8.5 Extension Methods

Let’s go through extension methods in C# — a way to add new methods to existing types without modifying them.

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📌 1. What Are Extension Methods?

• An extension method allows you to “extend” an existing class or interface with a new method.

• Useful for adding functionality to built-in types or third-party classes.

• Declared as static methods in a static class.

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📌 2. Basic Syntax

public static class StringExtensions
{
    public static bool IsCapitalized(this string str)
    {
        if (string.IsNullOrEmpty(str)) return false;
        return char.IsUpper(str[0]);
    }
}

• this string str → indicates extension for the string type

• Can now call .IsCapitalized() on any string

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📌 3. Using Extension Methods

string name = "Alice";
bool result = name.IsCapitalized();
Console.WriteLine(result); // True

• Looks like a normal method of the class, but it’s defined externally

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📌 4. Extension Methods with Parameters

public static class IntExtensions
{
    public static bool IsDivisibleBy(this int number, int divisor)
    {
        return number % divisor == 0;
    }
}
 
int num = 10;
Console.WriteLine(num.IsDivisibleBy(5)); // True
Console.WriteLine(num.IsDivisibleBy(3)); // False

• Extension methods can take additional parameters just like regular methods

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📌 5. Extension Methods with LINQ

• LINQ uses extension methods extensively

• Example: Where(), Select(), OrderBy() are all extension methods for IEnumerable<T>

int[] numbers = { 1, 2, 3, 4, 5 };
var evenNumbers = numbers.Where(n => n % 2 == 0); // LINQ extension

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📌 6. Rules for Extension Methods

1. Must be declared in a static class

2. Must be static method

3. First parameter specifies the type being extended with this

4. Cannot override existing instance methods

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📌 7. Advantages

• Add methods to existing classes without inheritance or modifying original code

• Makes code more readable and fluent

• Essential for fluent interfaces and LINQ operations

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📌 8. Summary Table

Feature	Description
Declaration	public static class, public static method
this keyword	Marks the type being extended
Usage	Call as if it’s an instance method
Benefits	Adds functionality without modifying original class, improves readability
Common Use Case	LINQ methods, helper methods for built-in types

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✅ Tip:

• Use extension methods to add small helper functions to classes you cannot modify

• Avoid adding too many unrelated methods to keep the API clean