Const, ReadOnly And Static Variables in C#

Const

Let's us look at an example.

Example 1

namespace Sample                                               {
    public class MyClass
    {
        private const int a;

    }
    public class Sample2
    {
        static void Main()
        {
        }
    }
}

Output 

Error 1 A const field requires a value to be provided.

A const variable must be initialized when declared and can't be modified later.

Private static const int a=9; // Const can’t be marked as static

Example 2

namespace Sample
{
    public class Sample1
    {
        private const Sample2 sample2 = new Sample2();//Error

    }
    public class Sample2
    {

    }
    public class Sample3
    {
        static void Main()
        {
        }
    }
}

Output 

Error 1. A const field of a reference type other than string can only be initialized with null.

A const field of a reference type other than string can only be initialized with null.

Example 3

namespace Sample
{
    public class Sample1
    {
        private const Sample2 sample2 = null;

    }
    public class Sample2
    {

    }
    public class Sample3
    {
        static void Main()
        {
        }
    }
}

Output 

No error

Example 4

namespace Sample
{
    public class Sample1
    {
        public const int a = 1;
        public const int b = b + 2;
    }
    public class Sample3
    {
        static void Main()
        {
        }
    }
}

The value of b evaluates at compile time, a constant can participate in a constant expression.

Static Field

Example 1

namespace Sample
{
    public class Sample1
    {
        public static int c1;
        static void Main()
        {
            Console.WriteLine(c1);
            Console.ReadKey();
        }
    }
}

Output 

0

Static variables are initialized as soon as the class loads with the default value. A variable in C# can never have an uninitialized value.

ReadOnly

namespace Sample
{
    public class Sample1
    {
        public static readonly int a=8;
        static void Main()
        {
            a = 10;
            Console.WriteLine(a);
            Console.ReadKey();
        }
    }
}

Output 

Error 1 A static readonly field cannot be assigned to (except in a static constructor or a variable initializer).

A Readonly field can be declared either when it is declared or in the constructor of its class.

Difference between const and Readonly keyword

A const field can only be initialized in the declaration of the field. A readonly field can be initialized either at the declaration or in a constructor. Therefore, readonly fields can have different values depending on the constructor used. A const field is a compile-time constant. A readonly field can be used for runtime constants.

namespace Sample
{
    public class Sample1
    {
        public  readonly int _a; //but const field show error if field not assigned
        public Sample1(int a)
        {
            _a = a;
        }
        static void Main()
        {
            Sample1 sample = new Sample1(5);

            Console.WriteLine(sample._a);
            Console.ReadKey();
        }
    }
}

Output 

5