Stopwatch ไม่มีใน VC# 2010 หรอครับ (พิมพ์ตามหนังสือ)

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 StringBuilder
{
    public partial class StringBuilder : Form
    {
        public StringBuilder()
        {
            InitializeComponent();
        }

        private void cmdNormal_Click(object sender, EventArgs e)
        {
            Stopwatch sw = new Stopwatch();
            sw.Start();

            string strTest = "";
            for (int i = 1; i <= 10000; i++)
            {
                strTest += i.ToString();
            }
            sw.Stop();
            lblResult.Text = sw.ElapsedMilliseconds + "ms.";
        }

        private void cmdStringBuilder_Click(object sender, EventArgs e)
        {
            Stopwatch sw = new Stopwatch();
            sw.Start();

            StringBuilder sb = new StringBuilder();
            for (int i = 1; i <= 10000; i++)
            {
                sb.Append(i.ToString());
            }

            sw.Stop();
            lblResult.Text = sw.ElapsedMilliseconds + "ms.";
        }
    }
}