Argthtjtr

I am getting into generics but there is something I don't understand Here I have this class;

    class Node<T>{

        T value;

        Node <T> next,prev;

        public Node(T vl)

        {

          this.value=vl;

          this.next=this.prev=null;

        }

        }

I have a class List and a method Insert and it is working fine :

 class Lista<T>

    {

        public Node<T> Head, Tail;

        public int Cnt;



        public Lista()

        {

            this.Head = this.Tail = null;

            this.Cnt = 0;

        }







public void Insert(T vl)

    {

        Node<T> nd = new Node<T>(vl);

        if (this.IsEmpty())

            this.Head = this.Tail = nd;

        else

        {

            this.Tail.next = nd;

            nd.prev = this.Tail;

            this.Tail = nd;

        }

        this.Cnt++;

    }





}

but I Have a method FindNode:

 public Node<T> FindNode(T vl)

        {

            if (this.IsEmpty()) return null;

            Node<T> tmp = this.Head;

            while (tmp != null && ((object)tmp.VL != (object)vl)) tmp = tmp.next;

            return tmp;

        }

which finds a Node inside the List now the problem is
if I have a Lista<string> it works fine but if I create a Lista<int>
even if the element in list exists while comparing it is ignoring the same values and going next until it goes null , why does that happen ?

Method isEmpty:

public bool IsEmpty()

        {

            if (this.Head == null && this.Tail == null) return true;

            return false;

        }

The problem is, that for valuetypes like int in your example you are boxing them by casting to object.
Per default, the == operator does ReferenceEquality and this means that it will never return true in your case because the boxes are never the same.

Easiest was to fix this would be to turn this line

while (tmp != null && ((object)tmp.VL != (object)vl)) tmp = tmp.next;

into this

 while (tmp != null && !Equals(tmp.VL , vl)) tmp = tmp.next;

This will use the default equality comparer for a certain type and makes your code work as intended.

You could go further by using a generic constraint declared for your class like so:

class Lista<T> where T : IEquateable<T>

and change the same line like this

while (tmp != null && (!tmp.VL.Equals(vl)) tmp = tmp.next;

Or make it possible to inject an IEqualityComparer<T> into your class

  class Lista<T>

  {

       IEqualityComparer<T> _comparer;

       public Lista(IEqualityComparer<T> comparer = null)

       {

           _comparer = comparer ?? EuqlityComparer<T>.Default;

       }



       public Node<T> FindNode(T vl)

       {

           if (this.IsEmpty()) return null;

           Node<T> tmp = this.Head;

           while (tmp != null && !_comparer.Equals(tmp.VL, vl) tmp = tmp.next;

           return tmp;

       }

  }