Insertion in binary search tree using recursion in c#

Csharp program for Insertion in binary search tree using recursion. Here problem description and explanation.

// Include namespace system
using System;
// Csharp program for
// Insertion in binary search tree by using recursion
public class TreeNode
{
	public int data;
	public TreeNode left;
	public TreeNode right;
	public TreeNode(int data)
	{
		this.data = data;
		this.left = null;
		this.right = null;
	}
}
public class BinarySearchTree
{
	public TreeNode root;
	public BinarySearchTree()
	{
		this.root = null;
	}
	// Insert a node element
	public TreeNode addNode(TreeNode node, int data)
	{
		if (node != null)
		{
			if (node.data >= data)
			{
				// When new element is smaller or
				// equal to current node
				node.left = this.addNode(node.left, data);
			}
			else
			{
				// When new element is higher to current node
				node.right = this.addNode(node.right, data);
			}
			// important to manage root node
			return node;
		}
		else
		{
			return new TreeNode(data);
		}
	}
	// Display preorder
	public void preorder(TreeNode node)
	{
		if (node != null)
		{
			// Display node value
			Console.Write("  " + node.data);
			// Visit to left subtree
			this.preorder(node.left);
			// Visit to right subtree
			this.preorder(node.right);
		}
	}
	public void inorder(TreeNode node)
	{
		if (node != null)
		{
			// Visit to left subtree
			this.inorder(node.left);
			// Display node value
			Console.Write("  " + node.data);
			// Visit to right subtree
			this.inorder(node.right);
		}
	}
	public void postorder(TreeNode node)
	{
		if (node != null)
		{
			// Visit to left subtree
			this.postorder(node.left);
			// Visit to right subtree
			this.postorder(node.right);
			// Display node value
			Console.Write("  " + node.data);
		}
	}
	public static void Main(String[] args)
	{
		var tree = new BinarySearchTree();
		/*
		         10
		        / \
		       /   \
		      4     15
		     / \   /
		    3   5 12
		    -------------
		    Build binary search tree
		*/
		tree.root = tree.addNode(tree.root, 10);
		tree.addNode(tree.root, 4);
		tree.addNode(tree.root, 3);
		tree.addNode(tree.root, 5);
		tree.addNode(tree.root, 15);
		tree.addNode(tree.root, 12);
		// Display tree nodes
		Console.WriteLine("Preorder ");
		tree.preorder(tree.root);
		Console.WriteLine("\nInorder ");
		tree.inorder(tree.root);
		Console.WriteLine("\nPostorder ");
		tree.postorder(tree.root);
	}
}

Output

Preorder
  10  4  3  5  15  12
Inorder
  3  4  5  10  12  15
Postorder
  3  5  4  12  15  10