为什么没有输出全部的值序列？

Question

《Visual C# 2010 Step by Step》中文版
，在P356第19章19.2.2 使用迭代器为Tree<TItem>类定义枚举器的时候，按书中的代码敲入后，调试执行后输出“5,10,11,15”而不是书中的“-12，-8,0,5,5,10,10,11,14，15”；
代码如下：两个cs文件：一个tree.cs；一个是Program.cs
namespace BinaryTree
{
    public class Tree<TItem> : IEnumerable<TItem> where TItem : IComparable<TItem>
    {
        public Tree(TItem nodeValue)
        {
            this.NodeData = nodeValue;
            this.LeftTree = null;
            this.RightTree = null;
        }
        public void Insert(TItem newItem)
        {
            TItem currentNodeValue = this.NodeData;
            if (currentNodeValue.CompareTo(newItem) > 0)
            {
                if (this.LeftTree == null)
                {
                    this.LeftTree = new Tree<TItem>(newItem);
                }
                else
                {
                    this.LeftTree.Insert(newItem);
                }
            }
            else
            {
                if (this.RightTree == null)
                {
                    this.RightTree = new Tree<TItem>(newItem);
                }
                else
                {
                    this.RightTree.Insert(newItem);
                }
            }
        }
        public void WalkTree()
        {
            if (this.LeftTree != null)
            {
                this.LeftTree.WalkTree();
            }
            Console.WriteLine(this.NodeData.ToString());
            if (this.RightTree != null)
            {
                this.RightTree.WalkTree();
            }
        }
        public TItem NodeData { get; set; }
        public Tree<TItem> LeftTree { get; set; }
        public Tree<TItem> RightTree { get; set; }
        #region IEnumerable<TItem> Members
        IEnumerator<TItem> IEnumerable<TItem>.GetEnumerator()
        {
            if (this.LeftTree != null)
            {
                foreach (TItem item in this.LeftTree)
                {
                    yield return item;
                }
            }
            yield return this.NodeData;
            if (this.RightTree != null)
            {
                foreach (TItem item in this.RightTree)
                {
                    yield return item;
                }
            }
        }
        #endregion
        #region IEnumerable Members
        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
        {
            throw new NotImplementedException();
        }
        #endregion
    }
}class Program
    {
        static void Main(string[] args)
        {
            Tree<int> tree1 = new Tree<int>(10);
            tree1.Insert(5);
            tree1.Insert(11);
            tree1.Insert(5);
            tree1.Insert(-12);
            tree1.Insert(15);
            tree1.Insert(0);
            tree1.Insert(14);
            tree1.Insert(-8);
            tree1.Insert(10);
            foreach (int item in tree1)
                Console.WriteLine(item);
        }
    }

Answer 1

奇怪了，我把代码复制到一个cs文件里也能正常输出，但是下图大家可以看到tree1里的结果视图只有4个元素，所以最后只输出了4个值，代码都是复制黏贴的，只是图片上分别位于不同的项目里面，难道是引用错了？大家经验丰富，看是什么问题啊?