I have a base class Animal and a derived class Lion. Each animal should hold a reference to the King of the Jungle, which is always a lion. So a read-only property Lion : KingOfTheJungle is added in the base class. This property is initialized in the Animal's constructor.
The idea is to create first the Lion object, and then pass a reference of it to the constructors of all other animals. The Jungle has only one lion, so when the lion is constructed should recognize itself as the King of the Jungle. The problem is that I can't create the Lion, because I get a compile time error when I pass this as a parameter to base():
abstract class Animal
{
public Lion KingOfTheJungle { get; }
public Animal(Lion theKing)
{
KingOfTheJungle = theKing;
}
}
class Lion : Animal
{
public Lion() : base(this) // Error CS0027
{
}
}
The error I get is:
Error CS0027 Keyword 'this' is not available in the current context
Searching for a workaround I ended up passing null instead of this in the Lion's constructor, and interpeting the null as this in the Animal's constructor:
abstract class Animal
{
public Lion KingOfTheJungle { get; }
public Animal(Lion theKing)
{
KingOfTheJungle = theKing ?? (Lion)this; // OK, but unsafe
}
}
class Lion : Animal
{
public Lion() : base(null) // OK, but not expressive
{
}
}
I don't like this workaround though, because it is neither expressive nor type-safe. Is there any safer or at least more elegant way to do the same thing?
Note: I want to keep the property KingOfTheJungle read-only, without a protected setter. The animals shouldn't be able to change this property after initialization.
I'd suggest KingOfTheJungle could be a virtual (or abstract) readonly property in the base class. Then override it in the inheriting classes.
Something along the lines of:
abstract class Animal
{
public abstract Lion KingOfTheJungle { get; }
}
class Lion : Animal
{
public override Lion KingOfTheJungle
{
get
{
return this;
}
}
}
The correct way to do this is with a static property:
public static Lion KingOfTheJungle { get; } = new Lion();
This solves several problems:
Non-deterministic crowning of the king. With this approach, there is always a king.
Too many kings. With this approach, there is always exactly one Lion that is king. Any other Lion instances are just plain Lion.
Different Animal instances consider different Lion instances their king, depending on which Lion was instantiated at what time.
Many copies of the same reference.
Can't pass this to base constructor. No need now.
If you really want to pursue your original design, IMHO it's worth keeping in mind that the constructor already has the this reference. Remember, this is the same object, no matter which class in the inheritance hierarchy the code exists.
That means a "better" way to do it but still following your original idea would look like this:
abstract class Animal
{
public Lion KingOfTheJungle { get; }
public Animal()
{
KingOfTheJungle = (this as Lion) ?? KingOfTheJungle;
}
}
This still has all but the last problem I mention above though. I would not implement it this way.
You mention that the jungle only ever has one Lion. This further suggests that you should make the Lion class a singleton. That would look something like this:
abstract class Animal
{
public static Lion KingOfTheJungle { get; } = Lion.Instance
public Animal()
{
// ...
}
}
class Lion : Animal
{
public static Lion Instance { get; } = new Lion();
private Lion() { }
}
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.
Right now I have a kind of dilemma. There is a class structure that is similar to the following one:
public interface IMammal
{
void Eat();
}
public interface IBarking
{
void Bark();
}
There are instances of IBarking and IMammal. Theoretically our animal can be either of them or just one. Cow is as you can see IMammal, and Dog is IMammal and IBarking. In theory we could even have someone who can bark, but isn't a mammal.
public class Mammal : IMammal
{
public void Eat()
{
Console.Write("Om-nom-nom");
}
}
public class Cow : Mammal
{
}
public class Dog : Mammal, IBarking
{
public void Bark()
{
Console.Write("Bark-bark!!!");
}
}
Here is a Farm, where only one animal lives:
public class Farm
{
private readonly IMammal _animal;
public Farm(IMammal animal)
{
_animal = animal;
}
public void Feed()
{
_animal.Eat();
}
public void Guard()
{
var dog = _animal as IBarking;
if (dog != null)
dog.Bark();
}
}
The problem, I can see here, is that we assume that IBarking is always IMammal. What is wrong with this design, how could it be fixed?
Such simplified examples hardly make any sense. You're missing the "problem case" and what you want to do then. Show a class that implements IBarking but not IMammal and what problems arise when you pass it to Farm. Anyway, given the premises:
Interface IMammal exists.
Interface IBarking exists.
A class implementing IBarking does not have to implement IMammal.
Class constructor Farm has to accept IBarking and IMammal.
Current class constructor accepts IMammal.
In this case you either need a new constructor, a new private member and more code to choose between the two, or an overlapping interface. I'd go with the latter: IFarmable.
Then you need:
public interface IMammal : IFarmable
public interface IBarking : IFarmable
public Farm(IFarmable farmable) { ... }
Most likely you have other constraints, like "but I want to call Eat() on the variable passed into the constructor", but then your description ("we assume that IBarking is always IMammal") is incorrect or incomplete and you need to move Eat() to the IFarmable interface.
I'll try to interpret your intention.
You want to have a farm where animals grow up (to later be slaughtered for food).
You might need another animal guarding them (might since you tried the cast).
A better design for that would be:
public class Farm
{
private readonly IMammal[] _animals;
public Farm(IMammal[] animals)
{
_animals = animals;
}
public void Feed()
{
foreach (var animal in _animals)
animal.Eat();
}
public IBarking GuardingAnimal { get; set; }
public void Guard()
{
if (GuardingAnimal != null)
GuardingAnimal .Bark();
}
}
Changes from your design:
I've made it crystal clear that there can be a guarding animal
The guarding animal is optional (as it's assign through a property instead of the constructor).
The reason that I wanted to make that distinction is that most animals are passive (you feed and harvest them) while the guard animal has a specific use case and should therefor not be hidden among the others.
If you want to feed the dog you, you should make that interface inherit IMammal (unless you introduce more functionality in mammal in which case you should extract IFeedable or similar).
I am implementing a collection of classes that exhibit the following pattern:
public class Animal {}
public abstract class AnimalToy
{
public AnimalToy(Animal owner)
{
Owner = owner;
}
public Animal Owner { get; private set; }
/* Various methods related to all toys that use the Owner property */
}
public class Dog: Animal
{
public void Bark() {}
}
public class PlasticBone: AnimalToy
{
public PlasticBone(Dog owner) : base(owner) {}
public void Throw()
{
((Dog)Owner).Bark();
}
}
I have a base class AnimalToy with a property that is a reference to another base class Animal.
I now want to implement a Dog and a PlasticBone toy for that Dog class. PlasticBone is a toy that's only valid for dogs, and in fact the constructor restricts the owner of PlasticBone to be of type Dog.
PlasticBone has a method Throw() that is unique to that class, that uses a method on Dog (Bark()) that is unique to the Dog class. Therefore I need to cast the generic property Owner to Dog before I can access it.
This works just fine, but in the project I am working on this situation comes up again and again and leads to quite ugly code where the methods of derived classes are full of downcasts of base class references. Is this normal? Or is there a better overall design that would be cleaner?
Here's one way to fix it:
public abstract class Animal
{
public abstract void MakeNoise();
}
Let dog implement the MakeNoise and you can just call that in your Toy class:
public void Throw()
{
Owner.MakeNoise();
}
You can make AnimalToy class generic, by this you can avoid casting.
public abstract class AnimalToy<TAnimal> where TAnimal : Animal
{
public AnimalToy(TAnimal owner)
{
Owner = owner;
}
public TAnimal Owner { get; private set; }
}
public class PlasticBone: AnimalToy<Dog>
{
public PlasticBone(Dog owner) : base(owner) {}
public void Throw()
{
Owner.Bark();
}
}
Worth noting that ((Dog)Owner) is not upcast, it is called downcast. upcast is over way around.
I'm going to try to elaborate a little on the discussion in the comments and hopefully provide you with a generalised, polymorphic solution. The general idea is functionally equivalent to what Carra was suggesting but, hopefully, this will help you to apply those concepts to the domain of your actual problem.
To work with your abstraction of your problem; suppose you have your abstract base class defined like so:
public abstract class Animal
{
public abstract void Catch();
}
This is a shift in the semantics of the abstract method; you no longer have an abstract Bark method, you simply have a Catch method, the semantics of which define it as, essentially "react to something being thrown." How the Animal reacts is entirely up to the animal.
So, you'd then define Dog like so:
public class Dog : Animal
{
public override void Catch()
{
Bark();
}
public void Bark()
{
// Bark!
}
}
And change your Throw method to:
public void Throw()
{
Owner.Catch();
}
Now, Bark is specific to Dog, while the generic Catch method is universal to all Animals. The Animal class now specifies that its subclasses must implement a method that reacts to items being thrown to it. The fact that the Dog barks is entirely up to the Dog.
This is the essence of polymorphism - it's not up to the toy to determine what the dog does; it's up to the dog. All the toy needs to know is that the dog can catch it.
More generally, though, I don't like to have two classes that extend independent base classes be logically coupled (parallel hierarchies, as Eric points out) unless one of them explicitly instantiates the other, though it's impossible to offer any real advice on that without seeing your actual solution architecture.
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.
In C# I have three classes: Person, Cat, and Dog.
Both the Cat and Dog classes have the method Eat().
I want the Person class to have a property ‘Pet’.
I want to be able to call the Eat method of both the Cat and Dog via the Person via something like Person.Pet.Eat() but I can’t because the Pet property needs to be either of type Cat or Dog.
Currently I’m getting round this with two properties in the Person class: PetDog and PetCat.
This is okay for now, but if I wanted a 100 different types of animal as pets then I don’t really want to have 100 different Pet properties in the Person class.
Is there a way round this using Interfaces or Inheritance? Is there a way I can set Pet to be of type Object but still access the properties of whichever animal class is assigned to it?
You could have the pets derive from a common base class:
public abstract class Animal
{
protected Animal() { }
public abstract void Eat();
}
And then have Cat and Dog derive from this base class:
public class Cat: Animal
{
public override void Eat()
{
// TODO: Provide an implementation for an eating cat
}
}
public class Dog: Animal
{
public override void Eat()
{
// TODO: Provide an implementation for an eating dog
}
}
And your Person class will have a property of type Animal:
public class Person
{
public Animal Pet { get; set; }
}
And when you have an instance of Person:
var person = new Person
{
Pet = new Cat()
};
// This will call the Eat method from the Cat class as the pet is a cat
person.Pet.Eat();
You could also provide some common implementation for the Eat method in the base class to avoid having to override it in the derived classes:
public abstract class Animal
{
protected Animal() { }
public virtual void Eat()
{
// TODO : Provide some common implementation for an eating animal
}
}
Notice that Animal is still an abstract class to prevent it from being instantiated directly.
public class Cat: Animal
{
}
public class Dog: Animal
{
public override void Eat()
{
// TODO: Some specific behavior for an eating dog like
// doing a mess all around his bowl :-)
base.Eat();
}
}
Is there a way round this using Interfaces or Inheritance?
Yes.
Interfaces: make an interface IEat or IPet or whatever concept you want to represent. Make the interface have an Eat method. Have Cat and Dog implement this interface. Have the Pet property be of that type.
Inheritance: Make an abstract base class Animal or Pet or whatever concept you want to represent. Make an abstract method Eat on the base class. Have Cat and Dog inherit from this base class. Have the Pet property be of that type.
What is the difference between these two?
Use interfaces to model the idea "X knows how to do Y". IDisposable, for example, means "I know how to dispose of the important resource that I am holding onto". That is not a fact about what the object is, it is a fact about what the object does.
Use inheritance to model the idea of "X is a kind of Y". A Dog is a kind of Animal.
The thing about interfaces is you can have as many of them as you want. But you only get to inherit directly from one base class, so you have to make sure you get it right if you're going to use inheritance. The problem with inheritance is that people end up making base classes like "Vehicle" and then they say "a MilitaryVehicle is a kind of Vehicle" and "A Ship is a kind of Vehicle" and now you're stuck: what is the base class of Destroyer? It is both a Ship and a MilitaryVehicle and it can't be both. Choose your "inheritance pivot" extremely carefully.
Is there a way I can set Pet to be of type Object but still access the properties of whichever animal class is assigned to it?
Yes, in C# 4 there is, but do not do so. Use interfaces or inheritance.
In C# 4 you can use "dynamic" to get dynamic dispatch at runtime to the Eat method on the object that is in Pet.
The reason you don't want to do this is because this will crash and die horribly should someone put a Fruit or a Handsaw in the Pet property and then try to make it Eat. The point of compile-time checks is to decrease program fragility. If you have a way to make more compile-time checks, use it.
Use an abstract class.
public abstract class Pet { public abstract void Eat(); }
public class Dog : Pet { }
public class Cat : Pet { }
public class Person {
public Pet Pet;
}
I’m sure you can do the rest.
Using an interface;
abstract class Animal
{
public virtual void DoSomething() { }
}
interface ICanEat
{
void Eat();
}
class Dog : Animal, ICanEat
{
public void Eat()
{
Console.Out.WriteLine("Dog eat");
}
}
class Cat : Animal, ICanEat
{
public void Eat()
{
Console.Out.WriteLine("Cat eat");
}
}
class Person
{
public Animal MyAnimal { get; set; }
}
Then you can call
(person.MyAnimal as ICanEat).Eat();
performing the correct null-checks as you go.
Why not create a base class of type Pet
public abstract class Pet{
public abstract void Eat();
}
Have this cat and dog inherit from an interface IPet
public interface IPet
{
bool hungry();
void Eat();
}