Extension Methods in C#

What are Extension Methods in C#?

Extension Methods in C# allow you to add new methods to an existing class without modifying its source code. This is useful when working with libraries or sealed classes.

Key Features of Extension Methods

• Allows adding methods to existing classes without altering their definition.
• Defined as static methods in a static class.
• The first parameter must use this keyword followed by the type being extended.
• Can be used with built-in types (e.g., string, int, List).

Example: Creating and Using an Extension Method

The following example demonstrates how to define and use an extension method.

Example: Adding an Extension Method to String

using System;

public static class StringExtensions
{
    public static string ToProperCase(this string str)
    {
        if (string.IsNullOrEmpty(str)) return str;
        return char.ToUpper(str[0]) + str.Substring(1).ToLower();
    }
}

// Usage
class Program
{
    static void Main()
    {
        string message = "hello WORLD!";
        Console.WriteLine(message.ToProperCase()); // Output: Hello world!
    }
}
        

The ToProperCase method extends the string class, capitalizing the first letter and making the rest lowercase.

Using Extension Methods with Generic Types

Extension methods can work with generic types to add functionality to collections and other types.

Example: Extension Method for Generic List

using System;
using System.Collections.Generic;
using System.Linq;

public static class ListExtensions
{
    public static void PrintAll(this List list)
    {
        list.ForEach(item => Console.WriteLine(item));
    }
}

// Usage
class Program
{
    static void Main()
    {
        List numbers = new List { 1, 2, 3, 4, 5 };
        numbers.PrintAll();
    }
}

// Output:
// 1
// 2
// 3
// 4
// 5
        

Here, PrintAll is an extension method that prints each element in a List.

Built-in Extension Methods in C#

Many extension methods are built into the C# framework. Some common ones include:

When to Use Extension Methods?

• When adding new functionality to existing classes without modifying them.
• For enhancing built-in .NET types (e.g., string, List).
• When working with libraries where modifying the source code is not possible.
• For improving code readability and reusability.