Extension Methods

Syntax

public static returnType MethodName(this Type target, ...) { }

Examples

public static class StringExtensions
{
    public static bool IsPalindrome(this string text)
    {
        string cleaned = text.ToLower().Replace(" ", "");
        return cleaned == new string(cleaned.Reverse().ToArray());
    }
}

string word = "racecar";
Console.WriteLine(word.IsPalindrome());  // true - called just like a regular method

public static class ListExtensions
{
    public static double Median(this List<int> numbers)
    {
        var sorted = numbers.OrderBy(n => n).ToList();
        int mid = sorted.Count / 2;
        return sorted.Count % 2 == 0
            ? (sorted[mid - 1] + sorted[mid]) / 2.0
            : sorted[mid];
    }
}

List<int> data = new List<int> { 5, 1, 4, 2, 3 };
Console.WriteLine(data.Median());  // 3

public static class IntExtensions
{
    public static bool IsPrime(this int n)
    {
        if (n < 2) return false;
        for (int i = 2; i * i <= n; i++)
            if (n % i == 0) return false;
        return true;
    }
}

List<int> numbers = Enumerable.Range(1, 20).ToList();
var primes = numbers.Where(n => n.IsPrime()).ToList();
Console.WriteLine(string.Join(", ", primes));  // 2, 3, 5, 7, 11, 13, 17, 19

Best practices

• Use extension methods to add genuinely reusable helper functionality to a type, especially one you don't own and can't otherwise modify
• Put extension methods in a well-named static class (StringExtensions, DateTimeExtensions) so they're easy to find and organize
• Don't overuse extension methods for logic that would fit more naturally as a regular method on a type you do control - reserve them for genuinely external types or cross-cutting helpers
• Remember an extension method never overrides a real instance method with the same signature - a genuine instance method always takes priority if one exists

At a glance

Purpose

Applications on the .NET platform

File extension

.cs

Runs in

.NET runtime

Usually used with

.NET SDK and libraries

Specifications & further reading

• C# language reference

Related C# documentation