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 lt string gt  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

User · Answer

There s a few ways of doing this  However  here is one of the easiest ways to do this and it s reusable  What is happening is that we re getting all the properties of the parent class and updating those same properties on the child class  Where baseObj would be the parent object and T would be the child class  public static T ConvertToDerived lt T gt  object baseObj  where T   new                 var derivedObj   new T            var members   baseObj GetType   GetMembers            foreach  var member in members                        object val   null              if  member MemberType    MemberTypes Field                                val     FieldInfo member  GetValue baseObj                     FieldInfo member  SetValue derivedObj  val                             else if  member MemberType    MemberTypes Property                                val     PropertyInfo member  GetValue baseObj                   if  val is IList  amp  amp  val GetType   IsGenericType                                        var listType   val GetType   GetGenericArguments   Single                        var list    IList Activator CreateInstance typeof List lt  gt   MakeGenericType listType                        foreach  var item in  IList val                                                list Add item                                               PropertyInfo member  SetValue baseObj  list  null                                     if    PropertyInfo member  CanWrite                        PropertyInfo member  SetValue derivedObj  val                                   return derivedObj

User · Answer

I don t think you can downcast an object  however there is a simple way to  quot downcast quot  the object outside the box   It isn t type safe  but it works   First serialize the object into json  then deserialize it into the child class object   It works the same as if you were passing the object between apis   So  while there are some people who may say  quot this doesn t work or isn t good quot   I would argue that it is exactly the way our internet currently works  so    why not use that method   No mapping required as long as parameter names are the same  and they will be if it is a child class   Note  This will likely not copy any private fields  if you have a constructor with parameters  this probably needs to be tested as well to ensure there aren t side effects  Here s my toolbox  public static string ConvertToJson lt T gt  this T obj        return JsonConvert SerializeObject obj     public static T ConvertToObject lt T gt  this string json        if  string IsNullOrEmpty json                 return Activator CreateInstance lt T gt               return JsonConvert DeserializeObject lt T gt  json      Here s how to use it  var sfcl   networkClient ConvertToJson   ConvertToObject lt SkyfilterClient gt

User · Answer

If you HAVE to  and you don t mind a hack  you could let serialization do the work for you  Given these classes  public class ParentObj       public string Name   get  set       public class ChildObj   ParentObj       public string Value   get  set       You can create a child instance from a parent instance like so  var parent   new ParentObj     Name    quot something quot     var serialized   JsonConvert SerializeObject parent   var child   JsonConvert DeserializeObject lt ChildObj gt  serialized    This assumes your objects play nice with serialization  obv  Be aware that this is probably going to be slower than an explicit converter

User · Answer

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

User · Answer

I would recommend identifying the functionality you need from any subclasses  and make a generic method to cast into the right subclass    I had this same problem  but really didn t feel like creating some mapping class or importing a library    Let s say you need the  Authenticate  method to take behavior from the right subclass   In your NetworkClient   protected bool Authenticate string username  string password              protected bool DoAuthenticate lt T gt  NetworkClient nc  string username  string password  where T   NetworkClient     Do a cast into the sub class    T subInst    T  nc    return nc Authenticate username  password

User · Answer

Use the cast operator  as such   var skyfilterClient    SkyfilterClient networkClient

User · Answer

You can copy value of Parent Class to a Child class  For instance  you could use reflection if that is the case

User · Answer

I m surprised AutoMapper hasn t come up as an answer   As is clear from all the previous answers  you cannot do the typecast   However  using AutoMapper  in a few lines of code you can have a new SkyfilterClient instantiated based on an existing NetworkClient   In essence  you would put the following where you are currently doing your typecasting   using AutoMapper         somewhere  your network client was declared var existingNetworkClient   new NetworkClient           now we want to type-cast  but we can t  so we instantiate using AutoMapper AutoMapper Mapper CreateMap lt NetworkClient  SkyfilterClient gt     var skyfilterObject   AutoMapper Mapper Map lt SkyfilterClient gt  existingNetworkClient     Here s a full-blown example     public class Vehicle         public int NumWheels   get  set        public bool HasMotor   get  set           public class Car  Vehicle         public string Color   get  set        public string SteeringColumnStyle   get  set           public class CarMaker            I am given vehicles that I want to turn into cars        public List lt Car gt  Convert List lt Vehicle gt  vehicles              var cars   new List lt Car gt           AutoMapper Mapper CreateMap lt Vehicle  Car gt        Declare that we want some automagic to happen       foreach  var vehicle in vehicles                  var car   AutoMapper Mapper Map lt Car gt  vehicle              At this point  the car-specific properties  Color and SteeringColumnStyle  are null  because there are no properties in the Vehicle object to map from             However  car s NumWheels and HasMotor properties which exist due to inheritance  are populated by AutoMapper          cars Add car                 return cars

User · Answer

You can use the as operator to perform certain types of conversions between compatible reference types or nullable types   SkyfilterClient c   client as SkyfilterClient  if  c    null          do something with it      NetworkClient c   new SkyfilterClient   as NetworkClient     c is not null SkyfilterClient c2   new NetworkClient   as SkyfilterClient     c2 is null

User · Answer

In OOP  you can t cast an instance of a parent class into a child class  You can only cast a child instance into a parent that it inherits from

User · Answer

You can t downcast  If the parent object is created  it cannot be cast to the child   One suggested workaround would be to Create an interface which the parent implements  Have the child override functionality if needed or just expose the parents functionality  Change the cast to be an interface and do the operations   Edit  May be could also check if the object is a SkyfilterClient using is keyword     if networkClient is SkyfilterClient

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