Add node to front of a link list

Question

I am working with a simple link list. I am trying to create a method that will insert an item to the front of my link list.

This Method works:

// Works
public static LinkedListNode AddItemAtFrontWorkingVersion(LinkedListNode node, int value)
{
  var newNode = new LinkedListNode(value);
  newNode.next = node;
  return newNode;
}

Called by:

LinkedListNode a = new LinkedListNode(5);
a = LinkedListNode.AddItemAtFrontWorkingVersion(a, 99);

Now I have a different version which doesn't work and i cant figure out why. The list is an object so it should be on the heap why cant i just set to the new node. When the method returns it is not changed.

Question: Why do I have to return the node why cant I just assign it like I am in AddItemAtEnd?

// Does Not work
public static void AddItemAtFront(LinkedListNode node, int value)
{
    var newNode = new LinkedListNode(value);
    newNode.next = node;
    node = newNode;
}

Called by:

LinkedListNode a = new LinkedListNode(5);
LinkedListNode.AddItemAtFront(a, 99);

Full code:

using System;

namespace DataStructure
{

    public class LinkedListNode
    {

        public int value;
        public LinkedListNode next;

        public LinkedListNode(int value)
        {
            this.value = value;
        }

        public static void AddItemAtEnd(LinkedListNode node, int value) {

            var list = node.next;

            while (node.next != null)
            {
                node = node.next;                
            }
            node.next = new LinkedListNode(value);
        }

        // Does Not work
        public static void AddItemAtFront(LinkedListNode node, int value)
        {
            var newNode = new LinkedListNode(value);
            newNode.next = node;
            node = newNode;
        }

        // Works
        public static LinkedListNode AddItemAtFrontWorkingVersion(LinkedListNode node, int value)
        {
            var newNode = new LinkedListNode(value);
            newNode.next = node;
            return newNode;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            LinkedListNode a = new LinkedListNode(5);
            LinkedListNode.AddItemAtFront(a, 99);
            a = LinkedListNode.AddItemAtFrontWorkingVersion(a, 99);
            LinkedListNode.AddItemAtEnd(a, 9);
            LinkedListNode.AddItemAtEnd(a, 10);
            LinkedListNode.AddItemAtEnd(a, 11);
            LinkedListNode.AddItemAtEnd(a, 12);
            LinkedListNode.AddItemAtFront(a, 99);

            a = LinkedListNode.AddItemAtFrontWorkingVersion(a, 99);
            Console.Read();
        }  
    }
}

Answer 1

You are passing your LinkedListNode by value. For your second version to be working you have to pass it by reference.

public static void AddItemAtFront(ref LinkedListNode node, int value)
{
    var newNode = new LinkedListNode(value);
    newNode.next = node;
    node = newNode;
}

More details may be found inside the documentation: Passing Reference-Type Parameters .