Question

Please help me find the defect in my logic. I have two variables named “prev” and “next”…What i am basically doing is reading the data from my database every 5s and printing it out using Websync server if next and prev are NOT equal. I have two rows in my database . It looks like

ID
8
10

Here is the link to the code http://pastebin.com/Hb3eH2Qv

When i run my program, i get the result as

8 10 8 10
8 10
8 10 8 10
8 10

….. (so on)

But, the result should be just

8 10

I dont know how 8 10 8 10 appears. Data gets concatenated twice.

NOTE: You can just see the code in PublishLoop() function

private void PublishLoop()
        {
            String prev=String.Copy("");
            String next=String.Copy("");
            String ConnectionString = ConfigurationManager.ConnectionStrings["MyDbConn"].ToString();
            SqlConnection connection = new SqlConnection(ConnectionString);
            SqlCommand command = connection.CreateCommand();
            command.CommandText = "select ID from Tab1";
            command.Notification = null;

            while (Running)
            {
                connection.Open();
                using (SqlDataReader reader = command.ExecuteReader(CommandBehavior.CloseConnection))
                {
                    StreamWriter sw1 = new StreamWriter("C:\\Users\\Thothathri\\Desktop\\next.txt");
                    while ((reader.Read()))
                    {
                        //Response.Write(reader[0].ToString());
                        next = String.Concat(next,reader[0].ToString());
                        sw1.WriteLine(next);

                    }
                    sw1.Close();


                    if (!prev.Equals(next))
                    {
                        Publisher publisher = new Publisher(new PublisherArgs
                        {
                            DomainKey = "c80cb405-eb77-4574-9405-5ba51832f5e6",
                            DomainName="localhost"
                        });
                        Publication publication = publisher.Publish("/test", JSON.Serialize(next));
                        if (publication.Successful == true)
                        {
                            StreamWriter sw = new StreamWriter("C:\\Users\\Thothathri\\Desktop\\error123.txt");
                            sw.WriteLine("success");
                            sw.WriteLine(next);
                            sw.Close();
                        }
                        else
                        {
                            StreamWriter sw = new StreamWriter("C:\\Users\\Thothathri\\Desktop\\error123.txt");
                            sw.Write("failed");
                            sw.Close();
                        }
                        prev = String.Copy(next);
                        next = String.Copy("");
                    }


                }
                Thread.Sleep(5000);

            }
        }

Answer 1

Renuiz answered it in a comment, but it is because you’re not clearing next.

So you build the string “8 10” in next, store it in prev. Next time you concat “8 10” with next, making “8 10 8 10”. Which is different so you print it.

                if (!prev.Equals(next))
                {
 ....
                    prev = String.Copy(next);
                    next = String.Copy("");
                }

This is the end of that loop. You should really be clearing next at the beginning of that loop.

Also you can just set the string

next = String.Empty;

I would declare next inside your while loop, as you don’t need it in the greater scope, and I would call it current rather than next.