What are Extension Methods in C#?

 

🔹 What are Extension Methods in C#?

Extension methods allow you to add new methods to an existing type (class, struct, interface) without modifying its source code or using inheritance.

👉 You can call them as if they are instance methods of that type.

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✅ Why do we need Extension Methods?

• Add functionality to existing .NET classes (e.g., string, int, DateTime)

• Keep code clean and readable

• Avoid inheritance for utility methods

• Widely used in LINQ

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🔑 Rules to Create Extension Methods

1. Must be created in a static class

2. Method must be static

3. First parameter must use this keyword

4. Namespace must be imported (using)

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🛠 How to Create an Extension Method (Step-by-Step)

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Example 1: Extension Method for string

Step 1️⃣ Create a static class

public static class StringExtensions
{
    public static int WordCount(this string str)
    {
        if (string.IsNullOrEmpty(str))
            return 0;

        return str.Split(' ').Length;
    }
}

Step 2️⃣ Use the extension method

class Program
{
    static void Main()
    {
        string text = "C sharp interview preparation";

        int count = text.WordCount();
        Console.WriteLine(count);
    }
}

🟢 Output:

3

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🧠 How it Works Internally

This call:

text.WordCount();

Is actually compiled as:

StringExtensions.WordCount(text);

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📌 Example 2: Extension Method for int

public static class IntExtensions
{
    public static bool IsEven(this int number)
    {
        return number % 2 == 0;
    }
}

Usage:

int n = 10;
Console.WriteLine(n.IsEven()); // true

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📌 Example 3: Extension Method for List<T>

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

Usage:

var list = new List<int> { 1, 2, 3 };
list.PrintAll();

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🔥 Extension Methods with Interfaces

public interface ILogger
{
    void Log(string msg);
}

public static class LoggerExtensions
{
    public static void LogInfo(this ILogger logger, string msg)
    {
        logger.Log("INFO: " + msg);
    }
}

Usage:

ILogger logger = new ConsoleLogger();
logger.LogInfo("Application started");

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❌ What Extension Methods CANNOT Do

• ❌ Cannot access private members

• ❌ Cannot override existing methods

• ❌ If method name already exists → instance method wins

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⚖ Extension Method vs Normal Method

Feature	Extension Method	Normal Method
Modify existing class	❌ No	✔ Yes
Uses this keyword	✔ Yes	❌ No
Belongs to class	Looks like it	Actually static
Access private data	❌ No	✔ Yes

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🎯 Real Interview Questions & Answers

❓ Q: Where are extension methods used in real projects?

✔ LINQ (Where, Select, OrderBy)
✔ Validation helpers
✔ Utility methods
✔ Clean architecture helper methods

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❓ Q: Can extension methods be static?

✔ Yes, they must be static

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❓ Q: Can we create extension methods for sealed classes?

✔ Yes (e.g., string)

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❓ Q: Can extension methods be overloaded?

✔ Yes

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❓ Q: What happens if both class method and extension method exist?

➡ Class method is called, not extension method.

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🧠 Easy Memory Trick

Static class + static method + this keyword = Extension Method

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