Difference between readonly and const keyword in C#

Csharp Server Side Programming Programming

readonly keyword

readonly keyword is used to define a variable which can be assigned once after declaration either during declaration or in constructor. const keyword is used to define a constant to be used in the program. Following is the valid usage of a readonly and const keywords in C#.

Example

Live Demo

using System.IO;
using System;

public class Program {
   public const int VALUE = 10;
   public readonly int value1;

   Program(int value){
      value1 = value;
   }
   public static void Main() {
      Console.WriteLine(VALUE);
      Program p1 = new Program(11);
      Console.WriteLine(p1.value1);
   }
}

Output

10
11

Following are some of the important differences between readonly and const keywords.

Sr. No.	Key	readonly keyword	const keyword
1	Purpose	readonly keyword is used to create a readonly fields.	const keyword is used to create constant fields.
2	Type	readonly is a constant defined at runtime.	const is used to create a constant at compile time.
3	Change	readonly field value can be changed after declaration.	const field value cannot be changed after declaration.
4	Method	readonly fields cannot be defined within a method.	const fields can be declared within a method.
5	Value assignment	readonly variables are declared as instance variable and assigned values in constructor.	const fields are to be assigned at the time of declaration.