页面指令中出现奇怪的运行时错误

这是页面指令：

<%@ Page Title="Edit Condition" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeBehind="ManageCondition.aspx.cs" Inherits="Backyard_Cures.ManageCondition" %>

那行抛出了这个运行时错误：

 Cannot implicitly convert type 'string' to 'System.Web.UI.WebControls.TextBox'

aspx文件：

<%@ Page Title="Edit Condition" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeBehind="ManageCondition.aspx.cs" Inherits="Backyard_Cures.ManageCondition" %>
<asp:Content ID="Content1" ContentPlaceHolderID="HeadContent" runat="server">
    <link href="Styles/ManageCondition.css" rel="stylesheet" type="text/css" />
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
    <div class="condition_block">
        <div class="input">
            <div class="label">Title</div>
            <asp:TextBox Text="" CssClass="textbox" ID="Title" runat="server"></asp:TextBox>
        </div>
        <div class="help">Text</div>
    </div>
    <br />
    <div class="condition_block">
        <div class="input">
            <div class="label">Description</div>
            <asp:TextBox Text="" CssClass="textbox" ID="Description" runat="server"></asp:TextBox>
        </div>
        <div class="help">Text</div>
    </div>
    <br />
    <div class="condition_block">
        <div class="input">
            <div class="label">Condition Tags</div>
            <asp:TextBox Text="" CssClass="textbox" ID="Tags" runat="server"></asp:TextBox>
        </div>
        <div class="help">Text</div>
    </div>
    <br />         
    <asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" />
</asp:Content>

代码隐藏文件：

namespace Backyard_Cures
{
    public partial class ManageCondition : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
        }
        protected void Button1_Click(object sender, EventArgs e)
        {
            Condition new_condition = new Condition();
            new_condition.Title = Title.Text;
            new_condition.Content = Description.Text;
            int new_conditionID = Backyard_Cures.AccessLayer.InsertCondition(new_condition);
            Response.Redirect("~/ViewCondition.aspx?ConditionID=" + new_conditionID, true);
        }
    }
}

主文件代码背后：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
namespace WebApplication2
{
    public partial class SiteMaster : System.Web.UI.MasterPage
    {
        protected void Page_Load(object sender, EventArgs e)
        {
        }
    }
}