ExplicitCastingExample

using System;

namespace ExplicitCasting
{
    class Person
    {
        protected Person instance;

        public Student ToStudent()
        {
            if (!(instance is Student))
                throw new Exception ("To return student, you have to create it before");

            return (Student)this;
        }

        public Trainer ToTrainder()
        {
            if (!(instance is Trainer))
                throw new Exception ("To return trainer, you have to create it before");

            return (Trainer)this;
        }
    }

    class Student : Person
    {
        public Student()
        {
            base.instance = this;
        }

        public void ApplyToCourse()
        {
            Console.WriteLine ("I have applied to a course");
        }
    }

    class Trainer : Person
    {
        public Trainer()
        {
            base.instance = this;
        }

        public void CreateCourse()
        {
            Console.WriteLine ("I have created a course");
        }
    }


    class MainClass
    {
        public static void Main (string[] args)
        {
            Person p;

            int type = int.Parse (Console.ReadLine());
            string cmd = Console.ReadLine();

            switch (type)
            {
                case 1:
                    p = new Student();
                    break;
                case 2:
                    p = new Trainer();
                    break;
                default:
                    throw new Exception("Invalid type");
            }

            if (cmd == "apply") {
                p.ToStudent().ApplyToCourse();
            } else if (cmd == "create") {
                p.ToTrainder().CreateCourse();
            } else {
                throw new Exception ("Invalid cmd");
            }
        }
    }
}