C# Async Sockets Server Receive Problems

Question

I have implemented my Server application regarding this post here: http://www.codeguru.com/csharp/csharp/cs_network/sockets/article.php/c8781#Client1

Sum up: I am using async Sockets ala BeginAccept(..), BeginReceive(..). My Server is capable of handling mutliple clients and everything works fine until a client performas two or more synchronous send operation without waiting some time. The client does not get any error and so is not notified, that the server does not get the second message! If the client waits approx. 100ms after the first send operation, everything works fine. I thought that when i use TCP i can ensure that the server receives the message. (Except there is an exception thrown)! Could you provide me a solution to fix this.

Here are the WaitForData(..) & OnDataReceive(..) Methods that i implemented in the server

public void WaitForData(MyClient client)
{
    try
    {
        if (pfnCallBack == null)
        {
            pfnCallBack = new AsyncCallback(OnDataReceived);
        }

        iarResult = client.Socket.BeginReceive(client.DataBuffer,
                                                0, client.DataBuffer.Length,
                                                SocketFlags.None,
                                                pfnCallBack,
                                                client);
    }
    catch (SocketException se)
    {
        MessageBox.Show("SocketException@WaitForData" + se.Message);
    }
}
public void OnDataReceived(IAsyncResult asyn)
{
    try
    {
        MyClient user= (MyClient)asyn.AsyncState;
        int iRx = user.Socket.EndReceive(asyn);

        byte[] receivedData = user.DataBuffer;

        MemoryStream memStream = new MemoryStream();
        BinaryFormatter binForm = new BinaryFormatter();
        memStream.Write(receivedData, 0, receivedData.Length);
        memStream.Seek(0, SeekOrigin.Begin);
        MyMessage msg = (MyMessage)binForm.Deserialize(memStream);

        switch (msg.Command)
        {
            case (MyMessage.MyCommand.ConnId):
                this.connId = (int) msg.MyObject;
                tsslConnStatus.Text += " | ID: " + connId.ToString();
            break;

            case (MyMessage.MyCommand.Text):
                MessageBox.Show(msg.MyObject.ToString());
                break;
        }
        WaitForData(server);
    }
    catch (ObjectDisposedException ode)
    {
        MessageBox.Show("ObjectDisposedException@OnReceiveData" + ode.Message);
    }
    catch (SocketException se)
    {
        MessageBox.Show("SocketException@OnReceiveData" + se.Message);
    }
}

The CLIENT calls a synchronous SEND METHOD TWICE or MORE! server INSTANCEOF MyClient

if (server.Socket.Connected)
{
    BinaryFormatter bf = new BinaryFormatter();
    MemoryStream ms = new MemoryStream();
    bf.Serialize(ms, message);
    MyMessage = new MyMessage(something);
    server.Socket.Send(ms.ToArray());
}

so, i think this code snippets must be enough for you to get the idea i was trying to use! If you need further details or code snippets, just tell me i will post it!

Thanx!

Answer 1

TCP is stream based and not message based. One Read can contain any of the following alternatives:

• A teeny weeny part of message
• A half message
• Excactly one message
• One and a half message
• Two messages

Thus you need to use some kind of method to see if a complete message have arrived. The most common methods are:

• Add a footer (for instance an empty line) which indicates end of message
• Add a fixed length header containing the length of the message

Update

Simple example having just length as header.

Server side:

var buffer = binaryFormmater.Serialize(myobj);
var length = buffer.Length;
networkStream.Send(length);
networkStream.Send(buffer, 0, buffer.Length);

Client side:

var header = new buffer[4];
// TODO: You need to make sure that 4 bytes have been read.
networkStream.Read(header, 0, 4);
var length = BitConverter.ToInt32(buffer);

var readbuffer= new byte[65535];
var bytesLeft = length;
var messageStream = new MemoryStream();
while (bytesLeft > 0)
{
     var read = networkStream.Read(readbuffer, 0, bytesLeft);
     messageStream.Write(readbuffer, 0, read);
     bytesLeft -= read,
}

messageStream.Seek(0, SeekOrigin.Begin);
MyMessage msg = (MyMessage)binForm.Deserialize(messageStream);