TCP/ip recive connection but locks up when trying to read data

Question

Please can someone tell me why my server locks up when my rclient connects and try to send data?

If I comment out:

Dim bytes(rclient.ReceiveBufferSiz         


        

Answer 1

You are assuming that reads return a specific amount of data such as ReceiveBufferSize. That is not true. A read returns at least one byte, that is all.

Just to clarify, TCP does not support message based transfer.

The correct way to read depends on the protocol. If the exact number of bytes expected is known you need to read in a loop until so many bytes are received (or use BinaryReader which does that for you).

For a line based protocol you can use StreamReader.ReadLine which again automates the looping.

ReceiveBufferSize is completely unrelated to how much data is available or will come.

DataAvailable is how much data can be read right now without blocking. But more data might come. It is almost always a bug to use it. Might return 0 at any time even if data comes in 1ms later.

Answer 2

EDIT: For those of you who have downloaded/tested this already, I made a bugfix to the classes so you'll need to redownload the sources if you're going to use them again.

If you want to perform proper data transfer you'll need to use a more reliable method than just simply reading random data. And as usr pointed out: the TcpClient.ReceiveBufferSize property does not tell you how much data there is to receive, nor how much data there is sent to you. ReceiveBufferSize is just a variable indicating how many bytes you are expected to get each time you read incoming data. Read the MSDN page about the subject for more info.

As for the data transfer, I've created two classes which will do length-prefixed data transfer for you. Just import them to your project and you'll be able to start using them immediatelly. Link: http://www.mydoomsite.com/sourcecodes/ExtendedTcpClient.zip

Example usage

Server side

1. First declare a new variable for ExtendedTcpClient, and be sure to include WithEvents in the declaration.

   Dim WithEvents Client As ExtendedTcpClient
   

2. Then you just need to use a normal TcpListener to check for incoming connections. The TcpListener.Pending() method can be checked in for example a timer.

   When you are to accept a new TcpClient, first declare a new instance of the ExtendedTcpClient. The class requires to have a form as it's owner, in this application Me is the current form.
   Then, use the ExtendedTcpClient.SetNewClient() method with Listener.AcceptTcpClient() as it's argument to apply the TcpClient from the listener.

   If Listener.Pending() = True Then
       Client = New ExtendedTcpClient(Me)
       Client.SetNewClient(Listener.AcceptTcpClient())
   End If
   

3. After that you won't be needing the timer anymore, as the ExtendedTcpClient has it's own thread to check for data.

   Now you need to subscribe to the PacketReceived event of the client. Create a sub like so:

   Private Sub Client_PacketReceived(sender As Object, e As ExtendedTcpClient.PacketReceivedEventArgs) Handles Client.PacketReceived
   
   End Sub
   

   In there you can for example output the received packet as text into a TextBox. Just check if the packet header is PlainText and then you can convert the received packets contents (which is an array of bytes, accessed via e.Packet.Contents) to a string and put it in the TextBox.

   If e.Packet.Header = TcpMessagePacket.PacketHeader.PainText Then
       TextBox1.AppendText("Message recieved: " & System.Text.Encoding.Default.GetString(e.Packet.Contents) & Environment.NewLine)
   End If
   

4. Lastly, when closing the form you just need to disconnect the client.

   Private Sub ServerWindow_FormClosing(sender As Object, e As System.Windows.Forms.FormClosingEventArgs) Handles Me.FormClosing
       If Client IsNot Nothing Then Client.Disconnect()
   End Sub
   

And that's it for the server side.

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Client side

1. For the client side you will only be needing a normal TcpClient (unless you don't want to receive data there too).

   Dim Client As New TcpClient
   

2. Then connect to the server via the IP and port you've given the listener.

   Client.Connect("127.0.0.1", 5555) 'Connects to localhost (your computer) at port 5555.
   

3. Now if you want to send plain text to the server you'd do something like this:

   Dim MessagePacket As New TcpMessagePacket(System.Text.Encoding.Default.GetBytes(TextBox2.Text), TcpMessagePacket.PacketHeader.PainText)
   MessagePacket.Send(Client) 'Client is the regular TcpClient.
   

And now everything should be working!

Link to a complete example project: http://www.mydoomsite.com/sourcecodes/TCP%20Messaging%20System.zip

If you want to add more headers to the class, just open TcpMessagePacket.vb and add more values in the PacketHeader enum (located in the region called Constants).

Hope this helps!

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Screenshot from the example project

(Click the image for larger resolution)