Difference Between Const, ReadOnly and Static in C#

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

namespace StaticReadOnlyConstant

{

    class Program

    {

        public static string str;                   //Ok

        //public static string str="Static";        //Ok

        static void Main(string[] args)

        {

            //----Static-------------------------------

            str = "Kunal Maurya";

            Console.WriteLine(str);

            //----ReadOnly-------------------------------

            //Console.WriteLine(ReadOnly.str);    //Ok cant access without object it is instance member

            ReadOnly ro = new ReadOnly();

            Console.WriteLine(ro.str);

            //----Constant-------------------------------

            Console.WriteLine(Constant.str);    //Ok

            Constant co = new Constant();

            //Console.WriteLine(co.str);          //Not Ok const variables are class members like static, it has one instance only

            Console.ReadKey();

        }

    }

    public class ReadOnly

    {

        //spublic readonly string str;     //Ok

        public readonly string str="ReadOnly";

        public ReadOnly()

        {

            str = "Kunal Maurya";  //Ok readonly can be modified inside the constructor

        }

        public void Modify()

        {

            //str = "Kunal Maurya";  //Ok readonly can not be modified any where except declaration time and inside constructor

        }

    }

    public class Constant

    {

        //public const string str;     //Not Ok must be initialize at declaration time

        public const string str = "ReadOnly"; //Ok

        public Constant()

        {

            //str = "Kunal Maurya";  //Not Ok const can not be modified any where except declaration time

        }

    }

}