Difference Between Constant And ReadOnly

In this article you will see the difference between Constant and ReadOnly keywords in C#.

Constants

• Constants are static by default.
• They must have a value at compilation-time (you can have e.g. 3.14 * 2 + 5, but cannot call methods).
• Could be declared within functions.
• These are copied into every assembly that uses them (every assembly gets a local copy of values).

using System;
namespace ConstantExample
{
    class ConstantExample
    {
        private const int PI = 3.14;
        public static void Main()
        {
            //You have to initialize Const variables while declaration
            const int RollNo = 1284;
            //Valid scenario
            const int Age = 10 + 13;
            const string Name = "Satish Kumar";

            // Reassigning a const variable is not allowed.
            RollNo = 1405;   // Will result compile time error.
            Age++;    // Will result compile time error.
            Console.WriteLine(Name);

            Console.Read();
        }
    }
}

Readonly

• Must have set value, by the time constructor exits.
• Are evaluated when instance is created.
• You can use static modifier for readonly fields.
• readonly modifier can be used with reference types.
• readonly modifier can be used only for instance or static fields, you cannot use readonly keyword for variables in the methods. 

using System;

namespace ReadOnlyExample
{
   class ReadOnlyTest
   {
      //You have to initilize readonly varabiles while declaration or in constructor
      readonly int RollNo = 1284;

      //Valid scenario
      readonly  int Age;

      readonly  string Name = "Satish Kumar";

      //readonly fields can be initlized in constructor
      public ReadOnlyTest (string name)
      {
          Age = 23;
          Name = name;
      }

      public changeName(string newName)
      {
          Name = "Satish Kumar Vadlavalli"; ;   // Will result error.
      }
   }

   class ReadOnlyExample
   {
        public static void Main()
        {
           ReadOnlyTest obj = new ReadOnlyTest("Satish");
           Console.Read();
        }
   }
}

Difference between const and readonly

• const fields has to be initialized while declaration only, while readonly fields can be initialized at declaration or in the constructor.
• const variables can declared in methods ,while readonly fields cannot be declared in methods.
• const fields cannot be used with static modifier, while readonly fields can be used with static modifier.
• A const field is a compile-time constant, the readonly field can be used for run time constants.

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