System.Threading.Tasks不包含方法Wait()；

在此处输入代码

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Threading;
namespace Threeeead
{
 class ThreadDemo
 {      
    public void Print1(Object obj1)
    {
        while (true)
        {
            Console.Write("1");
        }
    }
    public void Print2()
    {
        while(true)
        {
            Console.Write("2");
        }
    }
    static void Main(string[] args)
    {           
        ThreadDemo objDemo= new ThreadDemo();
       // Thread firstThread = new Thread (new ThreadStart(objDemo.Print1));
        //firstThread.Start();
       Thread secondThread = new Thread(new ThreadStart(objDemo.Print2));
       secondThread.Start();
        Thread thirdThread = new Thread(new ThreadStart (delegate()
        {
            while(true)
            Console.Write("XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX");
        }));
       thirdThread.Start();
       ThreadPool.QueueUserWorkItem(new WaitCallback(objDemo.Print1));
      // ThreadPool.QueueUserWorkItem(new WaitCallback(objDemo.Print2));

       secondThread.Wait();

嗨，我的问题是，为什么在最后一行我有错误，上面写着："错误1"System.Threading.Thread"不包含"Wait"的定义，也找不到接受类型为"System.Threadion.Thread"的第一个参数的扩展方法"Wait"（是否缺少using指令或程序集引用？）

看起来，此方法未包含在Sys.Thr.Task.命名空间中。

谢谢你抽出时间。