FtpWebRequest request = (FtpWebRequest)WebRequest.Create("ftp://url:port/in/test.txt");
            request.Method = WebRequestMethods.Ftp.UploadFile;
            request.Credentials = new NetworkCredential("login", "password");
            StreamReader sourceStream = new StreamReader("testfile.txt");
            byte[] fileContents = Encoding.UTF8.GetBytes(sourceStream.ReadToEnd());
            sourceStream.Close();
            request.ContentLength = fileContents.Length;

            Stream requestStream = request.GetRequestStream();//здесь ОШИБКА
      
            requestStream.Write(fileContents, 0, fileContents.Length);
            requestStream.Close();

            FtpWebResponse response = (FtpWebResponse)request.GetResponse();

            response.Close();