将有层级关系的DataTable改为Tree

     一般递归都要每层去查库，这样不仅浪费了连接、断开、处理数据库的资源，而且极大的消耗了计算时间，有了Linq之后，直接分析数据集合的父子层级关系，不仅提高了速度，更是可以将处理的的过程抽象出来实现复用。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Linq;
using System.Text;
using System.Xml.Linq;


namespace program
{
    /// <summary>
    /// 将DataTable改为Tree
    /// </summary>
    class DataTableToTree
    {
        DataTable dt;
        string parentField;
        string refID;
        string firstLever;
        StringBuilder sb = new StringBuilder("<root>");
        List<string> columnList = new List<string>();

        /// <summary>
        /// 初始化
        /// </summary>
        /// <param name="dt">数据表</param>
        /// <param name="parentField">关联字段[父]</param>
        /// <param name="refID">关联字段[子]</param>
        /// <param name="firstLever">顶级目录的父级值</param>
        public DataTableToTree(DataTable dt, string parentField, string refID,string firstLever="0")
        {
            this.dt = dt;
            this.parentField = parentField;
            this.refID = refID;
            this.firstLever = firstLever;
            fillColumn();
        }

        public XElement getXml()
        {
            if (dt.Rows.Count == 0 || parentField.Trim() == "" || refID.Trim() == "")
                return null;
            doLoop(firstLever);
            return XElement.Parse(sb.Append("</root>").ToString());
        }

        private void doLoop(string parent)
        {
            var list = from r in dt.AsEnumerable()
                       where r[parentField].ToString() == parent
                       select r;
            foreach (var r in list)
            {
                sb.Append(getXmlNode(r));
                doLoop(r[refID].ToString());
                sb.Append("</item>");
            }
        }

        private string  getXmlNode(DataRow r)
        {
            StringBuilder sb_=new StringBuilder("<item ");
            foreach (string columnName in columnList)
                sb_.Append(columnName + "='" + r[columnName].ToString() + "' ");
            return sb_.Append(">").ToString();
        }

        private void fillColumn()
        {
            foreach (DataColumn c in dt.Columns)
                columnList.Add(c.ColumnName);
        }
    }
}