本人扩展的可绑定Dataset的Treeview,遇到不能展开的问题

按以下代码扩展的TreeView,无论怎么设置ExpandDepth属性,都是折叠的,那位高手看一下是那的问题?如何解决?我想让其在默认情况下展开所有节点.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace SamWebControlLib
{
    [DefaultProperty("Text")]
    [ToolboxData("<{0}:PowerTreeView runat=server></{0}:PowerTreeView>")]
    public class PowerTreeView : TreeView
    {
        属性#region  属性
        private object _DataSource;
        [Bindable(true)]
        [Category("数据")]
        [DefaultValue("")]
        [Localizable(true)]
        [Description("数据源")]
        public override object DataSource
        {
            get
            {
                return _DataSource;
            }
            set
            {
                _DataSource = value;
            }
        }
        [Bindable(true)]
        [Category("数据")]
        [DefaultValue("")]
        [Localizable(true)]
        [Description("节点显示文本所在的字段")]
        public string DataTextField
        {
            get
            {
                String s = (String)ViewState["DataTextField"];
                return ((s == null) ? String.Empty : s);
            }
            set
            {
                ViewState["DataTextField"] = value;
            }
        }
        [Bindable(true)]
        [Category("数据")]
        [DefaultValue("")]
        [Localizable(true)]
        [Description("节点值所在的字段")]
        public string DataValueField
        {
            get
            {
                String s = (String)ViewState["DataValueField"];
                return ((s == null) ? String.Empty : s);
            }
            set
            {
                ViewState["DataValueField"] = value;
            }
        }
        [Bindable(true)]
        [Category("数据")]
        [DefaultValue("")]
        [Localizable(true)]
        [Description("上级节点值所在的字段")]
        public string ParentIDField
        {
            get
            {
                String s = (String)ViewState["ParentIDField"];
                return ((s == null) ? String.Empty : s);
            }
            set
            {
                ViewState["ParentIDField"] = value;
            }
        }
        [Bindable(true)]
        [Category("数据")]
        [DefaultValue("")]
        [Localizable(true)]
        [Description("根节点的值")]
        public string RootValue
        {
            get
            {
                String s = (String)ViewState["RootValue"];
                return ((s == null) ? String.Empty : s);
            }
            set
            {
                ViewState["RootValue"] = value;
            }
        }
        [Bindable(true)]
        [Category("数据")]
        [DefaultValue("")]
        [Localizable(true)]
        [Description("排序字段名")]
        public string SortField
        {
            get
            {
                String s = (String)ViewState["SortField"];
                return ((s == null) ? String.Empty : s);
            }
            set
            {
                ViewState["SortField"] = value;
            }
        }
        #endregion
        /**//// <summary>　绑定
        /// </summary>
        public void PowerTreeViewDataBind()
        {
            base.DataBind();
            this.Create_Tree(this.DataSource);
        }

        /**//// <summary>
        /// 构造树
        /// </summary>
        /// <param name="source"></param>
        private void Create_Tree(Object source)
        {
            DataTable dt = null;
            if ((source as DataSet) != null)
            {
                dt = ((DataSet)source).Tables[0];
            }
            else if ((source as DataTable) != null)
            {
                dt = ((DataTable)source);
            }
            else
            {
                throw new Exception("只允许绑定DataSet和DataTable");
            }
            if (dt != null && dt.Rows.Count > 0)
            {
                if (this.DataTextField == String.Empty) throw new ArgumentNullException("DataTextField属性不能为空!");
                if (this.DataValueField == String.Empty) throw new ArgumentNullException("DataValueField属性不能为空!");
                if (this.ParentIDField == String.Empty) throw new ArgumentNullException("ParentIDField属性不能为空!");
                if (this.RootValue == String.Empty) throw new ArgumentNullException("RootValue属性不能为空!");
                this.DataBindTreeView(null, dt, String.Format("{0}='{1}'", this.DataValueField, this.RootValue));
            }
        }
        /**//// <summary>
        /// 增加一级菜单
        /// </summary>
        /// <param name="CurrentNode"></param>
        /// <param name="dt"></param>
        /// <param name="sWhere"></param>
        private void DataBindTreeView(TreeNode CurrentNode, DataTable dt, string sWhere)
        {
            //string Text = "V_MENU_MC", Value = "V_MENU_DM", ToolTip = "V_MENU_EW", Menu_Top = "V_MENU_DM_SJ", NodeImg = "V_MENU_IMAGE", WHERE = "V_WHERE", Sort = "I_SORT", Url = "V_FILE";
            DataView dv = new DataView(dt);
            dv.RowFilter = sWhere;
            dv.Sort = SortField;
            //if (CurrentNode != null && CurrentNode.ImageUrl != "images/root.gif")
            //{
            //    if (dv.Count == 0)
            //    {
            //        CurrentNode.ImageUrl = "images/last.GIF";
            //    }
            //    else
            //    {
            //        CurrentNode.ImageUrl = "images/book.gif";
            //    }
            //}
            foreach (DataRowView row in dv)
            {
                TreeNode item = new TreeNode();
                if (row[this.DataTextField] != DBNull.Value) item.Text = row[this.DataTextField].ToString();
                //if (row[ToolTip] != DBNull.Value) item.ToolTip = row[ToolTip].ToString();
                string val = row[this.DataValueField].ToString();
                if (row[this.DataValueField] != DBNull.Value) item.Value = val;
                //if (row[Url] != DBNull.Value) item.NavigateUrl = row[Url].ToString();
                //if (row[WHERE] != DBNull.Value) item.NavigateUrl += row[WHERE].ToString();
                //item.Target = "main";

                if (CurrentNode == null)
                {
                    //item.ImageUrl = "images/root.gif";
                    this.Nodes.Add(item);
                }
                else
                {
                    CurrentNode.ChildNodes.Add(item);
                }

                string str = String.Format("{0}='{1}'", this.ParentIDField, row[this.DataValueField].ToString());
                //item..Style.Font.Name = "楷体_GB2312";
                //item.Style.Font.Size = new FontUnit("16px");
                //递归
                DataBindTreeView(item, dt, str);
            }
        }
        /**//// <summary>
        /// 获取节点的值
        /// </summary>
        /// <param name="node"></param>
        /// <returns></returns>
        public string GetNodeValue(TreeNode node)
        {
            if (node.Value == "01")
            {
                return "0"; 
            }
            else
            {
                return node.Value;
            }
        }
    }
}

使用item.Expanded = true;方法,只能在控件里面修改属性,以达到展开的显示.我的想法是在控件外面可以方便使用用ExpandDepth属性或ExpandAll方法,操作控件展开折叠.再次请教,有没有办法实现?
另,如果完善了些控件,别忘了通知我.我学习一下代码