Difference between Readonly and Const in C# with example

Difference between Readonly and Const in C# with example

Constant

Constant fields are defined at the time of declaration in the code snippet, because once they are defined they can't be modified. By default a constant is static, so you can't define them static from your side.

It is also mandatory to assign a value to them at the time of declaration otherwise it will give an error during compilation of the program snippet. That's why it is also called a compile-time constant.

Readonly

A Readonly field can be initialized either at the time of declaration or within the constructor of the same class. We can also change the value of a Readonly at runtime or assign a value to it at runtime (but in a non-static constructor only).

For that reason a Readonly field is also called a run-time constant.

public class ReadOnlyConstDynamicVar

    {

        public const int x = 10; // assigning value is mandatory

        public readonly int y ; // assigning value is optional

        public ReadOnlyConstDynamicVar()

        {

            //readonly takes default value if nothing is assigned

            y = 20;

        }

        public string GetreadOnlyValue()

        {           

            //y = 20;  // not allowed to change readonly variable

            const int g = 30;

            return g.ToString();

        }

    }

       static void Main(string[] args)

        {           

            ReadOnlyConstDynamicVar obj = new ReadOnlyConstDynamicVar();

            //reading const value

            Console.WriteLine(ReadOnlyConstDynamicVar.x.ToString());

            //reading readonly value

            Console.WriteLine(obj.y.ToString());

            Console.ReadLine();

        }