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C#: readonly

Handbook C# · Practical Guide

Prohibits field replacement after initialization or constructor; also describes immutable structures and read-only ref access.

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(new Item(7).Count);
class Item { public readonly int Count; public Item(int count) { Count = count; } }

Result:

7

Syntax

readonly Type field; / readonly struct Type / ref readonly Type

a readonly reference to a list does not make the list's elements immutable. For const, the value must be known at compile time.

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)
    {
        Settings settings = new("profile");
        Console.WriteLine(settings.Name);
    }
}
public class Settings(string name) { public readonly string Name = name; }

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)
    {
        Position position = new(2, 3);
        Console.WriteLine(position.Sum);
    }
}
public readonly struct Position(int x, int y) { public int X { get; } = x; public int Y { get; } = y; public int Sum => X + Y; }

Official source: Microsoft Learn.