What are extension methods and how are they used?

Question

ICSM Computer · Answer

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 classFirst parameter is prefixed with the this keywordCan 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 PrintAllandlt;Tandgt;(this Listandlt;Tandgt; list)
    {
        foreach (var item in list)
            Console.WriteLine(item);
    }
}
Usage:
var nums = new Listandlt;intandgt; { 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 methodsCannot access private members of the target typeOnly resolved at compile time, no virtual dispatch
Common Uses
LINQ (e.g. Where, Select, ToList)Fluent APIsUtility functions for built-in or library types
Example: LINQ Extension Method
var even = numbers.Where(n =andgt; n % 2 == 0);
Here, Where() is an extension method defined in 
System.Linq.Enumerable.

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What are extension methods and how are they used?

Anubhav Kumar

Can you answer this question?

1 Answers

Key Features

Example: Creating an Extension Method

How It Works Behind the Scenes

Practical Example: List Extensions

When to Use Extension Methods

Limitations

Common Uses

Example: LINQ Extension Method

Liked By

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)