TCPClient status no change

[qfchen]

I was able to open a connection using TCPClient, it is able to read and write data or string, however when I turn off the remote device or unplug the network cable, then check the client.connecte d property, it always shows "True". what could be happened? Thanks

[Plater]

The connection property is only reflective of the "last read or write operation", which I presonally feel makes it rather useless. You have to try to read/write on a closed socket before it the property changes.

Fortunatly I wrote the following function that I think works rather well.

//note: this only works if there is no data waiting to be read
//and in the case of a listener socket; no pending connections
public static bool isSocketConnected(Socket s)
{
   bool retval=true;
   bool part1 = s.Poll(1000, SelectMode.SelectRead);
   bool part2 = (s.Available == 0);
   if (part1 & part2)
   {//connection is closed
      retval=false;
   }
   return retval;
}
 

[qfchen]

Thank you,

But I'm using VB2005, is there a similar function as s.poll( ) in VB? by the way what is the poll function doing?

Thanks

[Plater]

All of that is available in .NET. So whether its VB.NET or C#, you can still use the calls. You can check the Poll() function on msdn
(Just need to change to VB.NET syntax)

EDIT: Quick online convert produced this, should be pretty close.

'note: this only works if there is no data waiting to be read 
'and in the case of a listener socket; no pending connections 
Public Shared Function isSocketConnected(ByVal s As Socket) As Boolean 
    Dim retval As Boolean = True 
    Dim part1 As Boolean = s.Poll(1000, SelectMode.SelectRead) 
    Dim part2 As Boolean = (s.Available = 0) 
    If part1 And part2 Then 
        'connection is closed 
        retval = False 
    End If 
    Return retval 
End Function 
 

[qfchen]

Thank you very much.

There are so many sokets from .net. Like Winsock, Sockets, TCPCLient. All of them can do the data transmission. only sockets has Poll( ) function. But I really don't know what are the difference between them? and how to choose one of them for my application? Just try one by one. Any comment?

Thanks.

[Plater]

Well quick overview
WinSock is COM implementation of sockets
Socket is a .NET impementation of sockets
TcpClient(and TcpListener and UDPClient as well) are wrapper classes around the .net Socket. They all contain properties for accessing the underlying sockeet directly.

[qfchen]

So, How can I Poll using Winsock and TCPClient, the Poll function is not implemented for this two objects. Is there any other way to do?

very appreciate for your help

[Plater]

Well winsock is a COM object, you shouldn't be using that anyway.
TcpClient, as I already said, is just a "wrapper" type class of the socket and has a property that is the instance of the socket.

myTcpClient.Cli ent is a Socket, so you can just pass that into the function.

[qfchen]

Thanks Plater,

However, after try the following code, the return value of s.Poll(1000, SelectMode.Sele ctRead) always 'false', and s.Available always '0'. no matter the remote device is powered on or off. Any other way to test the remote device is connected? or power off?. For your info, the device and the PC are connected via an ethernet switch. Thanks.

'note: this only works if there is no data waiting to be read
'and in the case of a listener socket; no pending connections
Public Shared Function isSocketConnect ed(ByVal s As Socket) As Boolean
Dim retval As Boolean = True
Dim part1 As Boolean = s.Poll(1000, SelectMode.Sele ctRead)
Dim part2 As Boolean = (s.Available = 0)
If part1 And part2 Then
'connection is closed
retval = False
End If
Return retval
End Function

[Plater]

I've never once had any trouble with it. It's always been dead on, not even any delay.
You are using a TcpClient so I'm not sure what your issue is.
Unless it has something to do with the linger state?

SelectRead
true if Listen has been called and a connection is pending;
-or-
true if data is available for reading;
-or-
true if the connection has been closed, reset, or terminated;

otherwise, returns false.

That would lead me to believe that your socket is still connected.