Difference Between Virtual Method and Abstract Method(CAN vs MUST)

CASE 1: No Compilation Error[Virtual Method Can be implemented]

using System;

namespace ConsoleApplication1

{

    public abstract class A

    {

        public abstract void Method1();

        public virtual void Method2()

        {

            Console.WriteLine("Method2Base");

        }

    }

    public class B : A

    {

        //public override void Method2()

        //{

        //    Console.WriteLine("Method2Derived");

        //}

        public override void Method1()

        {

            Console.WriteLine("Method1");

        }

    }

    class Program

    {

        static void Main(string[] args)

        {

            A obj = new B();

            obj.Method1();

            obj.Method2();

            Console.ReadLine();

        }

    }

}

CASE 2:Compilation Error[Abstract Method Must be implemented]

using System;

namespace ConsoleApplication1

{

    public abstract class A

    {

        public abstract void Method1();

        public virtual void Method2()

        {

            Console.WriteLine("Method2Base");

        }

    }

    public class B : A

    {

        public override void Method2()

        {

            Console.WriteLine("Method2Derived");

        }

        //public override void Method1()

        //{

        //    Console.WriteLine("Method1");

        //}

    }

    class Program

    {

        static void Main(string[] args)

        {

            A obj = new B();

            obj.Method1();

            obj.Method2();

            Console.ReadLine();

        }

    }

}