ASP.NET控件的下拉列表属性出错

最近在学习写控件，从最简单的日期选择器开始。
共有如下几个属性：
Minyear:最小年份
Maxyear:最大年份
Theme:样式

代码如下：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Web;
using System.Drawing;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;

namespace Calendar
{
    [ToolboxData("<{0}:calendar runat=\"server\"></{0}:calendar>")]
    public class calendar : TextBox
    {
        private int _minyear = 1900;
        private int _maxyear = 2999;

        [Browsable(true)]
        [Description("最小年份")]
        [DefaultValue("1900")]
        [Category("设定")]
        public virtual int Minyear
        {
            get { return _minyear; }
            set { _minyear = value; }
        }
        [Browsable(true)]
        [Description("最大年份")]
        [DefaultValue("2999")]
        [Category("设定")]
        public virtual int Maxyear
        {
            get { return _maxyear; }
            set { _maxyear = value; }
        }
        [Browsable(true)]
        [Description("样式设置")]
        [DefaultValue(0)]
        [Category("设定")]
        public calendarstyle Theme
        {
            get
            {
                if (this.ViewState["calendarstyle"] == null)
                {
                    return calendarstyle.blue;
                }
                return (calendarstyle) this.ViewState["calendarstyle"];
            }
            set
            {
                this.ViewState["calendarstyle"] = value;
            }
        }

        protected override void OnPreRender(EventArgs e)
        {
            this.Page.PreRenderComplete += new EventHandler(Page_PreRender);
            this.Attributes.Add("onFocus", "return getcalendar('" + this.ID + "','" + this.ID + "','','" + Minyear + "','" + Maxyear + "')");
            base.OnPreRender(e);
        }
        void Page_PreRender(object sender, EventArgs e)
        {
            Page.ClientScript.RegisterClientScriptResource(this.GetType(), "Calendar.javascript.calendar.js");
            Page.ClientScript.RegisterClientScriptResource(this.GetType(), "Calendar.javascript.cn_utf8.js");
            Page.ClientScript.RegisterClientScriptResource(this.GetType(), "Calendar.javascript.calendar-setup.js");
            RegCss("Calendar.style." + Theme + ".css");
        }
        /// <summary>
        /// 注册CSS样式表
        /// </summary>
        /// <param name="Path"></param>
        private void RegCss(string Path)
        {
            HtmlGenericControl css = new HtmlGenericControl("link");
            css.Attributes.Add("href", Page.ClientScript.GetWebResourceUrl(typeof(calendar), Path));
            css.Attributes.Add("type", "text/css");
            css.Attributes.Add("rel", "stylesheet");
            Page.Header.Controls.Add(css);
        }
    }
    public enum calendarstyle
    {
        blue,
        brown,
        green,
        system,
        tas,
        win2k,
        win2k_cold
    }
}

不设置属性的时候，也就是默认属性的时候正常:<Fting:calendar ID="Calendar1" runat="server"></Fting:calendar>

可是选择属性后<Fting:calendar ID="Calendar1" runat="server" Theme="win2k"></Fting:calendar>就会报如下错误:
CS0117: “System.Web.UI.WebControls.Calendar”并不包含“calendarstyle”的定义

网上的资料太少了，不知道这个问题该怎么解决。