¿Por qué no puedo encontrar _left y _right en BinarySearchTree?

Tengo un problema con el siguiente fragmento de código:

using System;
using System.Collections.Generic;
using System.Text;

namespace trees_by_firas
{
    class Program
    {
        static void Main(string[] args)
        {
            BinarySearchTree t = new BinarySearchTree();

            t.insert(ref t.root, 10);
            t.insert(ref t.root, 5);
            t.insert(ref t.root, 6);
            t.insert(ref t.root, 17);
            t.insert(ref t.root, 2);
            t.insert(ref t.root, 3);

            BinarySearchTree.print(t.root);
            Console.WriteLine(--------------------);
            Console.WriteLine(t.FindMax());
            Console.WriteLine(t.FindMin());
            Console.WriteLine(--------------------);
            Console.WriteLine(t.CountLeaves());
            Console.WriteLine(t.CountNodes());



        }

        public class TreeNode
        {
            public int n;
            public TreeNode _left;
            public TreeNode _right;


            public TreeNode(int n, TreeNode _left, TreeNode _right)
            {
                this.n = n;
                this._left = _left;
                this._right = _right;
            }


            public void DisplayNode()
            {
                Console.Write(n);
            }


        }


        public class BinarySearchTree
        {
            public TreeNode root;


            public BinarySearchTree()
            {
                root = null;
            }


            public void insert(ref TreeNode root, int x)
            {
                if (root == null)
                {
                    root = new TreeNode(x, null, null);
                }
                else
                    if (x < root.n)
                        insert(ref root._left, x);
                    else
                        insert(ref root._right, x);
            }

            public int FindMin()
            {
                TreeNode current = root;

                while (current._left != null)
                    current = current._left;

                return current.n;
            }

            public int FindMax()
            {
                TreeNode current = root;

                while (current._right != null)
                    current = current._right;

                return current.n;
            }



            public TreeNode Find(int key)
            {
                TreeNode current = root;

                while (current.n != key)
                {
                    if (key < current.n)
                        current = current._left;
                    else
                        current = current._right;
                    if (current == null)
                        return null;
                }
                return current;
            }



            public void InOrder(ref TreeNode root)
            {
                if (root != null)
                {
                    InOrder(ref root._left);
                    root.DisplayNode();
                    InOrder(ref root._right);
                }
            }

            public int CountNodes()
            {
                int count = 1; // me!        
                if (root._left != null)
                    count += _left.CountNodes();
                if (root._right != null)
                    count += _right.CountNodes();
                return count;
            }

            public int CountLeaves()
            {
                int count = (root._left == null && root._right == null) ? 1 : 0;
                if (root._left != null)
                    count += _left.CountLeaves();
                if (root._right != null)
                    count += _right.CountLeaves();
                return count;
            }

            public static void print(TreeNode root)
            {
                if (root != null)
                {
                    print(root._left);
                    Console.WriteLine(root.n.ToString());
                    print(root._right);
                }

            }



        }

    }
}

Obtengo los siguientes errores:

Error 1 The name '_left' does not exist in the current context 

// on the countnodes & countleaves

Error 2 The name '_right' does not exist in the current context 
// on the countnodes & countleaves

¿Alguna idea sobre cómo puedo solucionar estos errores?