Extension Methods in C#

Mishra Sourabh

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2 Jun 2014CPOL2 min read

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This article explains Extension methods.

Introduction

Extension methods are static methods that are always implemented in a static class. They don’t need a class object, they can be directly called. In C#, we already have some built-in extension methods like Union, Where and Zip. These extension methods need a System.Linq namespace declaration.

Background

In C#, extension methods can be identified by this logo. Extension methods can be defined in C# in a separate .cs file. The extension method is called like an instance method, but is actually a static method. The instance pointer "this" is a parameter and you must specify the "this" keyword before the appropriate parameter you want the method to be called upon. The difference between a regular static method and an extension method is that, in a static method we first provide the class name and then we call the method. In the extension method, the "this" keyword is in the parameter list. You can directly call this by a variable.

Using the Code

So let’s create an extension method.

Open Visual Studio
"File" ">>" New >> "Web Site..."
Select "ASP.NET Empty Web Site", then click "Ok"
Add a new WebForm
Add a new Class

ExtentionMethods.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
public static class ExtentionMethods
{
    public static string UpperCaseFirstLetter(this string value)
    {
        if (!string.IsNullOrEmpty(value))
        {
            string[] words = value.Split(' ');
            value = string.Empty;
            foreach (string word in words)
            {
                char[] getWord = word.ToCharArray();
                getWord[0] = char.ToUpper(getWord[0]);
                string getValue = new string(getWord);
                if (string.IsNullOrEmpty(value))
                {
                    value = getValue;
                }
                else
                {
                    value = value + ' ' + getValue;
                } 
            } 
        }
        return value; 
    }
}

In the code above, you can see we have a public static class, ExtentionMethods. In this class, there is a method "public static string UpperCaseFirstLetter(this string value)" in which the value is obtained and then this function converts the first letter to uppercase.

Call Extension Method in C# Program

Default.aspx

<%@ Page Language="C#" AutoEventWireup="true" 
CodeFile="Default.aspx.cs" Inherits="_Default" %>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
    Name <asp:TextBox ID="txtName" runat="server"></asp:TextBox>
 
    </div>
        <div>
            <asp:Button ID="Button1" runat="server" 
            Text="Call Extention Method" OnClick="Button1_Click" />
        </div>
    </form>
</body>
</html>

Default.aspx.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
 
public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        if (!string.IsNullOrEmpty(txtName.Text))
        {
            string name = txtName.Text;
            txtName.Text = name.UpperCaseFirstLetter();
        }
    }
}

Look at the code above, we have a textbox, txtName, and a Button. When the user enters text into the TextBox and clicks on the button, it executes the click event code and we have a name variable, that is a string type.
Look at this line:

txtName.Text = name.UpperCaseFirstLetter();

Here, we call our UpperCaseFirstLetter extension method in the name variable. You remember our method parameter was defined like this, the string value. That’s why when you use a name variable, it comes directly into the Intellisense.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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