WebClient仅下载一次源页面。请帮助C＃

我想下载HTML源代码并从中提取内容，但是Webclient只下载一次， 第二次它不起作用，直到我退出程序并再次重新启动它。

这是下载功能，我在一个按钮中调用了它：

public static async Task DownF(string[] Urls)
        {

            WebClient KeyClient = new WebClient();




            try
            {
                await Task.Run(() =>
                {

                    const string pattern = "<span.*?>(.*?)<\\/span>";


                    for (int i = 0; i < 3; i++)
                    {
                        while (KeyClient.IsBusy)
                        {
                            System.Threading.Thread.Sleep(1000);
                        }

                        string page = KeyClient.DownloadString(Urls[i]);

                        MatchCollection matchs = Regex.Matches(page, pattern);
                        string path = Environment.GetFolderPath(Environment.SpecialFolder.Desktop);

                        if (matchs.Count > 0)
                        {
                            StreamWriter wrt = new StreamWriter(path + "\\results.txt");


                            int KeyWordCounter = 0;
                            foreach (Match m in matchs)
                            {
                                KeyWordCounter += 1;

                                wrt.WriteLine(KeyWordCounter + "-" + m.Groups[1].Value);

                            }
                            wrt.Close();
                        }

                    }

                    MessageBox.Show("finich!");
                });
            }
            catch(Exception e)
            {
                MessageBox.Show(e.Message);

            }

// *********按钮中的呼叫功能

string site1 = "www.site1.com";
                string site2 = "www.site2.com";
                string site3 = "www.site3.com";

                string [] Urls = new string[3];
                Urls[0] = site1;
                Urls[1] = site2;
                Urls[2] = site3;
                 DownF(Urls);