Unable to Cast from Parent Class to Child Class

Question

I am trying to cast from a parent class to a child class but I get an InvalidCastException  The child class only has one property of type int  Does anyone know what I need to do

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You can t cast a mammal into a dog - it might be a cat   You can t cast a food into a sandwich - it might be a cheeseburger   You can t cast a car into a Ferrari - it might be a Honda  or more specifically  You can t cast a Ferrari 360 Modena to a Ferrari 360 Challange Stradale - there are differnt parts  even though they are both Ferrari 360s

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To cast   the actual object must be of a Type equal to or derived from the Type you are attempting to cast to        or  to state it in the opposite way   the Type you are trying to cast it to must be the same as  or a base class of  the actual type of the object    if your actual object is of type Baseclass  then you can t cast it to a derived class Type

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I have seen most of the people saying explicit parent to child casting is not possible  that actually is not true  Let s take a revised start and try proving it by examples   As we know in  net all castings have two broad categories    For Value type  For Reference type  in your case its reference type    Reference type has further three main situational cases in which any scenario can lie   Child to Parent  Implicit casting - Always successful      Case 1  Child to any direct or indirect parent   Employee e   new Employee    Person p    Person e    Allowed   Parent to Child  Explicit casting - Can be successful      Case 2  Parent variable holding parent object  Not allowed    Person p   new Person        p is true Person object Employee e    Employee p    Runtime err   InvalidCastException  lt -------- Yours issue      Case 3  Parent variable holding child object  Always Successful       Note  Because objects has polymorphic nature  it is possible for a variable of a parent class type to hold a child type    Person p   new Employee       p actually is Employee Employee e    Employee p     Casting allowed   Conclusion   After reading above all  hope it will make sense now like how parent to child conversion is possible Case 3       Answer To The Question        Your answer is    in case 2 Where you can see such casting is not allowed by OOP and you are trying to violate one of OOP s basic rule So always choose safe path       Further more  to avoid such exceptional situations  net has recommended using is as operators those will help you to take informed decisions and provide safe casting

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That would violate object oriented principles  I d say an elegant solution here and elsewhere in the project is using a object mapping framework like AutoMapper to configure a projection    Here s a slighty more complex configuration than is neccessary but is flexible enough for most cases   public class BaseToChildMappingProfile   Profile       public override string ProfileName               get   return  BaseToChildMappingProfile                protected override void Configure                 Mapper CreateMap lt BaseClass  ChildClassOne gt             Mapper CreateMap lt BaseClass  ChildClassTwo gt               public class AutoMapperConfiguration       public static void Configure                 Mapper Initialize x   gt                        x AddProfile lt BaseToChildMappingProfile gt                           When application starts call AutoMapperConfiguration Configure   and then you can project like this   ChildClassOne child   Mapper Map lt BaseClass  ChildClassOne gt  baseClass     Properties are mapped by convention so if the class is inherited the property names are exactly the same and mapping is configured automatically  You can add additional properties by tweaking the configuration  See the documentation

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As for me it was enough to copy all property fields from the base class to the parent like this   using System Reflection   public static ChildClass Clone BaseClass b        ChildClass p   new ChildClass               Getting properties of base class      PropertyInfo   properties   typeof BaseClass  GetProperties            Copy all properties to parent class      foreach  PropertyInfo pi in properties                if  pi CanWrite              pi SetValue p  pi GetValue b  null   null              return p      An universal solution for any object can be found here

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As of C  7 0  you can use the is keyword to do this    With those class defined    class Base      Define base class      class Derived   Base      Define derived class        You can then do somehting like    void Funtion Base b        if  b is Derived d                   Do something with d which is now a variable of type Derived              Which would be equivalent to    void Funtion Base b        Defined d      if  b is Derived                d    Defined b             Do something with d              You could now call    Function new Derived        Will execute code defined in if   As well as  Function new Base        Won t execute code defined in if   That way you can be sure that your downcast will be valid and won t throw an exception

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Paul  you didn t ask  Can I do it  - I am assuming you want to know how to do it    We had to do this on a project - there are many of classes we set up in a generic fashion just once  then initialize properties specific to derived classes  I use VB so my sample is in VB  tough noogies   but I stole the VB sample from this site which also has a better C  version   http   www eggheadcafe com tutorials aspnet a4264125-fcb0-4757-9d78-ff541dfbcb56 net-reflection--copy-cl aspx  Sample code   Imports System Imports System Collections Generic Imports System Reflection Imports System Text Imports System Diagnostics  Module ClassUtils      Public Sub CopyProperties ByVal dst As Object  ByVal src As Object          Dim srcProperties   As PropertyInfo   src GetType GetProperties         Dim dstType   dst GetType          If srcProperties Is Nothing Or dstType GetProperties Is Nothing Then             Return         End If          For Each srcProperty As PropertyInfo In srcProperties             Dim dstProperty As PropertyInfo   dstType GetProperty srcProperty Name               If dstProperty IsNot Nothing Then                 If dstProperty PropertyType IsAssignableFrom srcProperty PropertyType    True Then                     dstProperty SetValue dst  srcProperty GetValue src  Nothing   Nothing                  End If             End If         Next     End Sub End Module   Module Module1     Class base class         Dim  bval As Integer         Public Property bval   As Integer             Get                 Return  bval             End Get             Set ByVal value As Integer                   bval   value             End Set         End Property     End Class     Class derived class         Inherits base class         Public  dval As Integer         Public Property dval   As Integer             Get                 Return  dval             End Get             Set ByVal value As Integer                   dval   value             End Set         End Property     End Class     Sub Main             NARROWING CONVERSION TEST         Dim b As New base class         b bval   10         Dim d As derived class          d   CType b  derived class    invalidcast exception           d   DirectCast b  derived class    invalidcast exception          d   TryCast b  derived class    returns  nothing  for c         d   New derived class         CopyProperties d  b          d dval   20         Console WriteLine b bval          Console WriteLine d bval          Console WriteLine d dval          Console ReadLine       End Sub End Module   Of course this isn t really casting  It s creating a new derived object and copying the properties from the parent  leaving the child properties blank  That s all I needed to do and it sounds like its all you need to do  Note it only copies properties  not members  public variables  in the class  but you could extend it to do that if you are for shame exposing public members    Casting in general creates 2 variables pointing to the same object  mini tutorial here  please don t throw corner case exceptions at me   There are significant ramifications to this  exercise to the reader    Of course I have to say why the languague doesn t let you go from base to derive instance  but does the other way  imagine a case where you can take an instance of a winforms textbox  derived  and store it in a variable of type Winforms control  Of course the  control  can move the object around OK and you can deal with all the  controll-y  things about the textbox  e g   top  left   text properties   The textbox specific stuff  e g    multiline  can t be seen without casting the  control  type variable pointing to the textbox in memory  but it s still there in memory   Now imagine  you have a control  and you want to case a variable of type textbox to it  The Control in memory is missing  multiline  and other textboxy things  If you try to reference them  the control won t magically grow a multiline property  The property  look at it like a member variable here  that actually stores a value - because there is on in the textbox instance s memory  must exist  Since you are casting  remember  it has to be the same object you re pointing to  Hence it is not a language restriction  it is philosophically impossible to case in such a manner

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There are some cases when such a cast would make sense  I my case  I was receiving a BASE class over the network  and I needed more features to it  So deriving it to handle it on my side with all the bells and whistles I wanted  and casting the received BASE class into the DERIVED one was simply not an option  Throws InvalidCastException of Course   One practical think-out-of-the-box SOLUTION was to declare an EXTENSION Helper class that was NOT inheriting BASE class actually  but INCLUDING IT as a member   public class BaseExtension      Base baseInstance      public FakeDerived Base b             baseInstance   b             Helper methods and extensions to Base class added here     If you have loose coupling and just need a couple of extra features to base class without REALLY having an absolute need of derivation  that could be a quick and simple workaround

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The instance that your base class reference is referring to is not an instance of your child class   There s nothing wrong   More specifically   Base derivedInstance   new Derived    Base baseInstance   new Base     Derived good    Derived derivedInstance     OK Derived fail    Derived baseInstance     Throws InvalidCastException   For the cast to be successful  the instance that you re downcasting must be an instance of the class that you re downcasting to  or at least  the class you re downcasting to must be within the instance s class hierarchy   otherwise the cast will fail

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A simple way to downcast in C  is to serialize the parent and then deserialize it into the child    var serializedParent   JsonConvert SerializeObject parentInstance     Child c    JsonConvert DeserializeObject lt Child gt  serializedParent     I have a simple console app that casts animal into dog  using the above two lines of code over here

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The instance of the object should be created using the child class s type  you can t cast a parent type instance to a child type

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A variation on the serialization approach for those using ServiceStack   var child   baseObject ConvertTo lt ChildType gt       or the more verbose   var child   baseObject ToJson   FromJson lt ChildType gt       ServiceStack s serialization might be super fast and all  but clearly  this is not a solution for massive conversions in low-latency transfers  nor for highly complex types   That s likely obvious to anyone using ServiceStack  but thought I d clarify in anticipation of comments

[c#] Unable to Cast from Parent Class to Child Class

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