Object reference not set to an instance of an object.

Question

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

using System.Collections;

namespace ConsoleApplication2

{

class Program

{

static void Main(string[] args)

{

employee[] e1 = new employee[4];

e1[0] = new employee();

e1[1] = new parttimemeployee();

e1[2] = new fulltimeemployee();

foreach (employee e in e1)

{

e.add();

}

Console.ReadLine();

}

}

}

class employee

{

public string firstname="sai";

public string lastname="kumar";

public virtual void add()

{

Console.WriteLine("Base class method called");

}

}

class fulltimeemployee : employee

{

public new void add()

{

Console.WriteLine(firstname+lastname);

}

}

class parttimemeployee : employee

{

public override void add()

{

Console.WriteLine("partime employee called");

}

}

 

after executing above code i am getting object reference not set to an instance of an object. Please help sir im begineer