Creating and Using DLL (Class Library) in C#

Introduction

 

A Dynamic Link library (DLL) is a library that contains functions and codes that can be used by more than one program at a time. Once we have created a DLL file, we can use it in many applications. The only thing we need to do is to add the reference/import the DLL File. Both DLL and .exe files are executable program modules but the difference is that we cannot execute DLL files directly.

 

Creating DLL File

 

Step 1 - Open Visual Studio then select "File" -> "New" -> "Project..." then select "Visual C#"  -> "Class library".

 

 

 

(I give it the name "Calculation".)

 

Step 2 - Change the class name ("class1.cs") to "calculate.cs".

 

 

 

Step 3 - In the calculate class, write methods for the addition and subtraction of two integers (for example purposes).

 

 

 

Step 4 - Build the solution (F6). If the build is successful then you will see a "calculation.dll" file in the "bin/debug" directory of your project.

 

 

 

We have created our DLL file. Now we will use it in another application.

 

Using DLL File

 

Step 1 - Open Visual Studio then select "File" -> "New" -> "Project..." then select "Visual C#"  -> "Windows Forms application".

 

Step 2 - Design the form as in the following image:

 

 

 

Step 3 - Add a reference for the dll file, "calculation.dll", that we created earlier. Right-click on the project and then click on "Add reference".

 

 

 

Step 4 - Select the DLL file and add it to the project.

 

 

 

After adding the file, you will see that the calculation namespace has been added (in references) as in the following:

 

 

 

Step 5 - Add the namespace ("using calculation;") as in the following:

 

 

 

Step 6

1. using System;  
2. using System.Collections.Generic;  
3. using System.ComponentModel;  
4. using System.Data;  
5. using System.Drawing;  
6. using System.Linq;  
7. using System.Text;  
8. using System.Windows.Forms;  
9. using Calculation;  
10.   
11. namespace MiniCalculator  
12. {  
13.     public partial class Form1 : Form  
14.     {  
15.    
16.         public Form1()  
17.         {  
18.             InitializeComponent();  
19.         }  
20.         calculate cal = new calculate();  
21.         //Addition Button click event   
22.         private void button1_Click(object sender, EventArgs e)  
23.         {  
24.             try  
25.             {  
26.                 //storing the result in int i   
27.                 int i = cal.Add(int.Parse(txtFirstNo.Text), int.Parse(txtSecNo.Text));  
28.                 txtResult.Text = i.ToString();  
29.             }  
30.   
31.             catch (Exception ex)  
32.             {  
33.                 MessageBox.Show(ex.Message);  
34.             }  
35.         }  
36.    
37.         //Subtraction button click event  
38.   
39.         private void button2_Click(object sender, EventArgs e)  
40.         {  
41.             Try  
42.             {  
43.                 //storing the result in int i   
44.                 int i = cal.Sub(int.Parse(txtFirstNo.Text), int.Parse(txtSecNo.Text));  
45.                 txtResult.Text = i.ToString();  
46.             }  
47.             catch (Exception ex)  
48.             {  
49.                 MessageBox.Show(ex.Message);  
50.             }  
51.         }  
52.     }  
53. }