Csharp/CSharp Tutorial/Class/readonly

Creating a read-only property.

using System;
public class ClassWithReadOnlyProperty {
    int fAge;
    public int Age {
        get {
            return fAge;
        }
    }
    public ClassWithReadOnlyProperty() {
        fAge = 21;
    }
}
class MainClass {
    static public void Main() {
        ClassWithReadOnlyProperty crp = new ClassWithReadOnlyProperty();
        Console.WriteLine("Age {0}", crp.Age);
    }
}

Readonly and const

public class MyClass
{
   public MyClass()
   {
      this.y = 4;
      this.y = 6;
      SetField( ref this.y );
   }
   private void SetField( ref int val )
   {
      val = 8;
   }
   public readonly int x = 123;
   public readonly int y;
   public const     int z = 555;
}
public class MainClass{
   static void Main()
   {
      MyClass obj = new MyClass();
      System.Console.WriteLine( "x = {0}, y = {1}, z = {2}",
                                obj.x, obj.y, MyClass.z );
   }
}

x = 123, y = 8, z = 555

Readonly Fields

class Color
{
    public Color(int red, int green, int blue)
    {
        this.red = red;
        this.green = green;
        this.blue = blue;
    }
    
    int red;
    int green;
    int blue;
    
    public static readonly Color Red;
    public static readonly Color Green;
    public static readonly Color Blue;
    
    static Color()
    {
        Red = new Color(255, 0, 0);
        Green = new Color(0, 255, 0);
        Blue = new Color(0, 0, 255);
    }
}
class MainClass
{
    static void Main()
    {
        Color background = Color.Red;
    }
}

static readonly fields

public class Bicycle
{
  public readonly string make;
  public static readonly int wheels = 4;
  public Bicycle(string make)
  {
    System.Console.WriteLine("Creating a Bicycle object");
    this.make = make;
  }
}

class MainClass
{
  public static void Main()
  {
    System.Console.WriteLine("Bicycle.wheels = " + Bicycle.wheels);
    Bicycle myBicycle = new Bicycle("AAA");
    System.Console.WriteLine("myBicycle.make = " + myBicycle.make);
  }
}

Bicycle.wheels = 4
Creating a Bicycle object
myBicycle.make = AAA

Use readonly

You can create a read-only field in a class by declaring it as readonly.

A readonly field can be initialized, but not changed after that.

using System; 
 
class MainClass { 
  public static readonly int SIZE = 10; 
  
  public static void Main() { 
    int[] nums = new int[MainClass.SIZE]; 
 
    // MainClass.SIZE = 100; // Error!!! can"t change 
  } 
}