Derived TcpClient class problem

[Anders Berg]

Hi!
I'm developing a very simple chat application in VB.NET and I've stumbled
into a problem.
For simplicity I'm using the TcpListener and TcpClient classes. However, at
one point I discovered that I want to use the protected Active and Client
properties of the TcpClient class. To do this I made my own derived class:

Public Class MyTcpClient
Inherits TcpClient

Public ReadOnly Property IsActive() As Boolean
Get
Return Me.Active
End Get
End Property
Public ReadOnly Property IPAddress() As IPAddress
Get
Dim aSocket As Socket = Me.Client
Return CType(aSocket.R emoteEndPoint, IPEndPoint).Add ress
End Get
End Property
End Class

To wait for connections I start a TcpListener and use the AcceptTcpClient
method to assign a TcpClient. Originally this was done like this:
Dim Client As New TcpClient
Client = Listener.Accept TcpClient()

This worked really well, so when I changed to my derived class I use the
following code:
Dim Client As New MyTcpClient
Client = CType(Listener. AcceptTcpClient (), MyTcpClient)

However, this doesn't work but throws this exception:
System.InvalidC astException: Specified cast is not valid.

What am I doing wrong here? I'm quite new to programming in VB, but I know
several other languages where I could accomplish something similar without
problems. Maybe the problem is just that I don't know the correct VB syntax?
As I see it, there shouldn't be any problems assigning an instance of a
super class to an instance of a derived class.

Anyway, any help would be most appreciated.

Cheers, Anders

[Carlos J. Quintero [.NET MVP]]

> Dim Client As New MyTcpClient

Client = CType(Listener. AcceptTcpClient (), MyTcpClient)
Listener.Accept TcpClient() is a Microsoft function that returns a Microsoft
TcpClient, and since Microsoft does not know about your MyTcpClient type,
you can not do the cast.
As I see it, there shouldn't be any problems assigning an instance of a
super class to an instance of a derived class.
That's false. There can be casting problems if the instance of the super
class is not already an instance of the derived class. Only the opposite is
true: you can always assign a derived class object to a super class object.
That is, you can always convert a car into a vehicle, but you can not always
convert a vehicle into a car because of:

1) Maybe the vehicle is really a "disguised" truck.
2) Maybe the vehicle is neither a car nor an truck, it is just a generic
vehicle, and it wouldn´t know which is the aspect and behaviour of a car.

These concepts are independent of the programming language.

--

Carlos J. Quintero

The MZ-Tools all-in-one add-in, now for .NET: http://www.mztools.com
"Anders Berg" <an****@lbs.s e> escribió en el mensaje
news:41******** @news.wineasy.s e... Hi!
I'm developing a very simple chat application in VB.NET and I've stumbled
into a problem.
For simplicity I'm using the TcpListener and TcpClient classes. However,
at one point I discovered that I want to use the protected Active and
Client properties of the TcpClient class. To do this I made my own derived
class:

Public Class MyTcpClient
Inherits TcpClient

Public ReadOnly Property IsActive() As Boolean
Get
Return Me.Active
End Get
End Property
Public ReadOnly Property IPAddress() As IPAddress
Get
Dim aSocket As Socket = Me.Client
Return CType(aSocket.R emoteEndPoint, IPEndPoint).Add ress
End Get
End Property
End Class

To wait for connections I start a TcpListener and use the AcceptTcpClient
method to assign a TcpClient. Originally this was done like this:
Dim Client As New TcpClient
Client = Listener.Accept TcpClient()

This worked really well, so when I changed to my derived class I use the
following code:
Dim Client As New MyTcpClient
Client = CType(Listener. AcceptTcpClient (), MyTcpClient)

However, this doesn't work but throws this exception:
System.InvalidC astException: Specified cast is not valid.

What am I doing wrong here? I'm quite new to programming in VB, but I know
several other languages where I could accomplish something similar without
problems. Maybe the problem is just that I don't know the correct VB
syntax?
As I see it, there shouldn't be any problems assigning an instance of a
super class to an instance of a derived class.

Anyway, any help would be most appreciated.

Cheers, Anders