为什么Visual Studio不了解“线程”？

我包括using System.Windows;但线程仍然在名称空间中不存在吗？

            using System;
        using System.Collections.Generic;
        using System.Linq;
        using System.Web;
        using System.Web.UI;
        using System.Web.UI.WebControls;
        using System.Data.SqlClient;
        using System.Configuration;
        using System.Data;
        using System.Net.Mail;
        using System.Net;
        using System.ComponentModel;
        using System.Drawing;
        using System.Text;
        using System.Windows;
        using System.Timers;
        using Graduation_Project;




        namespace GP
        {
            public partial class Register : System.Web.UI.Page
            {
                static int count = 0;
                static int timer = 600;
                protected void Page_Load(object sender, EventArgs e)
                {

                    System.Windows.Threading.DispatcherTimer dt = new System.Windows.Threading.DispatcherTimer();
                    dt.Interval = new TimeSpan(0, 0, 0, 0, 600000); // 600000 Milliseconds
                    dt.Tick += new EventHandler(dt_Tick); 

                }

仍然有问题