此类错误@ client.DownloadFile不支持此方法

Question

我突然说System.Net.WebException : The request was abborted: The Request was canceled. ---> System.NotSupportedException : This method is not supported by this class这个奇怪的错误。我不知道为什么会发生因为它曾经工作得很好。

我尝试在线搜索，但没有找到有关此事的帮助。以下是我正在使用的方法，我已经指出了发出错误的行。

    //get column heading names from formatted text file
    public string getFTextColumnHead(string serverURL, string userName, string password, int headRow,
                                     string filePath, string reqFrom)
    {
        string line = "";
        //initialize WebClient object
        WebClient client = new WebClient();
        //if the request is for web
        if (reqFrom == "web")
        {
            //send web request to retreive the data from the url
            HttpWebRequest req = (HttpWebRequest) WebRequest.Create(serverURL);
            //parse credentials to the web server to authenticate
            client.Credentials = new NetworkCredential(userName, password);
            //Grab Data from the web server and save to the given path
            client.DownloadFile(new Uri(serverURL), filePath);//<---exception occurs here
        }
        //initialize input output stream and open the downloaded file
        StreamReader file = new StreamReader(filePath);
        //loop through the rows of the file until the column heading row is reached
        for (int i = 0; i < headRow; i++)
        {
            //read the line
            line = file.ReadLine();
        }
        //dipoase the client
        client.Dispose();
        //return the column heading string
        return line;
    }

谢谢你们。我正在关闭这个项目的截止日期。所有这一切都发生在最后一刻。非常感谢帮助：）

P.S。我安装了Resharper来查看代码。但是我并没有对这个特殊的班级做出任何改变：）。