Read only field

In this chapter you will learn:

How to create read only field
How to set the initial value to a read only field in constructor

Create read only field

A read only field declared with readonly modifier cannot change its value after declaration or outside the constructors.

using System; //from   j a va2 s  .c o  m
 
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 
  } 
}

The following code has a Rectangle with readonly width and height.

using System;/*from   j  av a  2s.  c  o m*/
class Rectangle{
   public readonly int Width = 3;
   public readonly int Height = 4;
}
class Program
{
    
    static void Main(string[] args)
    {
        Rectangle r = new Rectangle();

        Console.WriteLine(r.Width);

    }
}

The output:

Set read only field in constructor

The code above initialize the readonly fields when declaring them. We can also set the value in the contructor.

using System;/*from j a  va2  s  .c o  m*/
class Rectangle {
   public readonly int Width;
   public readonly int Height;
   
   public Rectangle(){
       Width = 5;
       
   }

}

class Program
{
    
    static void Main(string[] args)
    {
        Rectangle r = new Rectangle();

        Console.WriteLine(r.Width);
    }
}