using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

[assembly: TagPrefix("WebApplication", "MyControl")]
namespace WebApplication1
{
    
    [ToolboxData("<{0}:LinkedDropDownList runat=server></{0}:LinkedDropDownList>")]
    public class LinkedDropDownList : DropDownList//继承自基本的DropDownList
    {
        private DropDownList _linkedDropDownList;

        public LinkedDropDownList()
        {
            this.SelectedIndexChanged += new EventHandler(LinkedDropDownList_SelectedIndexChanged);//添加自己的SelectedIndexChanged事件
            this.AutoPostBack = true;//强制设置自己的AutoPostBack为true
        }
        //下面两个事件都作为获取数据源的事件
        public event EventHandler BindData;//这个事件为绑定父DropDownList
        public event Action<object, DropDownListIndexChangeEventArg> BindLinkedData;//这个事件为绑定子DropDownList

        protected void LinkedDropDownList_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (this.BindLinkedData != null)
            {
                this._linkedDropDownList = this.NamingContainer.FindControl(this.NextDropDownListID) as DropDownList;//从控件容器中找出子下拉菜单
                var newE = new DropDownListIndexChangeEventArg(this.SelectedValue, this.SelectedItem.Text, this.SelectedIndex,this._linkedDropDownList);
                BindLinkedData(this, newE);
            }
            //如果子DropDownList本身也为联动DropDownList 执行子DropDownList的绑定
            var next = this.NamingContainer.FindControl(this.NextDropDownListID) as LinkedDropDownList;
            if (next != null)
            {
                next.OnSelectedIndexChanged(EventArgs.Empty);
            }
        }
        //重写OnLoad使得可以发生数据绑定
        protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);
            if (this.BindData != null && !Page.IsPostBack)
            {
                BindData(this, EventArgs.Empty);
                this.SelectedIndex = 0;
                
            }
            if (this.BindLinkedData != null &&!Page.IsPostBack)
            {
                this._linkedDropDownList = this.NamingContainer.FindControl(this.NextDropDownListID) as DropDownList;
                var newE = new DropDownListIndexChangeEventArg(this.SelectedValue, this.SelectedItem.Text, this.SelectedIndex, this._linkedDropDownList);
                BindLinkedData(this, newE);
            }
        }
        //子DropDownList的ID
        public string NextDropDownListID
        {
            get
            {
                return ViewState[this.ID + "LinkedDropDownList"] as string;//加上自身的ID做唯一标示
            }
            set
            {
                ViewState[this.ID + "LinkedDropDownList"] = value;
               
            }
        }
    }
    //事件参数 方便绑定子菜单时候得到数据源绑定参数
    public class DropDownListIndexChangeEventArg : EventArgs
    {
        private readonly string _selectedValue = null;
        private readonly string _selectedText = null;
        private readonly int _selectedIndex;
        private readonly DropDownList _nextDropDownList = null;

        public DropDownListIndexChangeEventArg(string selectedValue, string selectedText, int selectedIndex,DropDownList nextDropDownList)
        {
            this._selectedValue = selectedValue;
            this._selectedText = selectedText;
            this._selectedIndex = selectedIndex;
            this._nextDropDownList = nextDropDownList;
        }

        public string SelectedValue
        {
            get
            {
                return this._selectedValue;