Question

I am new to both C# and to client-server programming. Right now, for class, I’m attempting to create an FTP client without using any pre-established FTP libraries. I feel like I have the project down for the most part, however I’m running into a problem when I make more than one call that requires use of the data port (list, retr, etc.) Here is a sample of the code that is breaking:

writer.WriteLine(portcmd);
writer.Flush();

GetServerMessage(stream);

writer.WriteLine("list ");
writer.Flush();


tmpserver = new TcpListener(IPAddress.Any, 3128);
tmpserver.Start(); 
tmpclient = tmpserver.AcceptTcpClient();
Console.WriteLine("gothere");

if (!tmpclient.Connected)
{
    tmpserver.Start();
}

StreamReader tmpreader = new StreamReader(tmpclient.GetStream());

GetServerMessage(stream);
while (tmpreader.Peek() != -1)
{
    Console.WriteLine(tmpreader.ReadLine());
}

tmpclient.Close();
tmpserver.Stop();

GetServerMessage(stream);

Getservermessage is a method that takes a network stream and prints out everything available within a .5 second timeout, stream is the NetworkStream for the current connection to the FTP server, and writer is that same network stream wrapped in a StreamReader for ease of writing ASCII characters to the server. In case you are wondering why I use a stream reader to read from the data connection, it is because the server closes the connection after it transmits the data so I could easily get an eof notification. My GetServerMessage method was for some reason breaking when I used the closed network stream.

This code is sending the port command to the FTP server to inform it that I will be requiring a data connection (first 2 lines) Then sending the list command, establishing the data connection to the server, getting the desired information, and then terminating the data connection (the rest of the code).

This code will execute without flaw the first time I run it but if I try it again, it hangs on the ‘tmpclient = tmpserver.AcceptTcpClient();’ line. It never reaches the “gothere” print statement. I believe this is because I am receiving the client from the same machine on the same port but I’m not sure. I tried adding a Boolean value to make sure the AcceptTcpClient() only ran once but then I got a runtime error and visual studio informed me that I may have ‘released resources before I was done with them’ I predicted this would be a problem but how can I tell if the server reestablishes the connection after it has closed it once?

At the end of the given code I stop tmpserver and close tmpclient. I originally did this because I knew the FTP server would close the connection when it was finished transmitting and thought it was the proper thing to do. I find if I comment out these lines, the code will execute more than once but the streams appear to be empty… I’m not sure if this information is helpful but I figured I’d mention it.

I apologize if I am unclear at all but my lack of knowledge with the subject makes it difficult to articulate my problem. If there is any confusion over what the problem is I’d be happy to attempt to clear it up.

Answer 1

To be able to accept another client you should execute tmpclient = tmpserver.AcceptTcpClient(); and waiting for the first client to finish its works(before accepting second client) may not be a good idea

Here is a sample server code that waits for the connections and echoes strings sent from each client. You can test it with telnet localhost 3128

Thread t = new Thread(Server);
t.IsBackground = true;
t.Start();

–

void Server()
{
    TcpListener listener = new TcpListener(IPAddress.Any, 3128);
    listener.Start();
    while (true)
    {
        var client = listener.AcceptTcpClient();
        new Thread(() =>
        {
            using (client)
            {
                var reader = new StreamReader(client.GetStream());
                var writer = new StreamWriter(client.GetStream());
                while (true)
                {
                    string line = reader.ReadLine();
                    if (line == "QUIT") break;
                    writer.WriteLine("From Thread[" + Thread.CurrentThread.ManagedThreadId + "] > " + line);
                    writer.Flush();
                }
            }
        }).Start();
    }
}