What happens if you break out of a Lock() statement?

Question

I'm writing a program which listens to an incoming TcpClient and handles data when it arrives. The Listen() method is run on a separate thread within the component, so it needs to be threadsafe. If I break out of a do while loop while I'm within a lock() statement, will the lock be released? If not, how do I accomplish this?

Thanks!

(Any other advice on the subject of Asynchronous TCP Sockets is welcome as well.)

private void Listen()
{
    do
    {
        lock (_clientLock)
        {
            if (!_client.Connected) break;
            lock (_stateLock)
            {
                if (!_listening) break;
                if (_client.GetStream().DataAvailable) HandleData();
            }
        }
        Thread.Sleep(0);
    } while (true);
}

Answer 1

Yes. The lock statement translates into a try/finally clause. In C# 4, for example, a lock statement like so:

lock(obj)
{
    // body
}

roughly translates ( taken from Eric Lippert's blog here ) to:

bool lockWasTaken = false;
var temp = obj;
try 
{ 
    Monitor.Enter(temp, ref lockWasTaken); 
    { 
       // body 
    }
}
finally 
{ 
    if (lockWasTaken) 
        Monitor.Exit(temp); 
}

When the execution leaves the scope of the lock {} , the underlying lock will be released automatically. This will happen no matter how you exit scope (break/return/etc), since the call to Monitor.Exit is wrapped, internally, inside of the finally block of a try/finally.

Answer 2

Yes, the lock will be released. You can use ILDASM or Reflector to look at the actual generated code. The lock statement is shorthand for the following code (roughly).

Monitor.Enter(_client);
try
{
  // do your stuff

}
finally {
  Monitor.Exit(_client);
}

Notice the finally block is always executed.

Answer 3

Once you exit the lock{} , it will unlock what you have locked (it's just like a using statement in that regard). It doesn't matter where you exit (the beginning, the end, or the middle), it's that you left the scope of the lock at all. Think about what would happen if you raised an exception in the middle.

Answer 4

Because you asked for other advice...I noticed that you are nesting locks. This, by itself, is not necessarily a bad thing. But, it is one my red flags I watch out for. There is the possibility of a deadlock if you ever acquire those two locks in a different order in another part of your code. I am not saying there is anything wrong with your code. It is just something else to watch out for because it is easy to get wrong.

Answer 5

To answer the other half of your question:

Any other advice on the subject of Asynchronous TCP Sockets is welcome as well

Simply put I wouldn't manage this in the fashion demonstrated by your original post. Rather seek help from the System.Net.Sockets.TcpClient and the System.Net.Sockets.TcpListener classes. Use the async calls like BeginAcceptSocket(...) and BeginRead(...) and allow the ThreadPool to do it's job. It's really pretty easy to put together that way.

You should be able to achieve all the server behavior you desire without ever coding the dreaded words "new Thread" :)

Here is a basic example of the idea, minus the idea of graceful shutdown, exception handling ect:

public static void Main()
{
    TcpListener listener = new TcpListener(new IPEndPoint(IPAddress.Loopback, 8080));
    listener.Start();
    listener.BeginAcceptTcpClient(OnConnect, listener);

    Console.WriteLine("Press any key to quit...");
    Console.ReadKey();
}

static void OnConnect(IAsyncResult ar)
{
    TcpListener listener = (TcpListener)ar.AsyncState;
    new TcpReader(listener.EndAcceptTcpClient(ar));
    listener.BeginAcceptTcpClient(OnConnect, listener);
}

class TcpReader
{
    string respose = "HTTP 1.1 200\r\nContent-Length:12\r\n\r\nHello World!";
    TcpClient client;
    NetworkStream socket;
    byte[] buffer;

    public TcpReader(TcpClient client)
    {
        this.client = client;
        socket = client.GetStream();

        buffer = new byte[1024];
        socket.BeginRead(buffer, 0, 1024, OnRead, socket);
    }

    void OnRead(IAsyncResult ar)
    {
        int nBytes = socket.EndRead(ar);
        if (nBytes > 0)
        {
            //you have data... do something with it, http example
            socket.BeginWrite(
                Encoding.ASCII.GetBytes(respose), 0, respose.Length, null, null);

            socket.BeginRead(buffer, 0, 1024, OnRead, socket);
        }
        else
            socket.Close();
    }
}

For a much more complicated example of how to do this see the SslTunnel Library I wrote a while ago.