Question

I have this:

public class ClientSession : TcpClient
{
    public int SessionGUID = 0;
}

And somewhere in server threads this:

ClientSession client = (ClientSession)tcpListener.AcceptTcpClient(); //cast failure

Ok, i understand that to cast like that, right side object must be instance of ClientSession (that possibly stored in pointer to base class)
but… how to construct ClientSession in that case?

I don’t want to make ClientSession like this:

public class ClientSession
{
    public int SessionGUID = 0;
    public TcpClient client;
}

Answer 1

TcpListener.AcceptTcpClient will return a TcpClient object and nothing else. The type of an object cannot be changed at runtime, so a direct conversion (that preserves object identity and data) is not possible.

I think composition (similar to your last code snippet) would be the best approach. You could enable conversion using cast syntax by implementing a custom conversion operator, but i would rather suggest using a constructor that takes a TcpClient here:

public ClientSession(TcpClient client) { /* ... */ }

so you could do this:

ClientSession client = new ClientSession(tcpListener.AcceptTcpClient());