Question

I’ve been doing a lot of research on how best to write ‘correct’ network code in C#.

I’ve seen a number of examples using the ‘using’ statement of C#, and I think this is a good approach, however i’ve seen inconsistent use of it with various expressions.

For instance, suppose I have some code like this:

TcpClient tcpClient = new TcpClient('url.com', 80); NetworkStream tcpStream = tcpClient.GetStream(); StreamReader tcpReader = new StreamReader(tcpStream); StreamWriter tcpWriter = new StreamWriter(tcpStream); 

Obviously, this code is going to be very flaky. So, i’ve seen some code that puts the using on the tcpClient, which seems good. However, doesn’t NetworkStream also have resources that need cleaning up? What about StreamReader/Writer?

Do I need to wrap all 4 statements in nested using statements?

And if so, what happens when the time has come to dispose? Won’t StreamWriter close the stream and consequently the socket? Then what happens when StreamReader, then NetworkStream, then TcpClient each go through their disposals?

Which brings up another question. WIth both StreamReader and StreamWriter composed of the same stream, who owns it? Don’t they both think they own it, and will thus both try to destroy it? Or does the framework know that the stream has already been destroyed and just silently ignore it?

It almost seems like the using statement is only necessary for the last statement in the chain, but then what happens if an exception is thrown in GetStream()? I don’t think it would properly clean up the socket then, so it seems redundant usings are necessary to ensure this doesn’t happen.

Does anyone know of any good, recent books on network programming with .net, and prefeably c# that include chapters on exception handling and resource management? Or maybe any good articles online? All the books I can find are from the .NET 1.1 era (Network Programming for the Microsoft .NET Framework, Network Programming in.NET, etc..), so this seems like a topic that needs some good resources.

EDIT:

Please, don’t let Marc’s very good comment stop anyone else from commenting on this 🙂

I’d like to hear anyone elses book recommendations or opinions on resource management, especially in regard to asynchronous usage.

Answer 1

Generally, objects should internally handle multiple Dispose() calls, and only do the main code once; so a stream getting Dispose()d multiple times is not usually a problem. Personally, I would use lots of using there; note that you don’t need to indent/nest, though (unless different levels have different life-times):

using(TcpClient tcpClient = new TcpClient('url.com', 80)) using(NetworkStream tcpStream = tcpClient.GetStream()) using(StreamReader tcpReader = new StreamReader(tcpStream)) using(StreamWriter tcpWriter = new StreamWriter(tcpStream)) {    ... } 

As you say, this ensures that if an error happens during initialization, everything is still cleaned up correctly. This also ensures that every level gets a chance (in the right order) to deal correctly with any buffered data, etc.

Re ownership; NetworkStream is actually an oddity in the first place… most streams are either input xor output. NetworkStream bends a few rules and shims two directions into one API; so this is an exception… normally the ownership would be clearer. Additionally, many wrappers have a flag to determine whether they should close the wrapped stream. StreamReader doesn’t, but some do (such as GZipStream, which has a leaveOpen ctor option). If you don’t want to flow ownership, this is an option – or use a non-closing stream intermediary – one is here (NonClosingStream or similar).

Re books; I picked up a copy of ‘TCP/IP Sockets in C#: Practical Guide for Programmers’ (here) – adequate, but not great.