downcast and upcast

Question

I am new to C   and OOP   When I have some code like the following   class Employee          some code     class Manager   Employee         some code     Question 1  If I have other code that does this      Manager mgr   new Manager       Employee emp    Employee mgr    Here Employee is a Manager  but when I cast it like that to an Employee it means I am upcasting it   Question 2   When I have several Employee class objects and some but not all of them are Manager s  how can I downcast them where possible

User · Answer

Upcasting is an operation that creates a base class reference from a subclass reference   subclass -  superclass   i e  Manager -  Employee  Downcasting is an operation that creates a subclass reference from a base class reference   superclass -  subclass   i e  Employee -  Manager    In your case  Employee emp    Employee mgr    mgr is Manager   you are doing an upcasting   An upcast always succeeds unlike a downcast that requires an explicit cast because it can potentially fail at runtime  InvalidCastException    C  offers two operators to avoid this exception to be thrown   Starting from   Employee e   new Employee      First     Manager m   e as Manager     if downcast fails m is null  no exception thrown   Second   if  e is Manager          the predicate is false if the downcast is not possible    Warning  When you do an upcast you can only access to the superclass  methods  properties etc

User · Answer

That is correct  When you do that you are casting it it into an employee object  so that means you cannot access anything manager specific  Downcasting is where you take a base class and then try and turn it into a more specific class  This can be accomplished with using is and an explicit cast like this    if  employee is Manager        Manager m    Manager employee        do something with it      or with the as operator like this   Manager m    employee as Manager   if  m    null          do something with it     If anything is unclear I ll be happy to correct it

User · Answer

Answer 1     Yes it called upcasting but the way you do it is not modern way  Upcasting can be performed implicitly you don t need any conversion  So just writing Employee emp   mgr  is enough for upcasting   Answer 2     If you create object of Manager class we can say that manager is an employee  Because class Manager   Employee depicts Is-A relationship between Employee Class and Manager Class  So we can say that every manager is an employee   But if we create object of Employee class we can not say that this employee is manager because class Employee is a class which is not inheriting any other class  So you can not directly downcast that Employee Class object to Manager Class object   So answer is  if you want to downcast from Employee Class object to Manager Class object  first you must have object of Manager Class first then you can upcast it and then you can downcast it

User · Answer

Upcasting and Downcasting   Upcasting  Casting from Derived-Class to Base Class Downcasting  Casting from Base Class to Derived Class  Let s understand the same as an example   Consider two classes Shape as My parent class and Circle as a Derived class  defined as follows   class Shape       public int Width   get  set        public int Height   get  set       class Circle   Shape       public int Radius   get  set        public bool FillColor   get  set        Upcasting   Shape s   new Shape     Circle c  s   Both c and s are referencing to the same memory location  but both of them have different views i e using  c  reference you can access all the properties of the base class and derived class as well but using  s  reference you can access properties of the only parent class   A practical example of upcasting is Stream class which is baseclass of all types of stream reader of  net framework   StreamReader reader   new StreamReader new FileStreamReader      here  FileStreamReader   is upcasted to streadm reder   Downcasting   Shape s   new Circle    here as explained above  view of s is the only parent  in order to make it for both parent and a child we need to downcast it  var c    Circle  s   The practical example of Downcasting is button class of WPF

User · Answer

Upcasting  using  Employee someInstance  is generally easy as the compiler can tell you at compile time if a type is derived from another    Downcasting however has to be done at run time generally as the compiler may not always know whether the instance in question is of the type given  C  provides two operators for this - is which tells you if the downcast works  and return true false  And as which attempts to do the cast and returns the correct type if possible  or null if not   To test if an employee is a manager   Employee m   new Manager    Employee e   new Employee     if m is Manager  Console WriteLine  m is a manager    if e is Manager  Console WriteLine  e is a manager      You can also use this  Employee someEmployee   e  as Manager      if someEmployee     null  Console WriteLine  someEmployee  e  is a manager     Employee someEmployee   m  as Manager      if someEmployee     null  Console WriteLine  someEmployee  m  is a manager

User · Answer

In case you need to check each of the Employee object whether it is a Manager object  use the OfType method   List lt Employee gt  employees   new List lt Employee gt        Code to add some Employee or Manager objects    var onlyManagers   employees OfType lt Manager gt      foreach  Manager m in onlyManagers         Do Manager specific thing

[c#] downcast and upcast

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