树的高度总是返回0

嗨，我有这个AVLTree，但我只是不能使高度方法工作。它总是给出零。你们谁能看一下，也许能发现我遗漏了什么。正如你所看到的，我尝试了不同的解决方案，但我不知道。那会节省时间的。谢谢!

     public class AVLTree2<TKey, TValue> : BinarySearchTree<TKey, TValue>
            where TKey : IComparable<TKey>
        {
    private AVLNode node;
     int count;
            /// <summary>
            /// Initializes a new instance of the AVLTree&lt;TKey, TValue> class that is empty.
            /// </summary>
            public AVLTree2() { node = null;}
            /// <summary>
            /// Initializes a new instance of the AVLTree&lt;TKey, TValue> class that contains elements copied from the specified IDictionary&lt;TKey, TValue>.
            /// </summary>
            /// <param name="collection">The IDictionary&lt;TKey, TValue> whose elements are copied to the new AVLTree&lt;TKey, TValue>.</param>
            public AVLTree2(IDictionary<TKey, TValue> collection) : base(collection)
            {
                foreach (KeyValuePair<TKey, TValue> t in collection)
                    this.Add(t.Key, t.Value);
            }
            /// <summary>
            /// Adds the specified key and value to the AVLTree&lt;TKey, TValue>.
            /// </summary>
            /// <param name="key">The key of the element to add.</param>
            /// <param name="value">The value of the element to add.</param>
            /// 
            public new int Count()
            {
                return count;
            }
            public override void Add(TKey key, TValue value)
            {
                AVLNode node = new AVLNode(key, value);
                base.Add(node);
                this.Rebalance(node.Parent);
                this.count++;
            }

            public override void Delete(TreeNode current)
            {
                base.Delete(current);
                this.Rebalance((AVLNode)current.Parent);
                this.count--;
            }
            private void Rebalance(AVLNode start)
            {
                int height, left, right, diff, leftC, rightC, diffC;
                while (start != null)
                {
                    height = start.Height;
                    left = start.Left != null ? start.Left.Height : -1;
                    right = start.Right != null ? start.Right.Height : -1;
                    diff = left - right;
                    if (diff == -2)
                    {
                        leftC = start.Right.Left != null ? start.Right.Left.Height : -1;
                        rightC = start.Right.Right != null ? start.Right.Right.Height : -1;
                        diffC = leftC - rightC;
                        if (diffC == 1)
                            this.RotateRight(start.Right);
                        this.RotateLeft(start);
                        start = start.Parent.Parent;
                        continue;
                    }
                    else if (diff == 2)
                    {
                        leftC = start.Left.Left != null ? start.Left.Left.Height : -1;
                        rightC = start.Left.Right != null ? start.Left.Right.Height : -1;
                        diffC = leftC - rightC;
                        if (diffC == -1)
                            this.RotateLeft(start.Left);
                        this.RotateRight(start);
                        start = start.Parent.Parent;
                        continue;
                    }
                    start.Height = Math.Max(left, right) + 1;
                    if (height == start.Height)
                        break;
                    else
                        start = start.Parent;
                }
            }
            protected void RotateLeft(AVLNode start)
            {
                base.RotateLeft(start);
                int left, right;
                left = start.Left != null ? start.Left.Height : -1;
                right = start.Right != null ? start.Right.Height : -1;
                start.Height = Math.Max(left, right) + 1;
                left = start.Height;
                right = start.Parent.Right != null ? start.Parent.Right.Height : -1;
                start.Parent.Height = Math.Max(left, right) + 1;
            }
            protected void RotateRight(AVLNode start)
            {
                base.RotateRight(start);
                int left, right;
                left = start.Left != null ? start.Left.Height : -1;
                right = start.Right != null ? start.Right.Height : -1;
                start.Height = Math.Max(left, right) + 1;
                left = start.Parent.Left != null ? start.Parent.Left.Height : -1;
                right = start.Height;
                start.Parent.Height = Math.Max(left, right) + 1;
            }
   public int Height()
            {
                return height(node);
            }
   public int height(AVLNode start)
   {
       AVLNode temp = start;
       if (temp == null)
           return 0;
       else if (start.Left == null && start.Right == null)
           return 0;
       else
           return 1 + Math.Max(height(start.Left), height(start.Right));


   }
            public class AVLNode : KeyValueTreeNode
            {
                public AVLNode(TKey key, TValue value) : base(key, value) { this.Height = 0; }
                public int Height { get; set; }
                public new AVLNode Parent { get { return (AVLNode)base.Parent; } set { base.Parent = value; } }
                public new AVLNode Left { get { return (AVLNode)base.Left; } set { base.Left = value; } }
                public new AVLNode Right { get { return (AVLNode)base.Right; } set { base.Right = value; } }
            }
        }
    }