递归树

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using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Collections.Generic;
using System.Data.SqlClient;
public partial class Default2 : System.Web.UI.Page
{
    IList<Category> list = new List<Category>();
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!this.IsPostBack)
        {
            SqlConnection con = new SqlConnection("server=.;uid=sa;pwd=;database=coll");
            SqlCommand cmd = new SqlCommand("select * from category", con);
            con.Open();
            SqlDataReader reader = cmd.ExecuteReader();
            while (reader.Read())
            {
                Category c = new Category();
                c.CategoryID = reader.GetInt32(0);
                c.ParentID = reader.GetInt32(1);
                c.Layer = reader.GetInt32(2);
                c.Name = reader.GetString(3);
                list.Add(c);
            }
            reader.Close();
            cmd.Dispose();
            con.Close();
            this.AddTreeNode(0, null);
        }
    }
    private void AddTreeNode(int parentId, TreeNode node)
    {
        List<Category> categorylist = new List<Category>();
        foreach (Category c in this.list)
        {
            if (c.ParentID == parentId)
                categorylist.Add(c);
        }
        if (categorylist.Count > 0)
        {
            foreach (Category cate in categorylist)
            {
                TreeNode tNode = new TreeNode();
                tNode.Text = cate.Name;
                if (node == null)
                {
                    this.TreeView1.Nodes.Add(tNode);
                }
                else
                {
                    node.ChildNodes.Add(tNode);
                }
                this.AddTreeNode(cate.CategoryID, tNode);
            }
        }
    }
}