Const, ReadOnly and Static Keywords in C#

This article will explain some important points about the const, readonly, and static keywords in C#. We often see most developers unable to understand the differences among the const, readonly, and static keywords in C#. I have recently also tried looking at tutorials (articles) around the web, and  I have seen some examples of these keywords. But I can't find a good solution, such as why and when. This article answers the question we often see around the web of why and when to use the const, readonly, and static keywords.

This is a very simple article but will clarify some of the concepts of C# for those who are learning or for those also who might have good knowledge but unfortunately miss these important keywords.

The following points define the const, readonly, and static keywords in C#.

1. const Keyword

The const fields or local variables must be assigned a value at the time of declaration.

They cannot be modified. const fields must be assigned a value at the time of declaration throughout the program.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace ConsoleApplication2
{
    class Program
    {
        public int a = 10;
        public int constant()
        {
            a = 20;
            return a;
        }
    }
    class run
    {
        static void Main(string[] args)
        {
           Program p = new Program();
           Console.WriteLine(p.constant());
        }
    }
}

Output

20

Now we declare the const keyword with a variable. It will show an error.

By default, a const is static. Hence you cannot define a const type as static. The following image shows the error when we use the static keyword with const.

A const field is a compile-time constant. A const field or local variable can be initialized with a constant expression that must be fully evaluated at compile time.

2. readonly Keyword

A readonly field can be initialized at the time of declaration.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace ConsoleApplication2
{
    class Program
    {
        public readonly int a = 10; // initialized at the time of declaration
        public int constant()
        {
            return a;
        }
    }
    class run
    {
        static void Main(string[] args)
        {
            Program p = new Program();
            Console.WriteLine(p.constant());
        }
    }
}

Output

10

A readonly field can be set within the constructor, or this value can only be changed in the constructor. It can't be changed in normal functions. readonly fields can be used for run-time constants.

class Program
{
    public readonly int a; // initialized at the time of declaration
    public Program()
    {
        a = 20;
    }
}

3. static keyword

The static keyword is used to specify a static member, which means static members are common to all the objects.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace ConsoleApplication2
{
    class Program
    {
        static int a; // initialized at the time of declaration
        public static int Hello()
        {
            a = 20;
            return a;
        }
    }
    class run
    {
        static void Main(string[] args)
        {
            //Program p = new Program();
            Console.WriteLine(Program.Hello());
        }
    }
}

Output

20

Now we take a variable without declaring the static keyword.

This keyword can be used with classes, fields, methods, properties, operators, events, and constructors, but it cannot be used with indexers, destructors, or types other than classes.