Qfyilyi

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.

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 and make and fix 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;

       }

  }