const and readonly keyword in C#

There is two way to declare constant variables one is by using const keyword and other is by using readonly keyword. The main difference between these two is, the first one is used to create a constant variable at compile time while the other one is used to create a constant variable at run time. Below is a detailed explanation of these two keywords:

Const

the const keyword is used to create the constant variable. Its value is given at compile time meaning that its value is given at the same time when we declare it. Its value will be the same during the lifetime of the program meaning that its value does not change once given. Static keyword can not be used with these type of variables because by default they are static.

Example:

using System; 
 
class Test { 
 
   // Constant variabless 
   public const int var1 = 100; 
   public const string str1 = "Rahul"; 
 
   static public void Main() 
   { 
       Console.WriteLine("The value of var1: {0}", var1); 
       Console.WriteLine("The value of str1: {0}", str1); 
   } 
}

Output:

The value of var1: 100
The value of str1: Rahul

ReadOnly:

readonly keyword is used to create the read-only variable. Its value is given at run time meaning that its value is given at the same time when we declare it or under the class constructor. Static keyword can be used with these type of variables because by default they are non-static.

Example

using System; 
 
class Test{ 
 
   // readonly variables 
   public readonly int var1; 
   public readonly int var2; 
 
   public Test(int v1, int v2) 
   {  
       var1 = v1; 
       var2 = v2; 
       Console.WriteLine("The value of var1: {0}", var1); 
	   Console.WriteLine("The value of var2: {0}", var2); 
   } 
 
   // Main method 
   static public void Main() 
   { 
       Test obj = new Test(100, 200); 
   } 
}

Output

The value of var1: 100
The value of var2: 200

Below are some differences between const and readonly keyword in C#: