provides you the option to create libraries (components) that are not “.exe” executables. Instead, the Class Library project final build output will be a “.dll” that can be referenced by other applications to expose its entire functionality.

Build a DLL

Step 1

First, create a project of type “Class Library” as shown here.

Step 2

 

Then we are implementing a Math class library that is responsible for calculating the sum of two numbers.

using System;

namespace LibraryUtil
{
    public class MathLib
    {
        public MathLib() { }

        public int Sum(int x, int y)
        {
            int z = x + y;

            return z;
        }
    }
}

Step 3

Build this code and you will see that a DLL file has been created rather than an exe in the root directory of the application (path = D:\temp\LibraryUtil\LibraryUtil\bin\Debug\ LibraryUtil.dll)

Consume the DLL

Step 1

 

Now create another console based application where we will utilize the class library functionality.

Step 2

 

Then you need to add the reference of the Math Class Library DLL file reference to access the declared class in the library DLL. (Right-click on the Reference then select Add reference then select the path of the DLL file.)

Step 3

 

When you add the class library reference then you will see in the Solution Explorer that a new LibraryUtil is added as in the following,

Step 4

 

Now add the namespace of the class library file in the console application and create the instance of the class declared in the library as follows,

using System;
using LibraryUtil; // add library namespace

namespace oops
{
    public class LibraryClass
    {
        static void Main()
        {
            //library class instance
            MathLib obj = new MathLib();

            //invoke Sum method
            Console.WriteLine(obj.Sum(12, 13));
        }
    }
}

Step 5

Finally, run the application and you must see the result as shown in the image below.

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