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);

        }

    }

}