EN CSharp Keyword extension

Українська · Русский · English · Deutsch · Français · Italiano · Español · 繁體中文 · 日本語 · 한국어

C#: extension

Handbook C# · Practical Guide

Declares an extension block: methods, properties, and other supported members for an existing type.

Simple example (Example.cs)

Save this example separately as Example.cs. Run it independently of the other examples on this page.

Example.cs:

// PROJECT-EXAMPLE
System.Console.WriteLine("ore".Size);
static class TextTools
{
    extension(string text) { public int Size => text.Length; }
}

Result:

3

Syntax

extension(Type receiver) { public ReturnType Member ... }

Extension block form added to C# 14. The classic extension method with this parameter also remains valid.

Example 1: simple form

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.IO;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

public static class Program
{
    public static async Task Main(string[] args)
    {
        Console.WriteLine("ore".Size);
    }
}
public static class TextTools { extension(string text) { public int Size => text.Length; } }

Example 2: another practical form

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.IO;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

public static class Program
{
    public static async Task Main(string[] args)
    {
        Console.WriteLine((12).IsEven());
    }
}
public static class NumberTools { extension(int value) { public bool IsEven() => value % 2 == 0; } }

Official source: Microsoft Learn.