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1 Answers

ICSM Computer
ICSM Computer 12 Jun 2025
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Extension methods allow you to "add" methods to existing types (classes, structs, interfaces) without modifying their source code or creating a derived type.

They are a type of syntactic sugar in C# that enables more natural, readable code.

Key Features

  • Declared as static methods in a static class
  • First parameter is prefixed with the this keyword
  • Can be called using instance method syntax

Example: Creating an Extension Method

public static class StringExtensions
{
    public static bool IsNullOrEmpty(this string input)
    {
        return string.IsNullOrEmpty(input);
    }
}

Now you can use it like this:

string name = "";
bool result = name.IsNullOrEmpty();  // Looks like a real method of string!

How It Works Behind the Scenes

The compiler converts this:

name.IsNullOrEmpty();

Into this:

StringExtensions.IsNullOrEmpty(name);

So the method is still static, just looks like an instance method.

Practical Example: List Extensions

public static class ListExtensions
{
    public static void PrintAll<T>(this List<T> list)
    {
        foreach (var item in list)
            Console.WriteLine(item);
    }
}

Usage:

var nums = new List<int> { 1, 2, 3 };
nums.PrintAll();  // Extension method used like a built-in method

When to Use Extension Methods

Use Case Use Extension Method?
Want to add methods to built-in types Yes
Cannot modify source of the type Yes
Need to enhance readability or reuse Yes
Need polymorphism or override behavior No (use inheritance instead)

Limitations

  • Cannot override existing methods
  • Cannot access private members of the target type
  • Only resolved at compile time, no virtual dispatch

Common Uses

  • LINQ (e.g. Where, Select, ToList)
  • Fluent APIs
  • Utility functions for built-in or library types

Example: LINQ Extension Method

var even = numbers.Where(n => n % 2 == 0);

Here, Where() is an extension method defined in System.Linq.Enumerable.

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