Is it possible to assign a base class object to a derived class reference with an explicit typecast

Question

Is it possible to assign a base class object to a derived class reference with an explicit typecast in C     I have tried it and it creates a run-time error

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No  A reference to a derived class must actually refer to an instance of the derived class  or null   Otherwise how would you expect it to behave   For example   object o   new object    string s    string  o  int i   s Length     What can this sensibly do    If you want to be able to convert an instance of the base type to the derived type  I suggest you write a method to create an appropriate derived type instance  Or look at your inheritance tree again and try to redesign so that you don t need to do this in the first place

User · Answer

You can do this using generic   public class BaseClass       public int A   get  set        public int B   get  set        private T ConvertTo lt T gt    where T   BaseClass  new                  return new T                         A   A               B   B                       public DerivedClass1 ConvertToDerivedClass1                  return ConvertTo lt DerivedClass1 gt                public DerivedClass2 ConvertToDerivedClass2                  return ConvertTo lt DerivedClass2 gt              public class DerivedClass1   BaseClass       public int C   get  set       public class DerivedClass2   BaseClass       public int D   get  set        You get three benefits using this approach    You are not duplicating the code You are not using reflection  which is slow  All of your conversions are in one place

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There actually IS a way to do this  Think about how you might use Newtonsoft JSON to deserialize an object from json  It will  or at least can  ignore missing elements and populate all the elements that it does know about   So here s how I did it  A small code sample will follow my explanation    Create an instance of your object from the base class and populate it accordingly  Using the  jsonconvert  class of Newtonsoft json  serialize that object into a json string  Now create your sub class object by deserializing with the json string created in step 2  This will create an instance of your sub class with all the properties of the base class    This works like a charm  So   when is this useful  Some people asked when this would make sense and suggested changing the OP s schema to accommodate the fact that you can t natively do this with class inheritance  in  Net    In my case  I have a settings class that contains all the  base  settings for a service  Specific services have more options and those come from a different DB table  so those classes inherit the base class  They all have a different set of options  So when retrieving the data for a service  it s much easier to FIRST populate the values using an instance of the base object  One method to do this with a single DB query  Right after that  I create the sub class object using the method outlined above  I then make a second query and populate all the dynamic values on the sub class object   The final output is a derived class with all the options set  Repeating this for additional new sub classes takes just a few lines of code  It s simple  and it uses a very tried and tested package  Newtonsoft  to make the magic work   This example code is vb Net  but you can easily convert to c      First  create the base settings object      Dim basePMSettngs As gtmaPayMethodSettings   gtmaPayments getBasePayMethodSetting payTypeId  account id      Dim basePMSettingsJson As String   JsonConvert SerializeObject basePMSettngs  Formatting Indented         Create a pmSettings object of this specific type of payment and inherit from the base class object     Dim pmSettings As gtmaPayMethodAimACHSettings   JsonConvert DeserializeObject Of gtmaPayMethodAimACHSettings  basePMSettingsJson

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You can cast a variable that is typed as the base-class to the type of a derived class  however  by necessity this will do a runtime check  to see if the actual object involved is of the correct type   Once created  the type of an object cannot be changed  not least  it might not be the same size   You can  however  convert an instance  creating a new instance of the second type - but you need to write the conversion code manually

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Is it possible to assign a base class object to a derived class reference with an explicit typecast in C      Not only explicit  but also implicit conversions are possible   C  language doesn t permit such conversion operators  but you can still write them using pure C  and they work  Note that the class which defines the implicit conversion operator  Derived  and the class which uses the operator  Program  must be defined in separate assemblies  e g  the Derived class is in a library dll which is referenced by program exe containing the Program class      In library dll  public class Base      public class Derived        System Runtime CompilerServices SpecialName      public static Derived op Implicit Base a            return new Derived a     Write some Base - gt  Derived conversion code here             System Runtime CompilerServices SpecialName      public static Derived op Explicit Base a            return new Derived a     Write some Base - gt  Derived conversion code here            In program exe  class Program       static void Main string   args            Derived z   new Base      Visual Studio can show squiggles here  but it compiles just fine            When you reference the library using the Project Reference in Visual Studio  VS shows squiggles when you use the implicit conversion  but it compiles just fine  If you just reference the library dll  there are no squiggles

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No  it is not possible   Consider a scenario where an ACBus is a derived class of base class Bus  ACBus has features like TurnOnAC and TurnOffAC which operate on a field named ACState  TurnOnAC sets ACState to on and TurnOffAC sets ACState to off  If you try to use TurnOnAC and TurnOffAC features on Bus  it makes no sense

User · Answer

No  see this question which I asked - Upcasting in  NET using generics  The best way is to make a default constructor on the class  construct and then call an Initialise method

User · Answer

You can use an Extention   public static void CopyOnlyEqualProperties lt T gt  this T objDest  object objSource  where T   class               foreach  PropertyInfo propInfo in typeof T  GetProperties                if  objSource GetType   GetProperties   Any z   gt  z Name    propInfo Name  amp  amp  z GetType      propInfo GetType                     propInfo SetValue objDest  objSource GetType   GetProperties   First z   gt  z Name    propInfo Name  amp  amp  z GetType      propInfo GetType    GetValue objSource            In Code   public class BaseClass     public string test  get  set     public Derived   BaseClass     Some properies    public void CopyProps        BaseClass baseCl  new BaseClass       baseCl test  Hello      Derived drv new Derived       drv CopyOnlyEqualProperties baseCl        Should return Hello to the console now in derived class     Console WriteLine drv test

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class Program       static void Main string   args                a a1   new b              a1 print              class a       public a                 Console WriteLine  base class object initiated              public void print                 Console WriteLine  base            class b a       public b                 Console WriteLine  child class object              public void print1                 Console WriteLine  derived                 when we create a child class object the base class object is auto initiated so base class reference variable can point to child class object   but not vice versa because a child class reference variable can not point to base class object because no child class object is created   and also notice that base class reference variable can only call base class member

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I had this problem and solved it by adding a method that takes a type parameter and converts the current object into that type   public TA As lt TA gt    where TA   Base       var type   typeof  TA       var instance   Activator CreateInstance type         PropertyInfo   properties   type GetProperties         foreach  var property in properties                  property SetValue instance  property GetValue this  null   null                return  TA instance      That means that you can use it in you code like this   var base   new Base    base Data   1  var derived   base As lt Derived gt     Console Write derived Data      Would output 1

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You can just serialize the base object to JSON and then deserialize it to the derived object

User · Answer

How about   public static T As lt T gt  this object obj                return JsonConvert DeserializeObject lt T gt  JsonConvert SerializeObject obj

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Expanding on  ybo s answer - it isn t possible because the instance you have of the base class isn t actually an instance of the derived class  It only knows about the members of the base class  and doesn t know anything about those of the derived class   The reason that you can cast an instance of the derived class to an instance of the base class is because the derived class actually already is an instance of the base class  since it has those members already  The opposite cannot be said

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Solution with JsonConvert  instead of typecast  Today i faced the same issue and i found a simple and quick solution to the problem using JsonConvert  var base   new BaseClass    var json   JsonConvert SerializeObject base   DerivedClass derived   JsonConvert DeserializeObject lt DerivedClass gt  json

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I combined some portions of the previous answers  thanks to those authors  and put together a simple static class with two methods that we re using     Yes  it s simple  no it doesn t cover all scenarios  yes it could be expanded and made better  no it s not perfect  yes it could possibly be made more efficient  no it s not the greatest thing since sliced bread  yes there are full-on robust nuget package object mappers out there that are way better for heavy use  etc etc  yada yada - but it works for our basic needs though      And of course it will try to map values from any object to any object  derived or not  only the public properties that are named the same of course - ignores the rest    USAGE   SesameStreetCharacter puppet   new SesameStreetCharacter     Name    Elmo   Age   5        creates new object of type  RealPerson  and assigns any matching property     values from the puppet object      this method requires that  RealPerson  have a parameterless constructor   RealPerson person   ObjectMapper MapToNewObject lt RealPerson gt  puppet       OR     create the person object on our own      so RealPerson can have any constructor type that it wants  SesameStreetCharacter puppet   new SesameStreetCharacter     Name    Elmo   Age   5    RealPerson person   new RealPerson  tall    Name    Steve        maps and overwrites any matching property values from     the puppet object to the person object so now our person s age will get set to 5 and    the name  Steve  will get overwritten with  Elmo  in this example ObjectMapper MapToExistingObject puppet  person     STATIC UTILITY CLASS   public static class ObjectMapper          the target object is created on the fly and the target type         must have a parameterless constructor  either compiler-generated or explicit       public static Ttarget MapToNewObject lt Ttarget gt  object sourceobject  where Ttarget   new                    create an instance of the target class         Ttarget targetobject    Ttarget Activator CreateInstance typeof Ttarget                map the source properties to the target object         MapToExistingObject sourceobject  targetobject            return targetobject                the target object is created beforehand and passed in     public static void MapToExistingObject object sourceobject  object targetobject                   get the list of properties available in source class         var sourceproperties   sourceobject GetType   GetProperties   ToList                loop through source object properties         sourceproperties ForEach sourceproperty   gt                 var targetProp   targetobject GetType   GetProperty sourceproperty Name                   check whether that property is present in target class and is writeable             if  targetProp    null  amp  amp  targetProp CanWrite                                   if present get the value and map it                 var value   sourceobject GetType   GetProperty sourceproperty Name  GetValue sourceobject  null                   targetobject GetType   GetProperty sourceproperty Name  SetValue targetobject  value  null

User · Answer

Best way to add all base properties to derived item is use reflection in costructor  Try this code  without creating methods or instances       public Derived Base item   base                  Type type   item GetType             System Reflection PropertyInfo   properties   type GetProperties            foreach  var property in properties                        try                               property SetValue this  property GetValue item  null   null                             catch  Exception

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Another solution is to add extension method like so    public static void CopyProperties this object destinationObject  object sourceObject  bool overwriteAll   true                        try                               if  sourceObject    null                                        PropertyInfo   sourceProps   sourceObject GetType   GetProperties                        List lt string gt  sourcePropNames   sourceProps Select p   gt  p Name  ToList                        foreach  PropertyInfo pi in destinationObject GetType   GetProperties                                                  if  sourcePropNames Contains pi Name                                                         PropertyInfo sourceProp   sourceProps First srcProp   gt  srcProp Name    pi Name                               if  sourceProp PropertyType    pi PropertyType                                  if  overwriteAll    pi GetValue destinationObject  null     null                                                                        pi SetValue destinationObject  sourceProp GetValue sourceObject  null   null                                                                                                                                 catch  ApplicationException ex                                throw                            then have a constructor in each derived class that accepts base class     public class DerivedClass  BaseClass                public DerivedClass BaseClass baseModel                        this CopyProperties baseModel                     It will also optionally overwrite destination properties if already set  not null  or not

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As everyone here said  that s not possible directly   The method I prefer and is rather clean  is to use an Object Mapper like AutoMapper   It will do the task of copying properties from one instance to another  Not necessarily the same type  automatically

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I disagree that it is not possible  You can do it like this   public class Auto         public string Make  get  set       public string Model  get  set      public class Sedan   Auto        public int NumberOfDoors  get  set      public static T ConvertAuto lt T gt  Sedan sedan  where T   class       object auto   sedan      return  T loc      Usage   var sedan   new Sedan    sedan NumberOfDoors   4  var auto   ConvertAuto lt Auto gt  sedan

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Not in the Traditional Sense    Convert to Json  then to your object  and boom  done   Jesse above had the answer posted first  but didn t use these extension methods which make the process so much easier   Create a couple of extension methods      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          Put them in your toolbox forever  then you can always do this  var derivedClass   baseClass ConvertToJson   ConvertToObject lt derivedClass gt      Ah  the power of JSON  There are a couple of gotchas with this approach   We really are creating a new object  not casting  which may or may not matter   Private fields will not be transferred  constructors with parameters won t be called  etc   It is possible that some child json won t be assigned   Streams are not innately handled by JsonConvert  However  if our class doesn t rely on private fields and constructors  this is a very effective method of moving data from class to class without mapping and calling constructors  which is the main reason why we want to cast in the first place

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As many others have answered  No   I use the following code on those unfortunate occasions when I need to use a base type as a derived type   Yes it is a violation of the Liskov Substitution Principle  LSP  and yes most of the time we favor composition over inheritance   Props to Markus Knappen Johansson whose original answer this is based upon   This code in the base class       public T As lt T gt                  var type   typeof T           var instance   Activator CreateInstance type            if  type BaseType    null                        var properties   type BaseType GetProperties                foreach  var property in properties                  if  property CanWrite                      property SetValue instance  property GetValue this  null   null                      return  T  instance          Allows       derivedObject   baseObect As lt derivedType gt      Since it uses reflection  it is  expensive    Use accordingly

User · Answer

In c  9 0 you can try to use records for this  They have default copy constructor that copy all fields - no need to use reflection   constructor with all fields  public record BaseR      public string Prop1   get  set       public record DerivedR   BaseR      public DerivedR BaseR baseR    base baseR         public string Prop2   get  set       var baseR   new BaseR   Prop1    quot base prob quot     var derivedR   new DerivedR baseR    Prop2    quot new prop quot

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You can use a copy constructor that immediately invokes the instance constructor  or if your instance constructor does more than assignments have the copy constructor assign the incoming values to the instance   class Person          Copy constructor      public Person Person previousPerson                Name   previousPerson Name          Age   previousPerson Age                Copy constructor calls the instance constructor      public Person Person previousPerson            this previousPerson Name  previousPerson Age                      Instance constructor      public Person string name  int age                Name   name          Age   age             public int Age   get  set         public string Name   get  set        Referenced the Microsoft C  Documentation under Constructor for this example having had this issue in the past

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This is how I solved this for fields  You can do the same iteration through properties if you want  You may want to do some checks for null etc  but this is the idea    public static DerivedClass ConvertFromBaseToDerived lt BaseClass  DerivedClass gt  BaseClass baseClass              where BaseClass   class  new               where DerivedClass   class  BaseClass  new                         DerivedClass derived    DerivedClass Activator CreateInstance typeof DerivedClass                derived GetType   GetFields   ToList   ForEach field   gt                                var base    baseClass GetType   GetField field Name  GetValue baseClass                   field SetValue derived  base                                  return derived

User · Answer

No  that s not possible since assigning it to a derived class reference would be like saying  Base class is a fully capable substitute for derived class  it can do everything the derived class can do   which is not true since derived classes in general offer more functionality than their base class  at least  that s the idea behind inheritance    You could write a constructor in the derived class taking a base class object as parameter  copying the values   Something like this   public class Base       public int Data       public void DoStuff                Do stuff with data          public class Derived   Base       public int OtherData       public Derived Base b            this Data   b Data          OtherData   0     default value            public void DoOtherStuff                Do some other stuff           In that case you would copy the base object and get a fully functional derived class object with default values for derived members  This way you can also avoid the problem pointed out by Jon Skeet   Base b   new Base     base class Derived d   new Derived     derived class  b DoStuff          OK d DoStuff          Also OK b DoOtherStuff          Won t work  d DoOtherStuff          OK  d   new Derived b       Copy construct a Derived with values of b d DoOtherStuff          Now works

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Might not be relevent  but I was able to run code on a derived object given its base   It s definitely more hacky than I d like  but it works   public static T Cast lt T gt  object obj        return  T obj             Invoke parent object s json function MethodInfo castMethod   this GetType   GetMethod  Cast   MakeGenericMethod baseObj GetType     object castedObject   castMethod Invoke null  new object     baseObj     MethodInfo jsonMethod   baseObj GetType    GetMethod   ToJSON    return  string jsonMethod Invoke  castedObject null

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No it is not possible  hence your runtime error   But you can assign an instance of a derived class to a variable of base class type

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I know this is old but I ve used this successfully for quite a while       private void PopulateDerivedFromBase lt TB TD gt  TB baseclass TD derivedclass                  get our baseclass properties         var bprops   baseclass GetType   GetProperties            foreach  var bprop in bprops                          get the corresponding property in the derived class             var dprop   derivedclass GetType   GetProperty bprop Name                 if the derived property exists and it s writable  set the value             if  dprop    null  amp  amp  dprop CanWrite                  dprop SetValue derivedclass bprop GetValue baseclass  null  null

[c#] Is it possible to assign a base class object to a derived class reference with an explicit typecast?

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