Convert base class to derived class

Question

Is it possible in C  to explicitly convert a base class object to one of it s derived classes  Currently thinking I have to create a constructor for my derived classes that accept a base class object as a parameter and copy over the property values  I don t really like this idea  so I d like to avoid it if possible   This doesn t seem like it should work  object is instantiated as new base  so memory shouldn t be allocated for extra members of the derived class  but C  seems to allow me to do it   class BaseClass         some stuff        class DerivedClass   BaseClass       public bool MyDerivedProperty  get  set        static void Main string   args        BaseClass myBaseObject   new BaseClass        DerivedClass myDerivedObject   myBaseObject as DerivedClass       myDerivedObject MyDerivedProperty   true

User · Accepted Answer

No  there s no built-in way to convert a class like you say   The simplest way to do this would be to do what you suggested  create a DerivedClass BaseClass  constructor   Other options would basically come out to automate the copying of properties from the base to the derived instance  e g  using reflection   The code you posted using as will compile  as I m sure you ve seen  but will throw a null reference exception when you run it  because myBaseObject as DerivedClass will evaluate to null  since it s not an instance of DerivedClass

User · Answer

No it is not possible  The only way that is possible is  static void Main string   args        BaseClass myBaseObject   new DerivedClass        DerivedClass myDerivedObject   myBaseObject as DerivedClass       myDerivedObject MyDerivedProperty   true

User · Answer

That s not possible  but you can use an Object Mapper like AutoMapper  Example   class A       public int IntProp   get  set      class B       public int IntProp   get  set        public string StrProp   get  set        In global asax or application startup   AutoMapper Mapper CreateMap lt A  B gt       Usage   var b   AutoMapper Mapper Map lt B gt  a     It s easily configurable via a fluent API

User · Answer

You can implement the conversion yourself  but I would not recommend that  Take a look at the Decorator Pattern if you want to do this in order to extend the functionality of an existing object

User · Answer

I have found one solution to this  not saying it s the best one  but it feels clean to me and doesn t require any major changes to my code  My code looked similar to yours until I realized it didn t work   My Base Class  public class MyBaseClass      public string BaseProperty1   get  set       public string BaseProperty2   get  set       public string BaseProperty3   get  set       public string BaseProperty4   get  set       public string BaseProperty5   get  set        My Derived Class  public class MyDerivedClass   MyBaseClass      public string DerivedProperty1   get  set       public string DerivedProperty2   get  set       public string DerivedProperty3   get  set        Previous method to get a populated base class  public MyBaseClass GetPopulatedBaseClass        var myBaseClass   new MyBaseClass        myBaseClass BaseProperty1    Something     myBaseClass BaseProperty2    Something else     myBaseClass BaseProperty3    Something more       etc        return myBaseClass      Before I was trying this  which gave me a unable to cast error  public MyDerivedClass GetPopulatedDerivedClass        var newDerivedClass    MyDerivedClass GetPopulatedBaseClass        newDerivedClass UniqueProperty1    Some One      newDerivedClass UniqueProperty2    Some Thing      newDerivedClass UniqueProperty3    Some Thing Else       return newDerivedClass      I changed my code as follows bellow and it seems to work and makes more sense now   Old  public MyBaseClass GetPopulatedBaseClass        var myBaseClass   new MyBaseClass        myBaseClass BaseProperty1    Something     myBaseClass BaseProperty2    Something else     myBaseClass BaseProperty3    Something more       etc        return myBaseClass      New  public void FillBaseClass MyBaseClass myBaseClass       myBaseClass BaseProperty1    Something     myBaseClass BaseProperty2    Something else     myBaseClass BaseProperty3    Something more       etc        Old  public MyDerivedClass GetPopulatedDerivedClass        var newDerivedClass    MyDerivedClass GetPopulatedBaseClass        newDerivedClass UniqueProperty1    Some One      newDerivedClass UniqueProperty2    Some Thing      newDerivedClass UniqueProperty3    Some Thing Else       return newDerivedClass      New  public MyDerivedClass GetPopulatedDerivedClass        var newDerivedClass   new MyDerivedClass        FillBaseClass newDerivedClass       newDerivedClass UniqueProperty1    Some One      newDerivedClass UniqueProperty2    Some Thing      newDerivedClass UniqueProperty3    Some Thing Else       return newDerivedClass

User · Answer

No  there is no built in conversion for this   You ll need to create a constructor  like you mentioned  or some other conversion method   Also  since BaseClass is not a DerivedClass  myDerivedObject will be null  andd the last line above will throw a null ref exception

[c#] Convert base class to derived class

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