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ALL,

Have the following problem. First, here is my code:

class InternetConnector { private static ManualResetEvent receiveDone = new ManualResetEvent( false ); public static ManualResetEvent processingDone = new ManualResetEvent( false ); public static ConcurrentQueue<string> messages = new ConcurrentQueue<string>(); public static bool Connected; public bool ReceiveMessage(Action<Socket> successHandler, Action<Exception> errorHandler) { bool receive = false; ClientStateObject obj = new ClientStateObject(); obj.server = client; var connectionData = new ConnectionData { ErrorHandler = errorHandler, SuccessHandler = successHandler, Socket = client, clientObj = obj }; if (Connected) { client.BeginReceive(connectionData.clientObj.buffer, 0, ClientStateObject.bufSize, 0, new AsyncCallback(ReceiveCallback), connectionData); receive = true; receiveDone.WaitOne(); } return receive; } private static void ReceiveCallback(IAsyncResult ar) { ConnectionData connectionData = new ConnectionData(); bool complete = false; try { connectionData = (ConnectionData)ar.AsyncState; Socket client = connectionData.Socket; int num = client.EndReceive(ar); { connectionData.clientObj.stringBuffer.Append(Encoding.ASCII.GetString(connectionData.clientObj.buffer, 0, num)); string response = connectionData.clientObj.stringBuffer.ToString(); if (response.EndsWith("&")) complete = true; string[] msgs = response.Split('&'); int j = 0; if (!complete) j++; for (int i = 0; i < msgs.Count() - j; i++) { string sts = msgs[i]; if (i == 0 && receivingMessage != String.Empty) { sts = receivingMessage + sts; messages.Enqueue(sts + "&" ); receivingMessage = String.Empty; } else messages.Enqueue(sts + "&"); } if (!complete) receivingMessage += msgs[msgs.Count() - 1]; else receivingMessage = String.Empty; receiveDone.Set(); if (connectionData.SuccessHandler != null) { connectionData.SuccessHandler(client); processingDone.WaitOne(); client.BeginReceive(connectionData.clientObj.buffer, 0, ClientStateObject.bufSize, 0, new AsyncCallback(ReceiveCallback), connectionData); } } } catch (Exception e) { if (connectionData.ErrorHandler != null) connectionData.ErrorHandler(e); } } }

And here is the success handler:

public partial class Form1 : Form { private void AsyncSuccessHandler(Socket socket) { if (InvokeRequired) { BeginInvoke(new Action( () => AsyncSuccessHandler( socket ) )); return; } if (InternetConnector.messages.Count() == 0) { status.Text = "Signals Receiver: Connected"; status.ForeColor = Color.Green; startStop.Text = "Stop"; isRunning = true; create.Enabled = true; SetGUIColorsAndValues(); client.SendMessageToConnector("First Signal Pass4&"); client.ReceiveMessage(AsyncSuccessHandler, AsyncErrorHandler); } else { SetGUIColorsAndValues(); GUIChangeOnConnection(); InternetConnector.processingDone.Set(); } } private void GUIChangeOnConnection() { for( int i = 0; i < InternetConnector.messages.Count; i++ ) { string message; InternetConnector.messages.TryDequeu( out message ); // process message } } }

Problem: I need to continuosly read the data from the server and display them in the GUI. I also need the GUI to be responsive to i.e. button clicks. I also need to start n read iteration when n-1 read iteration has been finished processing and the concurrentqueue object in my code is empty.

However going thru the code in debugger I can see that BeginReceive() call in the reading callback does not wait till the end of success handler and the queue is not empty.

What am I missing? Is there a better way to achieve this?

I am also aware of SignalR library but at this pont I don't want to use any 3rd party library.

Any help in this matter appreciated.

Thank you.

[EDIT]

Do I understand Amit's reply correctly:

class InternetConnector { private static ManualResetEvent processingDone = new ManualResetEvent( false ); private static void ReceiveCallback(IAsyncResult ar) { ConnectionData connectionData = new ConnectionData(); bool complete = false; try { connectionData = (ConnectionData)ar.AsyncState; Socket client = connectionData.Socket; int num = client.EndReceive(ar); { connectionData.clientObj.stringBuffer.Append(Encoding.ASCII.GetString(connectionData.clientObj.buffer, 0, num)); string response = connectionData.clientObj.stringBuffer.ToString(); if (response.EndsWith("&")) complete = true; string[] msgs = response.Split('&'); int j = 0; if (!complete) j++; for (int i = 0; i < msgs.Count() - j; i++) { string sts = msgs[i]; if (i == 0 && receivingMessage != String.Empty) { sts = receivingMessage + sts; messages.Enqueue(sts + "&" ); receivingMessage = String.Empty; } else messages.Enqueue(sts + "&"); } if (!complete) receivingMessage += msgs[msgs.Count() - 1]; else receivingMessage = String.Empty; receiveDone.Set(); if (connectionData.SuccessHandler != null) { processingDone.WaitOne(); connectionData.SuccessHandler(client); processingDone.Set(); client.BeginReceive(connectionData.clientObj.buffer, 0, ClientStateObject.bufSize, 0, new AsyncCallback(ReceiveCallback), connectionData); } } } catch (Exception e) { if (connectionData.ErrorHandler != null) connectionData.ErrorHandler(e); } } }

?

[/EDIT] [EDIT 2] Please see updated code. When I set a breakpoint at the very end of the GUIChangeOnConnection() - line with the "}" - I see that the queue does have some items. I will try to change ManualResetEvent, in the meantime. [/EDIT 2]

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  • It's not clear what's calling AsyncSuccessHandler to start with - so what thread it will start on. The loop at the end looks suspicious though. Commented Aug 17, 2012 at 6:06
  • Can you improve code formatting as it is very hard to read otherwise? I tried to edit but SO is not allowing me to do it as it is a trivial edit. Commented Aug 17, 2012 at 6:23
  • @JonSkeet, AsyncSuccessHandler is a function to call in case the socket operation is successful. Commented Aug 17, 2012 at 6:25

1 Answer 1

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If I have understood the problem correctly, you are referring to a problem at the following point of code:

if (connectionData.SuccessHandler != null) { connectionData.SuccessHandler(client); client.BeginReceive(connectionData.clientObj.buffer, 0, ClientStateObject.bufSize, 0, new AsyncCallback(ReceiveCallback), connectionData); }

If your intention is that you hit server only when you have finished processing previous data then the SuccessHandler must be blocking (as is obvious from your problem statement as well).

Now take a look at the following lines in your SuccessHandler

if (InvokeRequired) { BeginInvoke(new Action( () => AsyncSuccessHandler( socket ) )); return; }

The above code defers the execution of your SuccessHandler so that the UI remains responsive. Or in other words, your SuccessHandler is not blocking.

Since you are updating GUI from the SuccessHandler, it is also important to run it from UI thread (which it is correctly doing right now).

So in order to make the SuccessHandler blocking, you can make the following code itself run on UI thread using InvokeRequired mechanism

if (connectionData.SuccessHandler != null) { connectionData.SuccessHandler(client); client.BeginReceive(connectionData.clientObj.buffer, 0, ClientStateObject.bufSize, 0, new AsyncCallback(ReceiveCallback), connectionData); }

Since client.BeginReceive is async it will not block the UI thread and you anyway wanted to run SuccessHandler on UI thread..

If you don't have access to the Form in ReceiveCallback, you can use a waithandle to block call to client.BeginReceive till SuccessHandler has finished.

private static AutoResetEvent _processingDone = new AutoResetEvent( false ); //Introduce an internal property ProcessingDone to access _processingDone //This should be in a class which you can access both from 'Form1' and the class containing ReceiveCallBack //In ReceiveCallBack if (connectionData.SuccessHandler != null) { connectionData.SuccessHandler(client); ProcessingDone.WaitOne(); client.BeginReceive(connectionData.clientObj.buffer, 0, ClientStateObject.bufSize, 0, new AsyncCallback(ReceiveCallback), connectionData); }

ProcessingDone.Set() must be done from within SuccessHandler at a point where you can determine that SuccessHandler has finished and client.BeginReceive needs to be invoked. This is internal to your logic. From the code you have shared, it is difficult to pin point the place where you need to put ProcessingDone.Set().

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10 Comments

you understood correctly. Can you give me the proper C# syntax?
That will depend on whether 'ReceiveCallback' has access to the Form which contains AsyncSuccessHandler method...
No access. Why should it? It just a callback which should finish reading the socket. Remember, GUI runs on a different thread...
Yes, they run on different thread and that was my point. In order to block client.BeginReceive till you are done with SuccessHandler, you can simply make them both run on GUI thread. Currently you are running only SuccessHandler on GUI thread. If you don't have access to the Form in ReceiveCallback (and this fact is not very obvious from whatever little code you have shared) you can then easily block the receiveCallback till SuccessHandler is finished by using a waithandle just like receiveDone.
You mean something like this: "private static ManualResetEvent processingDone = new ManualResetEvent( false ); if( connectionData.SuccessHandler != null ) { processingDone.WaitOne(); connectionData.SuccessHandler( client ); processingDone.Set(); client.BeginReceive(); }". Thank you.
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