BackgroundWorker bw = null;



        public Form3()

        {

            InitializeComponent();



            bw = new BackgroundWorker();

            bw.WorkerSupportsCancellation = true;

            bw.DoWork += new DoWorkEventHandler(bw_DoWork);

            bw.RunWorkerCompleted += new RunWorkerCompletedEventHandler(bw_RunWorkerCompleted);

        }



        void bw_DoWork(object sender, DoWorkEventArgs e)

        {

            for (int i = 1; i < 100; i++)

            {

                Thread.Sleep(1);

                if (bw.CancellationPending)

                {

                    e.Cancel = true;

                    break;

                }



                this.Invoke((MethodInvoker)delegate

                {

                    this.richTextBox1.Text += String.Format("第{0}次测试是否联网：{1}", i.ToString(), IsWWW("http://www.baidu.com?guid=" + Guid.NewGuid())) + Environment.NewLine;

                });

                Thread.Sleep(2000);

            }

        }



        void bw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)

        {

            this.richTextBox1.Text += "执行完毕";

        }



        private void button1_Click(object sender, EventArgs e)

        {

            this.button1.Enabled = false;

            this.button2.Enabled = true;