Question

 class ListNode
{
    public object Data { get; private set; }

    public ListNode Next { get; set; }

    public ListNode(object Element)
    {
        Data = Element;
    }

    public ListNode(object Element, ListNode NextNode)
    {
        Data = Element;
        Next = NextNode;
    }

    public ListNode()
    {

    }
}

 class LinkedList
{
    ListNode first;
    ListNode last;

    public LinkedList()
    {
        first = null;
        last = null;
    }


   public ListNode Find(object After)
    {
        ListNode current = new ListNode();
        current= first;
        while (current.Data != After)
        current = current.Next;
        return current;
    }

   public void Add(object newItem, object After)
   {
       ListNode current=new ListNode();
       ListNode newNode=new ListNode();
       current = Find(After);
       newNode.Next = current.Next;
       current.Next = newNode;
   }

    public void InsertAtFront(object Element)
    {
        if (IsEmpty())
        {
            first = last = new ListNode(Element);
        }
        else
        {
            first = new ListNode(Element,first);
        }
    }

    bool IsEmpty()
    {
        return first == null;
    }

    public void Display()
    {
        ListNode current = first;
        while (current!=null)
        {
            Console.WriteLine(current.Data);
            current = current.Next;
        }           
    }
}

I implement Find method for Add After specific element, but when I debug it showing me the object reference not set to an instance of an object exception. Please point out my mistake in Find method or with Add After method. thanks

Answer 1

Problem is here

while (current.Data != After)
      current = current.Next;

When there is no After in your list you will eventually get current.Next equal to null
You need to check if current.Next is not null

   while (current.Next != null && current.Data != After)
      current = current.Next;

you should also fix your add logic (if you want to add elements into empty list)

   public void Add(object newItem, object After)
   {
       if(IsEmpty())
       {
            InsertAtFront(newItem);
            return;
       }

       ListNode newNode=new ListNode();
       newNode.Data = newItem; 
       ListNode current = Find(After);
       newNode.Next = current.Next;
       current.Next = newNode;
   }