What are extension methods and when should you use them?

Extension Methods allow you to add new methods to existing types without modifying the original type, creating a new derived type, or recompiling.

Key Characteristics:

• Must be defined in a static class
• Must be static methods
• First parameter uses the "this" keyword to specify the type being extended
• Called as if they were instance methods

Syntax:

public static class StringExtensions
{
    public static bool IsValidEmail(this string str)
    {
        if (string.IsNullOrWhiteSpace(str))
            return false;
        
        return str.Contains("@") && str.Contains(".");
    }
    
    public static string Truncate(this string str, int maxLength)
    {
        if (string.IsNullOrEmpty(str) || str.Length <= maxLength)
            return str;
        
        return str.Substring(0, maxLength) + "...";
    }
}

// Usage
string email = "user@example.com";
bool isValid = email.IsValidEmail();  // Called like an instance method

string longText = "This is a very long text";
string shortened = longText.Truncate(10);  // "This is a ..."

LINQ is built on extension methods:

// These are all extension methods on IEnumerable
var result = numbers
    .Where(n => n > 5)
    .Select(n => n * 2)
    .OrderBy(n => n);

When to use Extension Methods:

1. Adding utility methods to framework types you can't modify
2. Improving code readability with fluent APIs
3. Adding domain-specific functionality to existing types
4. Creating LINQ-like query operations

When NOT to use:

1. When you can modify the original class
2. When it breaks encapsulation (accessing private members)
3. For core business logic (prefer regular methods)
4. When it might conflict with existing or future methods

Best Practices:

• Use clear, descriptive names
• Keep them simple and focused
• Group related extensions in appropriately named static classes
• Be careful with common types (string, int) to avoid naming conflicts
• Document them well