DropDownList 年月日联动

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

namespace Text
{
    public partial class InsusDate : System.Web.UI.UserControl
    {
        private int? _year;
        private int? _month;
        private int? _day;

        public int? Year
        {
            get
            {
                if (ViewState["Year"] != null)
                {
                    return Convert.ToInt32(ViewState["Year"]);
                }

                if (this.DropDownListYear.SelectedIndex == -1 || string.IsNullOrEmpty(this.DropDownListYear.SelectedItem.Value))
                    return null;
                else
                    return Convert.ToInt32(this.DropDownListYear.SelectedItem.Value);
            }
            set
            {
                ViewState["Year"] = value;
            }
        }

        public int? Month
        {
            get
            {
                if (ViewState["Month"] != null)
                {
                    return Convert.ToInt32(ViewState["Month"]);
                }
                if (this.DropDownListMonth.SelectedIndex == -1 || string.IsNullOrEmpty(this.DropDownListMonth.SelectedItem.Value))
                    return null;
                else
                    return Convert.ToInt32(this.DropDownListMonth.SelectedItem.Value);

            }
            set
            {
                ViewState["Month"] = value;
            }
        }

        public int? Day
        {
            get
            {
                if (ViewState["Day"] != null)
                {
                    return Convert.ToInt32(ViewState["Day"]);
                }
                if (this.DropDownListDay.SelectedIndex == -1 || string.IsNullOrEmpty(this.DropDownListDay.SelectedItem.Value))
                    return null;
                else
                    return Convert.ToInt32(this.DropDownListDay.SelectedItem.Value);
            }
            set
            {
                ViewState["Day"] = value;
            }
        }

        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                Data_Binding();
            }
        }
        /// <summary>
        /// 数据的绑定
        /// </summary>
        private void Data_Binding()
        {
            DropDownlistParse(this.DropDownListYear, i_Year.Select(y => new { v = y }).ToList(), "v", "v");
            DropDownlistParse(this.DropDownListMonth, i_Month.Select(m => new { v = m }).ToList(), "v", "v");
            this.DropDownListDay.Items.Insert(0, new ListItem(string.Empty, string.Empty));
        }

        protected void DropDownListYear_SelectedIndexChanged(object sender, EventArgs e)
        {
            DayDataBinding();
        }

        protected void DropDownListMonth_SelectedIndexChanged(object sender, EventArgs e)
        {
            DayDataBinding();
        }
        /// <summary>
        /// 在年或月DropDownlist控件有异动时，作相应对日的DropDownList控件数据绑定：
        /// </summary>
        private void DayDataBinding()
        {
            if (this.DropDownListYear.SelectedIndex == -1 || string.IsNullOrEmpty(this.DropDownListYear.SelectedItem.Value))
                return;
            if (this.DropDownListMonth.SelectedIndex == -1 || string.IsNullOrEmpty(this.DropDownListMonth.SelectedItem.Value))
                return;

            int y = Convert.ToInt32(this.DropDownListYear.SelectedItem.Value);
            int m = Convert.ToInt32(this.DropDownListMonth.SelectedItem.Value);
            ///i_Day(y, m).Select(d => new { v = d }).ToList()把原来的1  转为 v=1,这个数据源就有了v属性
            DropDownlistParse(this.DropDownListDay, i_Day(y, m).Select(d => new { v = d }).ToList(), "v", "v");
        }
        /// <summary>
        /// DropDownLIst数据绑定
        /// </summary>
        /// <param name="ddl">DropDownList</param>
        /// <param name="dataSource">数据源</param>
        /// <param name="dataTextField"></param>
        /// <param name="dataValueField"></param>
        private void DropDownlistParse(DropDownList ddl, object dataSource, string dataTextField, string dataValueField)
        {
            ddl.DataSource = dataSource;
            ddl.DataTextField = dataTextField;
            ddl.DataValueField = dataValueField;
            ddl.DataBind();
            ddl.Items.Insert(0, new ListItem(string.Empty, string.Empty));
        }

        List<int> i_Year
        {
            get
            {
                List<int> y = new List<int>();
                int Ny = DateTime.Now.Year;
                for (int i = 1953; i <= Ny; i++)
                {
                    y.Add(i);
                }
                return y;
            }
        }

        List<int> i_Month
        {
            get
            {
                List<int> m = new List<int>();
                for (int i = 1; i <= 12; i++)
                {
                    m.Add(i);
                }
                return m;
            }
        }
        /// <summary>
        /// 根据年月获得Day的List
        /// </summary>
        /// <param name="year"></param>
        /// <param name="month"></param>
        /// <returns></returns>
        public List<int> i_Day(int year, int month)
        {
            int days = DateTime.DaysInMonth(year, month);
            List<int> d = new List<int>();

            for (int i = 1; i <= days; i++)
            {
                d.Add(i);
            }
            return d;
        }
    }
}

控件的前端页面是

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="InsusDate.ascx.cs" Inherits="Text.InsusDate" %>
<asp:DropDownList ID="DropDownListYear" runat="server" AutoPostBack="true" OnSelectedIndexChanged="DropDownListYear_SelectedIndexChanged"
    Width="60px">
</asp:DropDownList>
年<asp:DropDownList ID="DropDownListMonth" runat="server" AutoPostBack="true" OnSelectedIndexChanged="DropDownListMonth_SelectedIndexChanged"
    Width="40PX">
</asp:DropDownList>
月<asp:DropDownList ID="DropDownListDay" runat="server" Width="40PX">
</asp:DropDownList>
日

 用户控件，当使用的时候，拖到页面就可以了