用流读写文本

我得到了一个c# windows窗体应用程序，它从sharepoint读取文本文件，然后做一些修改，然后它应该更新。听起来很简单，但我有点失去了流/凭证的使用。到目前为止，我能够阅读没有任何问题(所以没有凭据问题)，但当尝试这样做时:

System.IO.StreamWriter file2 = new System.IO.StreamWriter(s);

这是我遇到的错误:

• n{"流不可写。"}系统。异常{系统。ArgumentException}

                        WebRequest request = WebRequest.Create(rutaCoinsDiscount);
                    request.Timeout = 30 * 60 * 1000;
                    request.UseDefaultCredentials = true;
                    request.Proxy.Credentials = request.Credentials;
                    WebResponse response = (WebResponse)request.GetResponse();
                    using (Stream s = response.GetResponseStream())
                        if (s != null)
                        {
                            string linea;
                            System.IO.StreamReader file = new System.IO.StreamReader(s);
                            while ((linea = file.ReadLine()) != null)
                            {
                                coinsLines[contador] = linea;
                                contador++;
                                if (linea != "") {
                                    lastline++;
                                }
                            }
                            file.Close();
                            int index2 = coinsLines[1].IndexOf(":") + 1;
                            string Gcoins = coinsLines[1].Substring(index2);
                            giveBalanceOld = Convert.ToInt32(Gcoins);
                            giveBalanceOld = giveBalanceOld - giveAmount;
                            coinsLines[1] = "GIVE:" + giveBalanceOld.ToString();
                            coinsLines[lastline] = DateTime.Today.ToString("d") + "+GIVE to:+" + destination + "+Coins: " + giveAmount;
                            System.IO.StreamWriter file2 = new System.IO.StreamWriter(s);
                            for (int j = 0; j < lastline; j++)
                            {
                                file2.WriteLine(coinsLines[j]);
                            }
                            file2.Close();