What happen when GetStream.Read is executed? - vb.net

Question

I have the following code as part of TCP server
Private Sub StartTcpClient(ByVal client As TcpClient)
Dim bytesRead As Integer
Dim RxBuffer(1024) As Byte
Dim RxDataStr As String = ""
Dim BeginTime As Date = Date.Now
Dim CurrentTime As Date
Dim ElapsedTicks As Long = -1
'Dim elapsedSpan As New TimeSpan(elapsedTicks)
While True
bytesRead = client.GetStream.Read(RxBuffer, 0, RxBuffer.Length)'What happen here?
If bytesRead > 0 Or ElapsedTicks < 3 * 10000000.0 Then 'Espera hasta 3 segundos
CurrentTime = Date.Now
ElapsedTicks = CurrentTime.Ticks - BeginTime.Ticks
'RxDataStr = System.Text.ASCIIEncoding.ASCII.GetString(RxBuffer, 0, bytesRead) 'Original
RxDataStr += System.Text.ASCIIEncoding.Default.GetString(RxBuffer, 0, bytesRead) 'UTF8
Else
client.Close()
AckString = RxDataStr
AckReady = True
AckPending = False
Exit Sub
End If
End While
End Sub
I wonder about what happen when the line GetStream.Read is executed.
It goes away from my code and doesn't come back until some data is collected or an error happens or something else?
What I need to do is close the current connection if the time between the data arrivals is bigger than 3 seconds.

Answer 1

It appears that you are trying to read from a stream in a loop until no more data is received, or a certain amount of time has passed since you started reading.
The call you use to do this Stream.Read is a blocking call. So if you don't receive any data, this will block indefinitely.
Because in your case, the the stream instance you have is a NetworkStream, you can specify its ReadTimeout property to prevent this. This causes the following behavior:
If the read operation does not complete within the time specified by
this property, the read operation throws an IOException.
Thus, you will have to catch the IOException and check if it is due to a read timeout. Your code would then look like this:
Imports System.Diagnostics
// ... other stuff
Dim stream As Stream = client.GetStream
Dim maxTime As TimeSpan = TimeSpan.FromSeconds(3)
Dim elapsed As Stopwatch = Stopwatch.StartNew()
Dim done As Boolean = False
While Not done
Dim timeout As Long = CLng((maxTime - elapsed.Elapsed).TotalMilliseconds))
If (timeout > 0) Then
stream.ReadTimeout = timeout
Try
bytesRead = stream.Read(RxBuffer, 0, RxBuffer.Length)
If bytesRead > 0 Then 'Espera hasta 3 segundos
RxDataStr += System.Text.ASCIIEncoding.Default.GetString(RxBuffer, 0, bytesRead) 'UTF8
Else
done = True
End If
Catch ioEx As IOException
If elapsed.Elapsed > maxTime Then
done = True ' due to read timeout
Else
Throw ' not due to read timeout, rethrow
End If
End Try
Else
done = True
End If
End While
client.Close()
AckString = RxDataStr
AckReady = True
AckPending = False
Since you are doing I/O, I would also recommend performing it as an asynchronous operation using async/await and Stream.ReadAsync. In this case you should make all the methods in your call chain do async/await.