C# Readonly fields

“Read-only members are values which are set at compile time or at runtime by using constructor, but cannot be modified later”

Read-only members are initialized in either at declaration or at constructor. Readonly member can be instance member or static member.

Syntax:

public const [data type] [field name]= value; (or)

public const [data type] [field name]; public Class_name() // Instance constructor         {             [field name] = value;         }

In the below example we will see

•          How to initialize readonly member at the time declaration
•          Assigning value to readonly member from the instance, parameterized constructor.
•          How to initialize static readonly member at the time declaration
•          Assigning value to static readonly member from the static constructor.
•          What happens if we  try to reassign readonly value other than variable initialization and constructor

Example:

class ReadOnlyMember     {         public readonly double pi=3.14;         public readonly double root2;         public readonly int weekDays;         public static readonly int months = 12;         public static readonly int yearDays;         public ReadOnlyMember() // Instance constructor         {             root2 = 1.414;         }         public ReadOnlyMember(int valweekDays) //parameterized constructor         {             weekDays = valweekDays;         }         static ReadOnlyMember() //static constructor         {             yearDays = 365;             Console.WriteLine("called static constructor and readonly member yearDays value is {0}", yearDays);         }     }     class ReadOnlyMemberProgram     {         public static void Main()         {             ReadOnlyMember obj=new ReadOnlyMember();             Console.WriteLine("readonly field pi intilized at declaration and value is {0}", obj.pi);             Console.WriteLine("readonly field root2 intilized in constructor and value is {0}", obj.root2);             //obj.root2 = 5; //Compile Error : A readonly field can not be assigned (except in a constructor or a variable intializer)             ReadOnlyMember obj1 = new ReadOnlyMember(7);             Console.WriteLine("readonly field weekDays. value is passed from object creation and value is {0}", obj1.weekDays);             //obj1.weekDays = 5; //Compile Error             Console.WriteLine("static readonly field months intilized at declaration and value is {0}",ReadOnlyMember.months);                         Console.WriteLine("\nObject obj values are obj.pi={0} ,obj.root2={1}, obj.weekDays={2}", obj.pi, obj.root2, obj.weekDays);             Console.WriteLine("Object obj1 values are obj1.pi={0} ,obj1.root2={1}, obj1.weekDays={2}", obj1.pi, obj1.root2, obj1.weekDays);             Console.Read();                      }     }

Output:

Error:

If you try to re-assign the value of readonly field other than at variable initialization and constructor we will get compile time error as “A readonly field can not be assigned (except in a constructor or a variable intializer)”