Question

I am having some trouble with a TCP Client , the service connects to the server and listens for messages, which are received and processed 90% of the time. After a bit of debugging I found that on some occasions I get the “Unable to read whole packet of data!” error.(in code)

I don’t have control over what the server sends.
My thinking is it is not receiving the complete message (I think it fails on longer messages but unsure due to volume).

What is the best way I can account for this?, or any other tips. I have my receive method below which is where all the action and problems are.

I am sorry this is my first attempt with TCP , so don’t be afraid

    public void Receive(IAsyncResult ar)
        {
            if (ar != null)
            {
                try
                {

                    byte[] mLenBytes = (byte[])ar.AsyncState;
                    byte[] mDataBytes = null;
                    int bytes_read = mTcpClient.Client.EndReceive(ar);
                    if (bytes_read != 4)
                        throw new Exception("Unable to read whole header!");
                    int len = mLenBytes[3] + ((mLenBytes[2] + (mLenBytes[1] + (mLenBytes[0] << 8)) << 8) << 8);
                    mDataBytes = new byte[len];
                    bytes_read = mTcpClient.Client.Receive(mDataBytes, len, SocketFlags.None);
                    if (bytes_read != len)
                        throw new Exception("Unable to read whole packet of data!" + "Expected " + len + " Got "  + Convert.ToString(bytes_read) + "\r\n" + (System.Text.Encoding.ASCII.GetString(mDataBytes)));
                        //This is the error that is raised!. 

                    // raise an event
                    PhoneBoxEventArgs e1 = new PhoneBoxEventArgs(System.Text.Encoding.UTF8.GetString(mDataBytes));
                    Console.WriteLine("Data received is = " + e1.Data);
                    OnPassEvent(e1);

                }
                catch (Exception ex)
                {

                }

            }
            byte[] mLenBytes_new = new byte[4];
            mTcpClient.Client.BeginReceive(mLenBytes_new, 0, 4, SocketFlags.None, new AsyncCallback(Receive), mLenBytes_new);
        }

Answer 1

You can not assume to get the entire “message” in one read from the TCP server. That is usually why you do the entire header-stuff.

The header is supposed to be 4 bytes long and tell you, how many bytes are to follow. So what you need to do is:

1. Read asynchronously until you have 4 bytes
2. Keep reading asynchronously until you have as many bytes as you’ve been told to receive in the header

What you’re doing is:

1. Assume you get 4 bytes in one go and fail if you don’t (problem: you can not assume that you get 4 bytes in one receive)
2. Receive as many bytes as you can, fail if you didn’t get enough bytes (problem again: you can not be sure to receive all bytes in one go)

Also I can tell from your code that you’ve started receiving asynchronously. Why do you then switch to synchronous receiveing after you got the header?