Csharp/CSharp Tutorial/Class/readonly

Creating a read-only property.

<source lang="csharp">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);
   }

}</source>

Readonly and const

<source lang="csharp">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 );
  }

}</source>

x = 123, y = 8, z = 555

Readonly Fields

<source lang="csharp">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;
   }

}</source>

static readonly fields

<source lang="csharp">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);
 }

}</source>

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.

<source lang="csharp">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 
 } 

}</source>