//using System.ComponentModel;
private void Form1_Load(object sender, EventArgs e)
{
    using (BackgroundWorker bgWorker = new BackgroundWorker())
    {
        int i = 0;

        //将会引起界面卡死的操作放在单独线程中执行
        bgWorker.DoWork += delegate(object sender1, DoWorkEventArgs e1)
        {
            foreach (string item in Directory.GetFiles(@"E:\www", "*.*", SearchOption.AllDirectories))
            {
                File.AppendAllText(@"E:\list.txt", item + "\r\n");
                i++;
            }
        };

        //在 RunWorkerCompleted 中操作线程间控件,这是允许的
        bgWorker.RunWorkerCompleted += delegate(object sender1, RunWorkerCompletedEventArgs e1)
        {
            textBox1.Text = string.Format("统计完成,总共{0}个文件.", i);
        };

        //开始执行
        bgWorker.RunWorkerAsync();
    }
}