C# Newbie - Asynch Socket Problem

Question

I am in the process of becoming a self taught programmer...so patience with me...and if I ask any dumb questions...I apologize in advance. I am trying to be as concise and clear as possible.

1. There is a server application that is installed on my PC. This application is an intermediary. It connects to an internet server, so in that respect it is a client. But from my perspective, it is a server. I connect to this "server app" on my PC, with the application I am writing, using socket(s), locally at 127.0.0.1.
2. My application instantiates a socket, or sockets, configures them, connects to this local server app on my PC...and then communicates. I have all this working.
3. The basic structure of the communication between my App, and this local Server is an Async Loop. The fields/objects/methods are live on the main form of my basic WinForms App.
4. Here is the basic structure. I have exclude minutae like the socket initiation....connection...and event handlers that start events....since I dont want to post a ton of code which will bore you all...
5. Basically...my problem is I can't seem to figure out how to close the socket without crashing my App....

//******************************************************************************//
public partial class MainForm : Form
{
private Socket A1Socket;
private AsyncCallback A1Callback;
private byte[] a1SocketBuffer = new byte[65536];
private bool a1BeginReceiveFlag = true;
private byte[] a1CopyBuffer = new byte[65536];
bool stopWfdLoop = false;

public MainForm()
{InitializeComponent();}

//Lot of stuff I skipped....

//Event that kicks off data download
private void SomeButton_Click(object sender, EventArgs e) //At this point, my socket is up and running, and I want to star the loop that will pull data.
{
waitForData(CM.A1Cfg.SocketType); //The argument CM.A1Cfg.SocketType is just a descriptive string. In the future it will help differentiate between different socket types.
}

//The waitForData() method. If the stopWfdLoop flag is set true...this stops the loop.
private void waitForData(string SocketType)
{
if (!stopWfdLoop)
{
if (A1Callback == null)
{
A1Callback = new AsyncCallback(onReceive);
}

if (a1BeginReceiveFlag) //This flag make sure we never call BeginReceive until we have copied the buffer
{
a1BeginReceiveFlag = false;
A1Socket.BeginReceive(a1SocketBuffer, 0, a1SocketBuffer.Length, SocketFlags.None, A1Callback, SocketType);
}
}
}

//The OnReceive() method...which is automatically called by the .NET framework when data is ready to be copied on my end.
private void onReceive(IAsyncResult argOr)
{
if (argOr.AsyncState.ToString().Equals("A1"))
{
int rxByteCount = A1Socket.EndReceive(argOr);
for (int i = 0; i < rxByteCount - 1; i++){a1CopyBuffer[i] = a1SocketBuffer[i]; }
a1BeginReceiveFlag = true; //OK to set the Flag True again

//Some processing methods...I skipped....

waitForData(CM.A1Cfg.SocketType); //This sends us right back into the loop. So this loop just goes on and on...till the flag kicks us out
}
}

//**************************************************************************************************************//

So...again...my problem is when I try to close the socket..the app crashes. I thought that by having the stopWfdloop flag...and setting it to true...I would stop the loop. Then I could shut down and stop the sockets....but it does not work. Here is what I do to try and shut down the socket.

private void DisconnectButton_Click(object sender, EventArgs e)
{
stopWfdLoop = true;
waitForData(CM.A1Cfg.SocketType); //This idea here is to call the loop once...with the flag set true, to make sure the loop is shut down before we proceed
A1Socket.Shutdown(SocketShutdown.Both);
A1Socket.Close();
A1Socket = null;
stopWfdLoop = true;
}

App dies here...

What am I missing?

Sam Hobbs · Answer

One possibility is to create a flag that is set to true or false initially. It remains the same value until you change the value in DisconnectButton_Click. Then in waitForData you check the flag and if it indicates that you are disconnecting then do not do another receive.

Sam Hobbs · Answer

Perhaps I misunderstand but I think there is not any recursion here. The important thing is that I think you are saying that you need to prevent OnReceive from doing another BeginReceive (which is actually done in waitForData ) and if that is what you are saying then I agree.

Brant Williams · Answer

Sam, Thanks. Yes...your conclusion is similar to what mine was. The data download, in cpu terms...takes a LONG time. That is also the time when the main thread is free to work. So initially I was killing the socket...while it was in the middle of the BeginReceive method...on another thread...before the AsyncCallback calls onReceive(). That is never going to work. So the key is to set a flag that causes the BeginReceive to get skipped. Once it gets skipped...then it never goes into that loop. The main thread is no longer at the mercy of the AsyncCallback which takes over when it comes back (ie...it calls onReceive()...which then hijacks the main thread). So...set a flag that causes either the waitForData call never to be made...or results in the BeginReceive() method getting skipped when waitForData() is called. Then I will have broken the recursive loop. The socket will be dormant...and I can shut it down, then kill it. So...that said...the hard part is actually getting the flag set in a manner that will go into effect....not as easy as it sounds. I think I need to have a step in onReceive that always updates a local flag to the status of a flag set by the Disconnect Button event handler. In other words...a statement in onReceive polls for status each cycle. That local flag can then be used on an if that encapsulates the recursive call to waitForData.... Hey...I think I will try that right now... This was quite a valuable learning experience...because it taught me a lot about program flow.

Sam Hobbs · Answer

I have done very little network programming. I have done a bit of asynchronous IO but my attempt has not been totally successful. I have tried to write an email reader that can work asynchronously. I don't know why it does not work. I suspect that I am not using try/catch enough and I suggest that you do that; that is, use try/catch a lot to catch errors. C# has a very nasty tendency to just quit a method if there is an uncaught error. I think one problem is that you are doing a BeginReceive (in waitForData) before a shutdown, which I think you do not want to do. So in DisconnectButton_Click I think you can use the IAsyncResult.IsCompleted Property and if it is false then use the IAsyncResult.AsyncWaitHandle Property to wait for completion. Note that the BeginReceive method returns an IAsyncResul. You should save that as a member variable so you can use it as I describe above. I hope that helps. there are very many details to be concerned about and as I said I am not very experienced. One more thing you need to research is how to do a forced shutdown when a receive fails. I think you need to use a timer to determine that an asynchronous IO has failed; the timeout value I think is never used for asynchronous operations.

Brant Williams · Answer

Sam, Thanks for the follow up. Some times the intermediary software (server to my app) loses connection to its internet server. Sometimes everything reconnects...but some times I have to shut it all down, and start over. Re-start the intermediary program...and thus I need to kill all the sockets, null them out, and start over. I think my underlying problem is that I may not have fully understood the process flow for an asynch call recursive loop like this. I still may not...but here goes... The only time code OTHER THAN the BeignReceive and onRecive loop pair can do anything, is DURING the BeginReceive to EndReceive period...correct? Once BegicReceive is done....and onReceive is automatically called...THAT is the process....and with the recursive call...that jumps straight back into BeginReceive. Now...realistically...that is a lot of time. But any methods or flag sets to kill the loop have to occur with the understanding that they will be called while BeginReceive is executing on its separate thread. Again...this is a LOT of time. That little section of code executes FAST...and this data comes in pretty slow...so probably 99.9% of the time...that BeginReceive thread is active. So...here is what I did. 1. I declared a delegate which takes no arguments and returns void. 2. I added an Invoke to the "Disconnect" button event handler code as follows: this.Invoke(new Delay(confirmDataLoopExit)); if (A1Socket != null) { A1Socket.Shutdown(SocketShutdown.Both); A1Socket.Close(); A1Socket = null; } 3. The encapsulated method, confirmDataLoopExit, looks like this: do { stopDataFlag = true; } while (!adminBeginReceiveFlag); It seems to work...but now I think I have a new problem. I have chewed up ALL my available main thread process time with an infinite loop.... I am not very good at catch and flag logic...but with a little more time...I will figure something out. At least I think I am beginning to get a handle on process flow. Nothing like using asynch calls to show that you really dont know what is going on in your code....

Sam Hobbs · Answer

Let us first start at the beginning. Why do you need to close the socket? I am not saying you do nt need to; I am tryig to understand. Are you doing that at the end of your program and it will not try to do any more reads? Or do you need to close the connection because that is what the other end expects? Perhaps you do not need to close the connection so for the purposes of being methodical I want to ensure you that you do.

Suthish Nair · Answer

I think only Brant having knowledge about this particular topic here..

Brant Williams · Answer

Tried the delegate and Invoke... DID NOT WORK. Next, I thought that problem was the very long period of time where the BeginReceive call is away on a different thread. I tried an Invoke that does not complete until a flag is set in the onReceive method. Made a flag that is true only when BeginReceive is not executing. The idea is that the Invoke loops till it knows you are in onReceive...then it comes back. The flag is also set, so you can never re-enter the BeginReceive. Still does not work.... man this is frustrating... How do you safely kill one of these socket loops?

Brant Williams · Answer

Do I need to use a Invoke(), and make a call to the waitForData after setting the stopflag? This will have the effect of stopping the event handler statement sequence until the call to waitForData (which should just loop through and do nothing once) is done. This should insure that no data loop is running....

Brant Williams · Answer

I have now found that if I break the events from the DisconnectButton_Click call between two buttons...it sort or works. Instead of disconnecting in one step...I have to disconnect in two. This is not acceptable, but may help to figure out what the underlying problem is. I can kill A1Socket without crash, if I do the following: private void STOPDATALOOPBUTTON_Click(object sender, EventArgs e) { stopWfdLoop = true; waitForData(CM.A1Cfg.SocketType) } private void DISCONNECTSOCKETBUTTON_Click(object sender, EventArgs e) { if (A1Socket != null) { A1Socket.Shutdown(SocketShutdown.Both); A1Socket.Close(); A1Socket = null; } } So I hit the STOPDATALOOPBUTTON...then hit the DISCONNECTSOCKETBUTTON...and the socket is closed and disposed of, without killing the app. I am baffled as to why I can do this in two steps...but not one.... If I put all these statements in the same button event handler call...it crashes. But I can divide it up...and it does NOT.

C# Newbie - Asynch Socket Problem

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Answers (10)