C#窗体控件实现一个多线程序出现了点小问题，、请大家指点一下

刚刚学习C#，想创建一个时间的窗口，但是运行不了说是Thread没有声明，不知道怎么改，下面是程序的具体，请大神们看看 谢谢
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace 多线运行
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
delegate void SetTextCallback(string text);

    private void button1_Click(object sender, EventArgs e)
    {
        Thread t = new Thread(new ThreadStart(Show));
        t.Start();
    }
    private void SetText(string text)
    {
        if (textBox1.InvokeRequired)
        {
            SetTextCallback d = new SetTextCallback(SetText);
            this.Invoke(d, new object[] { text });
        }
        else
        {
            textBox1.Text = text;
        }
    }


    private void Show()
    {
        for (int i = 0; i < 500; i++)
        {
            SetText(DateTime.Now.ToString());
            Thread.Sleep(100);
        }
    }
}

}

using System.Threading;