Convert given binary tree to doubly linked list in c#

Csharp program for Convert given binary tree to doubly linked list. Here problem description and explanation.

// Include namespace system
using System;
/* 
  Csharp program for
  Convert a given binary tree to doubly linked list
*/
// Binary Tree Node
public class LinkNode
{
	public int data;
	public LinkNode left;
	public LinkNode right;
	public LinkNode(int data)
	{
		// Set node value
		this.data = data;
		this.left = null;
		this.right = null;
	}
}
public class BinaryTree
{
	public LinkNode root;
	public BinaryTree()
	{
		// Set initial values
		this.root = null;
	}
	// Display node element of doubly linked list
	public void display()
	{
		if (this.root == null)
		{
			Console.WriteLine("Empty Linked List");
		}
		else
		{
			Console.WriteLine("Linked List Head to Tail :");
			// Get first node of linked list
			var temp = this.root;
			LinkNode tail = null;
			// iterate the linked list
			while (temp != null)
			{
				tail = temp;
				// Display node value
				Console.Write("  " + temp.data);
				// Visit to next node
				temp = temp.right;
			}
			Console.WriteLine("\nLinked List Tail to Head :");
			// Get last node of linked list
			temp = tail;
			// iterate the linked list
			while (temp != null)
			{
				// Display node value
				Console.Write("  " + temp.data);
				// Visit to prev node
				temp = temp.left;
			}
		}
	}
	// Function which is convert binary tree to DLL
	public void transform(LinkNode node, LinkNode x, LinkNode y)
	{
		if (node != null)
		{
			// Visit to left subtree
			this.transform(node.left, x, node);
			if (this.root == null)
			{
				// Set the root node when is empty
				this.root = node;
			}
			// Visit to right subtree
			this.transform(node.right, node, y);
			if (node.left == null)
			{
				// Connect left node
				node.left = x;
				if (x != null)
				{
					// Connect right to left node
					x.right = node;
				}
			}
			if (node.right == null)
			{
				// Connect right node
				node.right = y;
				if (y != null)
				{
					// Connect left to right node
					y.left = node;
				}
			}
		}
	}
	public void doublyList()
	{
		var node = this.root;
		if (node != null)
		{
			// Reset the root value
			this.root = null;
			// Covert into Doubly linked list
			this.transform(node, null, null);
		}
	}
	public static void Main(String[] args)
	{
		// Create new tree
		var tree = new BinaryTree();
		/*
		  Binary Tree
		-----------------------
		        1
		       /  \
		      2    3
		     /    /  \
		    4    5    6
		     \    \
		      7    9
		     / 
		    8
		*/
		// Add binary tree nodes
		tree.root = new LinkNode(1);
		tree.root.left = new LinkNode(2);
		tree.root.right = new LinkNode(3);
		tree.root.right.right = new LinkNode(6);
		tree.root.right.left = new LinkNode(5);
		tree.root.right.left.right = new LinkNode(9);
		tree.root.left.left = new LinkNode(4);
		tree.root.left.left.right = new LinkNode(7);
		tree.root.left.left.right.left = new LinkNode(8);
		tree.doublyList();
		tree.display();
	}
}

Output

Linked List Head to Tail :
  4  8  7  2  1  5  9  3  6
Linked List Tail to Head :
  6  3  9  5  1  2  7  8  4