suppose I have two following classes
public class Animal
{
public string name { get; set; }
}
public class Cat: Animal
{
public int age { get; set; }
public string type { get; set; }
}
and now I want to convert my derived class "Cat" match property(e.g. Name) to my base class "Animal" with implicit conversion as following way.
Cat cat = new Cat();
cat.name = "diana";
cat.age = 2;
cat.type = "Siamese-American Shorthair";
Animal animal = new Animal();
animal = (Animal)cat;
// or
animal = cat as Animal;
so while doing above coding it will work fine and will get name property implicit in Animal class object but if I check object of Animal class that is animal actually contains the object of Cat class that is cat and not actually get the object of Animal class.
So please help me to come over this situation so I can directly implicit convert my child class property to my matched class property with proper parent class object.
A cat will always be a cat, even if you are only looking at animals. You cannot remove the actual type of an object; casting it to a parent type will only affect the variable you store it to but the underlying object remains the same.
Cat cat = new Cat(); // cat is a cat
Animal catAnimal = cat; // a cat is also an animal
Console.WriteLine(catAnimal.GetType()); // still a cat
Cat newCat = (Cat)catAnimal;
Console.WriteLine(newCat == cat); // the same cat
Console.WriteLine(animal == cat); // the same cat
First, this is not an implicit conversion, but an explicit one.
An implicit conversion example is this:
int x = 123;
string y = "asdf" + x; // this is the implicit conversion.
Second, you can't "uncat" the cat. it will still be a cat even if your reference is of type object,
Third, any object from any class will keep the properties and fields of it's parent class, unless they are declared as private, so casting a cat to an animal to get it's name property is redundant.
So, can such a cast be useful?
The answer is yes, it might, in the following situations:
Your derived class hides base class functionality using the new keyword.
Your derived class explicitly implements an interface, and the implicit implementation is different then the explicit one.
Here are examples of these situations:
Casting an object to it's base class to use a property or method is when the property or method in the derived class is declared as new:
public class Base {
internal virtual string X() {
return "Base";
}
}
public class Derived1 : Base
{
internal new string X()
{
return "Derived 1";
}
}
public class Derived2 : Base
{
internal override string X()
{
return "Derived 2";
}
}
Derived1 a = new Derived1();
Base b = new Derived1();
Base c = new Derived2();
Console.WriteLine("Derived1 as Derived1: "+ a.X()); // Derived1 as Derived1: Derived 1
Console.WriteLine("Derived1 as Base: " + b.X()); // Derived1 as Base: Base
Console.WriteLine("Derived2 as Base: " + c.X()); // Derived2 as Base: Derived 2
See fiddle here
Casting an object to one of the interfaces it implements, when the class overloads the explicit implementation with an implicit one.
public interface IBlabla {
string bla();
}
public class BlaBla : IBlabla
{
public string bla() {
return "implicit";
}
string IBlabla.bla()
{
return "EXPLICIT";
}
}
BlaBla Myclass = new BlaBla();
Console.WriteLine(Myclass.bla()); // implicit
Console.WriteLine(((IBlabla)Myclass).bla()); // EXPLICIT
See fiddle here
You cannot change an objects type. As Yuval Itzchakov already mentioned you need to create an instance of type Animal. You may only create some kind of copy-constructor for your Animal-class that copied all properties and variables from the given Animal to the new one:
public Animal(Animal animal) {
this.Name = animal.Name;
// further variables and properties to reset the state exactly to the state of the given animal
}
Now you can create an instance of type Animal from every derived type like so:
Animal ani = new Animal(cat);
However that still sounds like a design-flaw (propbably an XY-problem) to me. If you need to access the Name-property of your cat you won´t need to cast to its base-type.
If you really wanted to, you could do something along the lines of
internal class Program
{
public class Animal
{
public string Name { get; set; }
}
public class Cat : Animal
{
public int Age { get; set; }
public string Type { get; set; }
}
public static void Main()
{
var cat = new Cat();
cat.Name = "Puss";
var animal = cat.ToBaseClass<Animal, Cat>();
Debug.Assert(!(animal is Cat));
Debug.Assert(animal.Name == "Puss");
}
}
public static class ReflectionHelper
{
public static TBase ToBaseClass<TBase, TDerived>(this TDerived from)
where TBase : new()
where TDerived : TBase
{
var result = new TBase();
foreach (PropertyDescriptor propertyDescriptor in TypeDescriptor.GetProperties(result))
{
propertyDescriptor.SetValue(result, propertyDescriptor.GetValue(from));
}
return result;
}
}
... but it would be pretty ugly ;)
Thanks to all of you who help me to solve my problem and I take everyone answers and comments very seriously.
I have also find another solutions which may be useful to someone. As this questions is regarding conversion with same property from child to parents class, we can use AutoMapper which can do conversion for any class from child to parent class or parent to child class. For that first you can download or update AutoMapper from nuget at give link and add as reference in your project or application.
https://www.nuget.org/packages/AutoMapper/
Now as per my question if I want to convert property from child to parent class than coding using AutoMapper functionality would be as below.
Mapper.CreateMap<Cat, Animal>();
Animal animal = Mapper.Map<Animal>(cat);
I hope this solutions will work for you as it has solved my issues.
Related
Let's say that i have a base class Animal.
public abstract class Animal;
This animal class has the abstract method:
public abstract T Copy<T>() where T : Animal
When this method is overridden in the Lion class:
public class Lion : Animal
{
string roar = "Roar";
}
i wish to return a copy of this lion without its references. So what i think it should be overridden like is this:
public abstract T Copy<T>()
{
return new Lion(){
roar = this.roar;
}
}
but this is not allowed, because Lion cannot be converted to the T type(which is an Animal).
This is a understandable error, becouse not every Animal is necessarily a Lion.
But how can i set this situation up where the copy method knows it supposed to return the subclass, Lion in this case, and not have to do all kinds of casting to the correct type? And still have every animal have a method Copy().
You don't need to use a generic method in this case, because every Lion is an Animal, so you can simply make Copy return an Animal
public abstract class Animal
{
public abstract Animal Copy();
}
public class Lion : Animal
{
public string Roar {get; set;} = "Roar";
public override Animal Copy()
{
return new Lion
{
Roar = this.Roar
};
}
}
Then testing it like so:
public static void Main()
{
var lion1 = new Lion();
var lion2 = lion1.Copy();
Console.WriteLine(lion1 == lion2);
}
Prints false to the console, as expected because C# classes are reference types and the default == implementation checks for reference equals on classes and our two lions aren't the same object in memory.
Now every subclass of Animal must implement the method Copy returning an Animal. Which animal that is doesn't matter
You can not use generics in this situation, they don't support what you want to do.
However, if you're on C# 9 you can use the new "Covariant return types" support.
Specifically, you can write your code like this:
public abstract class Animal
{
public abstract Animal Copy();
}
public class Lion : Animal
{
public override Lion Copy() => new Lion();
// ^
// notice that I used Lion here, not Animal
}
If you're calling Copy through a reference declared as being of type Animal but referencing an instance of Lion, you will get back a value typed to Animal, holding a Lion reference.
However, if you're calling Copy through a reference typed to Lion, you will get back a reference typed to Lion as well.
Example:
Animal a = new Lion();
Animal c = a.Copy(); <-- holding a Lion, but type is Animal
Lion l = new Lion();
Lion l2 = l.Copy(); <-- type is now Lion as well
If you're not on C# 9, a different option would be to make the Copy method know the type of the derived class, by making Animal generic:
void Main()
{
Animal<Lion> a = new Lion();
Lion c1 = a.Copy(); // <-- notice that this is now typed to Lion as well
Lion l = new Lion();
Lion c2 = l.Copy();
}
public abstract class Animal<T> where T : Animal<T>
{
public abstract T Copy();
}
public class Lion : Animal<Lion>
{
public override Lion Copy() => new Lion();
}
This, however, means you always have to declare Animal as the right type of animal so you lose some of the benefits of having this base class.
I'm facing a bit of problem with the following casting:
class A
{
}
class B : A
{
}
class C<T> where T : A
{
protected T property { get; set; }
}
class D : C<B>
{
}
class MainClass
{
public static void Main (string[] args)
{
C<A> x = new D();
// Error CS0029: Cannot implicitly convert type `SampleApp.D' to `SampleApp.C<SampleApp.A>' (CS0029) (SampleApp)
}
}
I don't understand why this is failing since D is wider than C<A> since it implements C<B>, and B : A. Any workarounds?
If you can use C# 4.0, you can write the following code.
class A { }
class B : A {}
interface IC<out T> {}
class C<T> :IC<T> where T : A { protected T property { get; set; } }
class D : C<B> {}
class MainClass {
public static void Main()
{
IC<A> x = new D();
}
}
Let's name your classes Animal for A, Barker for B, and Dog for D.
Actually C<Animal> is wider than Dog : C<Barker>. Assume you have public property Me of type T and assignment possible:
C<Animal> a = new Dog();
a.Me = Elephant; // where Elephant inherited from Animal
Oops! Dog is parametrized with Barker. Have you seen barking elephants?
You need to declare some covariant interface to allow assignment of class instantiated with more derived type argument C<Barker> to object instantiated with less derived type argument C<Animal>. You can use empty interface, like #NickW suggested, but you will not be able to do something with instance of that interface (it's empty!). So, let's do something like that:
interface IC<out T>
where T : Animal
{
IEnumerable<T> Parents(); // IEnumerable is covariant
T Me { get; } // no setter
}
class C<T> : IC<T>
where T: Animal
{
// implementation
}
class D : C<Barker>
{
// implementation
}
Above scenario is still impossible, but now you can
IC<Animal> a = new Dog();
foreach(var parent in a.Parents)
Console.WriteLine(parent);
Console.WriteLine(a.Me);
You can't do that because the Generics are actualy templates and they don't act like what you want to do with them. Let me show you by this:
When you say "C<A>" it means a generic class by a "parameter" of "A".
BUT
When you say "D" it means exactly "D"!
So D is not equal to a generic class by a parameter of A. As you can simply see it in the result of ToString function on both types (by using typeof).
Hope it helps
Cheers
I'm running into an issue when trying to access an interface property on an implementing class. The problem is, I only have the specific (Cat) type at runtime, so my app breaks when it tries to cast.
Here's what I have:
public class Animal {}
public class Cat : Animal {}
public interface IPetSitter {}
public interface IPetSitter<T> IPetSitter where T : Animal {
T Pet { get; set; }
}
public class Kid { }
public class NeighborhoodKid : Kid, IPetSitter<Animal> {
Animal Pet { get; set; }
}
// --- Implementation ---
// Kid Timmy is instantiated elsewhere
// Animal type "A" is passed in dynamically
if (Timmy is IPetSitter) {
((IPetSitter<A>)Timmy).Pet = new A();
}
This cast will error if the types don't match. I'd love to do something like this:
public interface IPetSitter {
object Pet { get; set; }
}
public interface IPetSitter<T> : IPetSitter where T : Animal {
new T Pet { get; set; }
}
// --- Implementation ---
NeighborhoodKid Timmy = new NeighborhoodKid();
((IPetSitter)Timmy).Pet = new Cat();
But that forces anything implementing IPetSitter to have both [object Pet] and [Cat Pet] properties.
I'd appreciate any ideas. Thanks.
UPDATE: I should have made it more clear initially, but sometimes I will create a Kid class and sometimes a NeighborhoodKid class. That's why I need to cast to IPetSitter<T>. Not all kids I create will be sitting pets. This is starting to sound creepy.
The problem is that you defined
public class NeighborhoodKid : IPetSitter<Animal>
{
Animal IPetSitter<Animal>.Pet { get; set; }
}
and not
public class NeighborhoodKid : IPetSitter<Cat>
{
Cat IPetSitter<Animal>.Pet { get; set; }
}
or
public class NeighborhoodKid<T> : IPetSitter<T> where T : Animal
{
Cat IPetSitter<T>.Pet { get; set; }
}
Timmy, ultimately, was initialized as a NeighborhoodKid. Which means that Pet, for him, as Animal. Timmy is an IPetSitter<Animal>, and you can't cast it to IPetSitter<Cat>.
You could do it the other way around, though, assuming Cat : Animal.
This:
((IPetSitter<Animal>)Timmy).Pet = new Cat();
Actually works simply because Timmy really is IPetSitter<Animal>, since NeighborhoodKid : IPetSitter<Animal>, so you aren't really doing anything with that cast - just accessing the pet property.
The problem with the line after that, isn't accessing to Pet, nor putting Cat into it - it is casting Timmy to IPetSitter<Cat> that is the problem. You are downcasting it to something that it isn't.
You can always up-cast, but you can only down-cast into what you initialized the object with.
If you want NeighborhoodKid to be an IPetSitter of any sort of animal, including animal itself, you should do:
public class NeighborhoodKid<T> : IPetSitter<T> where T : Animal
{
...
}
That way, it is generic, and constraining it to being something that is either an Animal or something that derives from Animal, whether directly or indirectly.
Still, if you initialized it as new NeighborhoodKid<Animal>(), you will not be able to look at it as (aka cast it to) IPetSitter<Cat>, because it was INITIALIZED as an IPetSitter<Animal> (since the generic T parameter given to the NeighborhoodKid constructor was Animal, and is passed to the IPetSitter generic parameter).
Why not just Timmy.Pet = new Cat();?
Just make it public and youll be all set:
public class NeighborhoodKid : Kid, IPetSitter<Animal>
{
public Animal Pet { get; set; }
}
If you create a NeighborhoodKid that doesnt inherit from IPetSitter, the setter wont be availabe.
public class LazyNeighborhoodKid : Kid
{
// Nothing here, hes not a Pet Sitter, can access Pet
}
I'm not really a fan of generics outside of their usefulness in collections, and this is why. You are forcing every NeighborhoodKid to be bound to a single specific type of Animal. What if Timmy can watch cats or dogs? Are you going to create different Timmy instances for each?
Instead, I'm thinking you enforce animal types at at the instance level. For example (I've truncated some of the types for the sake of brevity):
public interface IAnimal {...}
public class Cat : IAnimal {...}
public interface IPetSitter
{
IAnimal Pet { get; set; }
}
public class Kid : IPetSitter
{
public Kid (params Type[] allowedPets) {
_allowedPets = allowedPets;
}
readonly IEnumerable<Type> _allowedPets;
IAnimal _pet;
public IAnimal Pet
{
get {
return _pet;
}
set {
if (!_allowedPets.Contains(value.GetType()) {
throw new InvalidArgumentException("This instance does not support " + value.GetType().Name + ".");
}
_pet = value;
}
}
}
If you leave your enforcement at the instance level, then you don't necessarily need to use concrete casting just to set a property.
I have an interface and two types that derive from it.
However, I cannot do the following:
B objectB = (B) objectA
Where B derives from Interface1 (I am making up the name of classes but the point still stands), and likewise for objectA (which is of type A). I get the following error message:
Cannot cast expression of type A to B.
Both types are deriving from the interface, what am I missing?
Types do not derive from an interface. They implement an interface.
The fact that both an Elephant and a Spider are Animals doesn't mean
that you can convert one to the other.
An object is assignable to an ancestor (direct or indirect base type) or to an interface it implements, but not to siblings (i.e. another type deriving from a common ancestor); however, you can declare your own explicit conversions:
class FooObject : IObject
{
public string Name { get; set; }
public int Value { get; set; }
public static explicit operator FooObject(BarObject bar)
{
return new FooObject { Name = bar.Name, Value = bar.Value };
}
}
class BarObject : IObject
{
public string Name { get; set; }
public int Value { get; set; }
public static explicit operator BarObject(FooObject bar)
{
return new BarObject { Name = bar.Name, Value = bar.Value };
}
}
Now you can write
var foo = new FooObject();
var bar = (BarObject)foo;
or
var bar = new BarObject();
var foo = (FooObject)bar;
without getting errors.
You can also create implicit conversions, if it feels natural. E.g. int is implicitly convertible to double: int i = 5; double x = i;.
(This is also an answer to the closed question How do I cast Class FooObject to class BarObject which both implement interface IObject?).
You cannot cast or convert from A to B if all they share is a common interface unless you actually define your own conversion operator, assuming you control the source for one of the types, or use another provided user-defined conversion supplied by someone who does control the source. (However, such user-defined conversions would not preserve the original object. One object goes into the conversion, a different object comes out.)
You can convert from A to Interface1, and B to Interface1. But two types simply sharing a common parent does not make those two types convertible to one another.
A a = new A();
B b = new B();
Interface1 obj1 = a; // legal
Interface1 obj2 = b; // legal
B obj3 = (B)a; // not legal, a is simply not a B
tobias86 put in well in a comment below, you have a cat and a dog. Both derive from Animal. But a cat just isn't a dog.
As an expansion, you might be struggling with how and why you would use an interface. You do not use an interface to substitute an A for a B, or a B for an A. You use it to substitute either A or B for Interface1. It's the interface you expect, and the A or B you might supply. Given:
public void DoSomething(Interface1 obj) { } // expects
DoSomething(new A()); // you can supply A
Or
public Interface1 GetSomething() // callers expect to get
{
return new B(); // you can supply a B
}
It's the interface you are programming towards, The A and B are merely implementations. You might be thinking you can pass a B to something that expects A. The expectation possibly needs to change.
The fact that both types implement the same interface (or have the same base-type, for that matter) does not make them interchangeable; an A is always an A, and a B is always a B. In an inheritance chain, an object can be cast as itself or any parent type. You have:
A : ISomeInterface
B : ISomeInterface
which lets you cast an A as A or ISomeInterface, and a B as B or ISomeInterface
or (depending on your meaning of "derived from")
SomeBaseType
> A
> B
which lets you cast an A as A or SomeBaseType, and a B as B or SomeBaseType
(plus object, in each case)
You need to cast as the interface.
interface IBase { }
class A : IBase { }
class B : IBase { }
With this, the only thing the two types have in common is the interface members. B might have items that A does not.
A a = new A();
B b = new B();
IBase aBase = a;
IBase bBase = b;
You can then call anything on the IBase Interface.
When casting from A to B B must be a super type for A or the runtime type of the object must be B
that is if you have
class A : B{}
you can cast an object of compile time type A to B. You can also cast a type of B to A if the runtime type of the object is A
in your case the two types does not share super-subtype relationship. They only share a common super type but that's not sufficient.
As an example of why this can't work (generically) how would you have the compiler cast from Point[] to a Dictionary<string,HashSet<byte>>? (both implement IEnumerable)
What you want to do doesn't make sense. objectA is not a B.
You can only cast them to the interface type. A is not B but they are both I. this means you can take A and cast to I or B and cast to I but not B and cast to A
Imagine the following setup:
public interface Human
{
bool Male { get; }
}
public class Man : Human
{
public bool HasABeard { get { return true; } }
public bool IsMale { get { return true; } }
}
public class Woman : Human
{
public bool IsMale { get { return false; } }
public List<Pair<Shoe>> Shoes { get; set; }
}
What would you expect the compiler to produce from the following code? What will the output be?
Man a;
Woman b = new Woman();
a = (Man)b;
Console.WriteLine(a.HasABeard ? "Beard ON" : "Beard OFF");
With the help of a static generic method defined in the interface, a desired result could be achieved using a reflection:
public interface IPerson
{
string Name { get; set; }
static TDest ChangeType<TDest, TSource>(TSource source) where TSource : IPerson where TDest : IPerson, new()
{
var instance = new TDest();
foreach (var property in typeof(IPerson).GetProperties())
{
property.SetValue(instance, property.GetValue(source));
}
return instance;
}
}
Classes implementing IPerson:
public class Worker : IPerson
{
public string Name { get; set; }
}
public class Manager : IPerson
{
public string Name { get; set; }
}
Usage:
var worker = new Worker { Name = "John" };
var manager = IPerson.ChangeType<Manager, Worker>(worker);
Alternatively using Aggregate:
public interface IPerson
{
string Name { get; set; }
static TDest ChangeType<TDest, TSource>(TSource source) where TSource : IPerson where TDest : IPerson, new()
{
return typeof(IPerson).GetProperties()
.Aggregate(new TDest(), (dest, prop) => {
prop.SetValue(dest, prop.GetValue(source));
return dest;
});
}
}
I'd appreciate your advice on the following:
I'm using polymorphism. I have a base class and 30 sub classes that inherit this base class. I'm up casting instances of these sub classes to the base class type so that they can be handled in a more generic fashion.
My question is this.
I need to access a public property that is specific to a particular sub class. Do I need to write a giant case statement where I check the type and down cast accordingly in order to access the property I need or is there a more elegant solution?
static void Main(string[] args)
{
animal slyvester = new cat();
animal lassie = new dog();
animal silver = new horse();
// Big ugly type checking code. If I have 30 types to check is there a better way?
if (slyvester.GetType() == typeof(cat)) {
Console.WriteLine(((cat)(animal)slyvester).PurrStrength);
}
else if(slyvester.GetType() == typeof(dog)) {
}
else if (slyvester.GetType() == typeof(horse))
{
}
Console.ReadLine();
}
}
public class animal {
}
public class cat : animal {
private string _purrStrength = "Teeth Shattering";
public string PurrStrength {
get { return _purrStrength; }
set { _purrStrength = value; }
}
}
public class dog : animal {
}
public class horse : animal {
}
You should consider an interface based approach. With interfaces, you define a set of operations (a contract by which implementers must conform) which your types must define. E.g, we could define a base interface, IAnimal
public interface IAnimal
{
string GetSound();
}
From which we can define some animal types:
public class Cat : IAnimal
{
public string GetSound()
{
return "Meow!";
}
}
public class Dog : IAnimal
{
public string GetSound()
{
return "Woof!";
}
}
Now, when we want to declare our animal, we declare it of type IAnimal:
IAnimal cat = new Cat();
IAnimal dog = new Dog();
Console.WriteLine(cat.GetSound());
Console.WriteLine(dog.GetSound());
You could go one step further, and specialise your animals:
public class Cat : IAnimal
{
public virtual string GetSound()
{
return "Meow!";
}
}
public class BigCat : Cat
{
public override string GetSound()
{
return "Roar!";
}
}
In the latter example, I can make a default implementation of the cat's GetSound method, and then override it for my big cat.
Interface based programming hides away the need to horrible type conversions, because an interface guarantees a set of operations that will be provided.
If you don't have to know the exact type of the passed object, you just need a property value in case it doesn't exist in the base type, but it may or may not exists in the actual type, you can use reflection:
static void Main(string[] args)
{
animal slyvester = new cat();
animal lassie = new dog();
animal silver = new horse();
DoSomething(slyvester);
DoSomething(lassie);
DoSomething(silver);
Console.ReadLine();
}
static void DoSomething(animal entity)
{
string INeedThisProperty = "PurrStrength";
Type type = entity.GetType();
PropertyInfo property = type.GetProperty(INeedThisProperty);
if (property != null && property.CanRead)
{
Console.WriteLine("Found: {0}", property.GetValue(entity, null));
}
}
If in precise moment of property access you dont't have any clue what type is it, somehow you have to figure out it.
Or, what I personaly would do, is try to create virtual functions/properties on base class that describes my child classes action in more generic way, override them in child classes with concrete implementation and after call that functions/properties using upper casted objects.
The answer is to use polymorphism. The idea is to introduce a method in the base interface or as in this case the base class. Then just call this method! The runtime will automagically delegate the call to the correct type.
Look at the modified implementation below:
public abstract class Animal
{
public abstract void OutputInterestingFact();
}
public class Cat : Animal {
private string _purrStrength = "Teeth Shattering";
public string PurrStrength {
get { return _purrStrength; }
set { _purrStrength = value; }
}
public override void OutputInterestingFact()
{
Console.WriteLine(PurrStrength);
}
}
public class Dog : Animal {
public override void OutputInterestingFact()
{
// Do stuff for dog here
}
}
public class Horse : Animal {
public override void OutputInterestingFact()
{
// Do stuff for horse here
}
}
I made the Animal into an abstract class. You could also make the OutputInterestingFact method virtual with an empty method body.
I've also renamed your classes to begin with an upper case letter. Make this a habit since this is practice in C# and other programmers will find your code easier to read.
Now, to use this just call the method.
slyvester.OutputInterestingFact();
Is that elegant enough?
Your code does not cover all the cases that I can think of, but just 2 possible solutions:
class Animal {
public abstract string PropertyValue { get; set; }
}
class Cat : Animal {
public override string PropertyValue {
get { return PurrStrength; }
set { PurrStrength = value; }
}
}
or, for multiple properties:
class Animal {
public virtual string[] GetPropertyValues() { return null; }
}
class Cat : Animal {
public override string[] GetPropertyValues() {
return new string[] { PurrStrength };
}
}