oop - C# : Converting Base Class to Child Class

Question

I have a class, NetworkClient as a base class :

using System.IO;
using System.Net.Sockets;
using System.Threading.Tasks;

namespace Network
{
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

public class NetworkClient
{
    public NetworkClient()
    {
        tcpClient = new TcpClient();
    }
    public NetworkClient(TcpClient client)
    {
        tcpClient = client;
    }

    public virtual bool IsConnected
    {
        get;
        private set;
    }
    private StreamWriter writer { get; set; }
    private StreamReader reader { get; set; }

    private TcpClient tcpClient
    {
        get;
        set;
    }

    public virtual NetworkServerInfo NetworkServerInfo
    {
        get;
        set;
    }

    public async virtual void Connect(NetworkServerInfo info)
    {
        if (tcpClient == null)
        {
            tcpClient=new TcpClient();
        }
        await tcpClient.ConnectAsync(info.Address,info.Port);
        reader = new StreamReader(tcpClient.GetStream());
        writer = new StreamWriter(tcpClient.GetStream());
    }

    public virtual void Disconnect()
    {
        tcpClient.Close();            
        reader.Dispose();

        writer.Dispose();
    }

    public async virtual void Send(string data)
    {
        await writer.WriteLineAsync(data);
    }

    public async virtual Task<string> Receive()
    {
        return await reader.ReadLineAsync();
    }

}
}

And also have a child class derived from NetworkClient :

using System.Net;

namespace Network
{
using Data;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

public class SkyfilterClient : NetworkClient
{
    public virtual IPAddress Address
    {
        get;
        set;
    }

    public virtual int Port
    {
        get;
        set;
    }

    public virtual string SessionID
    {
        get;
        set;
    }

    public virtual User UserData
    {
        get;
        set;
    }

    protected virtual bool Authenticate(string username, string password)
    {
        throw new System.NotImplementedException();
    }

}
}

The problem is, that when im trying to cast NetworkClient into SkyfilterClient. An exception is thrown, Unable to cast object of type 'Network.NetworkClient' to type 'Network.SkyfilterClient'.

Whats wrong with my code ? I see that Stream can be converted to NetworkStream, MemoryStream. Why NetworkClient can't be converted to Skyfilter Client?

Answer 1

As long as the object is actually a SkyfilterClient, then a cast should work. Here is a contrived example to prove this:

using System;

class Program
{
    static void Main()
    {
        NetworkClient net = new SkyfilterClient();
        var sky = (SkyfilterClient)net;
    }
}

public class NetworkClient{}
public class SkyfilterClient : NetworkClient{}

However, if it is actually a NetworkClient, then you cannot magically make it become the subclass. Here is an example of that:

using System;

class Program
{
    static void Main()
    {
        NetworkClient net = new NetworkClient();
        var sky = (SkyfilterClient)net;
    }
}

public class NetworkClient{}
public class SkyfilterClient : NetworkClient{}

HOWEVER, you could create a converter class. Here is an example of that, also:

using System;

class Program
{
    static void Main()
    {
        NetworkClient net = new NetworkClient();
        var sky = SkyFilterClient.CopyToSkyfilterClient(net);
    }
}

public class NetworkClient
{  
  public int SomeVal {get;set;}
}

public class SkyfilterClient : NetworkClient
{
    public int NewSomeVal {get;set;}
    public static SkyfilterClient CopyToSkyfilterClient(NetworkClient networkClient)
    {
        return new SkyfilterClient{NewSomeVal = networkClient.SomeVal};
    }
}

But, keep in mind that there is a reason you cannot convert this way. You may be missing key information that the subclass needs.

Finally, if you just want to see if the attempted cast will work, then you can use is:

if(client is SkyfilterClient)
    cast