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Does C# support return type covariance?
Why can’t I implement an Interface this way?
Consider the following:
public interface IAnimal {
}
public class Animal : IAnimal {
}
public interface ICage {
IAnimal SomeAnimal {get;}
}
public class Cage : ICage{
public Animal SomeAnimal { get; set; }
}
I've read a lot of stuff on covariance and contravariance for IEnumerable, but I'm not sure how to get the above code to work. I get the error "Cage does not implement interface member IAnimal". Since it defined Animal, which is more defined than IAnimal, it seems like covariance should take care of me.
What am I missing? Thanks in advance.
That's not currently possible in C#.
It theoretically possible for the language designers to add it, they just haven't [yet]. They may or may not decide to add it to a potential future version of C#.
The best workaround would probably be:
public class Cage : ICage
{
public Animal SomeAnimal { get; set; }
IAnimal ICage.SomeAnimal
{
get { return SomeAnimal }
}
}
Quoted from Eric Lippert, in response to me asking the same question here:
Why can't I implement an Interface this way?
C# does not support return type covariance for the purposes of
interface implementation or virtual method overrding. See this
question for details:
Does C# support return type covariance?
C# does support generic covariance and contravariance of interfaces
and delegate types that are constructed wtih reference types for the
type arguments as of C# 4.
And C# does support return type covariance when converting a method
that returns a reference type to a delegate type whose return type is
a compatible reference type. (And similarly it supports parameter type
contravariance.)
If this subject interests you, I have written a great many articles
discussing various versions of variance that C# does and does not
support. See
http://blogs.msdn.com/b/ericlippert/archive/tags/covariance+and+contravariance/
for details.
To get covariance you should have something like this:
public interface IAnimal {
}
public class Lion : IAnimal {}
public class Sheep : IAnimal {}
// note the "out" on the T type parameter
public interface ICage<out T> where T:IAnimal {
T SomeAnimal {get;}
}
public class Cage<T> : ICage<T> where T:IAnimal {
public T SomeAnimal { get; set; }
}
you can now do this:
// without covariance on ICage you can't assign a 'Cage<Sheep>' to 'ICage<IAnimal>'
ICage<IAnimal> sheeps = new Cage<Sheep>() {SomeAnimal=new Sheep()};
ICage<IAnimal> lions = new Cage<Lion>() {SomeAnimal=new Lion()};
or this (creating a heterogeneous list of Cage<IAnimals> with both Cages of Sheeps and Cages of Lions), which is an equivalent but probably more useful example:
// without covariance on ICage it errors: cannot convert from 'Cage<Sheep>' to 'ICage<IAnimal>'
var zoo = new List<ICage<IAnimal>>{
new Cage<Sheep> {SomeAnimal=new Sheep()},
new Cage<Lion> {SomeAnimal=new Lion()},
};
if you try removing the out from the declaration of ICage you will see the difference.
public interface IAnimal {
}
public class Animal : IAnimal {
}
public interface ICage {
IAnimal SomeAnimal {get;}
}
public class Cage : ICage{
public Animal SomeAnimal { get; set; }
}
public class AnotherAnimal : IAnimal {
}
Cage c = new Cage();
ICage ic = (ICage)c;
ic.Animal = new AnotherAnimal();
this would be invalid because AnotherAnimal implements IAnimal, but it is not an Animal.
EDIT
the above would only be relevant if there was a setter defined in the interface. Because there is not, the proper answer here is that the desired functionality cannot be achieved in C#; it is a feature not included in the language.
Related
This question already has answers here:
Why an inherited interface can't be converted to its base interface in generic context?
(2 answers)
Closed 3 years ago.
There is an interface which inherits the generic interface.
Here is generic interface
public interface IBaseService<TEntity> where TEntity : BaseEntity
{
}
Interface which inherits generic interface
public interface IChatService :IBaseService<Messages>
{
}
Then i have a class which takes the generic interface in the constructor as a parameter.
public class BaseApi
{
IBaseService<BaseEntity> _baseService;
public BaseApi(IBaseService<BaseEntity> baseService)
{
_baseService = baseService;
}
}
When i pass the interface object which implements the generic interface it doesn't allow me to pass the object of the interface, it asks to pass generic interface.
Here is error:
Any solution?
Thanks in advance, sorry for bad english!
This is quite logical. Let's simplify matters a bit by renaming our types and creating a new example:
public class BaseApi
{
public BaseApi(IBucket<Fruit> baseService) { ... }
}
And our IBucket<Messages> represents a concrete type of fruit (apple):
public interface IChatService :IBucket<Apple>
{
}
A bucket of apples is not the same as a bucket of fruit because you can put an orange in the latter and you cannot do this with the former.
To deal with this you could either change the signature of IChatService:
public interface IChatService :IBaseService<BaseEntity>
{
}
Or pass the expected IBaseService<Messages> type to BaseApi's ctor.
You can fix this with covariance. But only if your argument can be covariant
public interface IBaseService<out TEntity> where TEntity : BaseEntity
{
}
https://learn.microsoft.com/en-us/dotnet/standard/generics/covariance-and-contravariance
Just another small C# training app, and just another Compilation Error, but it cannot just go away from me... I am just wondering, what I am doing wrong here:
public abstract class Material
{
}
public abstract class Cloth<T> where T:Material
{
public T Prop { get; set; }
}
public class Cotton : Material
{
}
public class Dress<T> : Cloth<T> where T : Material
{
}
public class Test
{
private Cloth<Material> cloth;
public Test()
{
/* below won't compile */
cloth = new Dress<Cotton>();
}
}
I want to get the base class object from a closed constructed class. Anyone ?
When trying to compile I get the error:
Cannot implicitly convert type Dress<Cotton> to Cloth<Material>
What you want to achieve is called covariance (see the following article for samples).
Unfortunately, there's no variance support for classes: it's restricted to interfaces and delegates.
Thus and alternatively, you might design an interface called ICloth<T> with T covariant:
public interface ICloth<out T>
{
T Prop { get; set; }
}
And implement it in any of your possible cloths, including Cloth<T>.
Now type cloth as ICloth<T> and your assignment should work (i.e. cloth = new Dress<Cotton>();), because Dress<Cotton> is ICloth<out T>, which is an interface with a T covariant generic parameter.
Learn more about generic interface with variance in the following article on MSDN.
I have class MockRepository which implements interface IRepository and I have a class Technology which implements interface IIdentifiable.
I want to cast an object from MockRepository<Technology> to IRepository<IIdentifiable> and also cast back again after some operations are complete. Is this possible? my code compiles but when i try to run it i get an invalid cast exception.
Short answer is no.
If you have an interface IMyInterface<T> then the compiler will create a new interface for each type of T that you use and substitute for all values of T.
To give an example:
I have an interface:
public interface IMyInterface<T> {
public T Value {get; set;}
}
I have 2 classes:
public class Foo {}
public class Bar : Foo {}
I then define the following
public class Orange : IMyInterface<Foo> {}
public class Banana : IMyInterface<Bar> {}
The compiler will automatically create 2 new interfaces using a specific name convention, I'm going to use different names to highlight that they are different
public interface RandomInterface {
Foo Value { get; set; }
}
public interface AlternativeInterface {
Bar Value { get; set; }
}
public class Orange : RandomInterface {
}
public class Banana : AlternativeInterface {
}
As you can see there is no relationship between RandomInterface and AlternativeInterface. So a class inheriting from RandomInterface cannot be cast to AlternativeInterface
UPDATE AFTER READING QUESTION COMMENTS
If you wish to pass MockRepository to a function that expects IRepository you can do the following
public void MyFunction<T>(IRepository<T> repo) where T: IIdentifiable {
}
I take it you have:
class MockRepository<T> : IRepository<T> // note: same T
{
}
interface IRepository<out X> // note: "out" means covariance in X
{
}
Then because a Technology is an IIdentifiable, covariance gives that an IRepository<Technology> is also an IRepository<IIdentifiable>.
So if your IRepository<> interface is (or can be made) covariant, it should work.
Given these base classes and interfaces
public abstract class Statistic : Entity, IStatistic
{
protected abstract IStatisticsRepository<IStatistic> Repository {get;}
...
public class AverageCheckTime : Statistic
...
public interface IStatisticsRepository<T> : IRepository<T> where T : IStatistic
...
public interface IAverageCheckTimeRepository : IStatisticsRepository<AverageCheckTime>
...
public class AverageCheckTimeRepository : StatisticRepository<AverageCheckTime>, IAverageCheckTimeRepository
...
public class RepositoryFactory
{
public static IAverageQueueTimeRepository AverageQueueTimeRepository
{
get { return CurrentServiceLocator.GetInstance<IAverageQueueTimeRepository>(); }
}
Why does AverageCheckTime's implementation throw an invalid cast exception:
protected override IStatisticsRepository<IStatistic> Repository
{
get { return (IStatisticsRepository<IStatistic>)RepositoryFactory.AverageCheckTimeRepository; }
}
How do I cast an instance of IAverageCheckTimeRepository as an IStatisticsRepository<IStatistic> which I assumed it already was?
OK, I've made these changes...which makes me wonder if I've gone over the top with the generics in the first place
public interface IStatisticsHelper
{
void GenerateStatistics();
List<IStatistic> BuildReport();
}
...
public interface IStatisticsRepository<T> : IRepository<T>, IStatisticsHelper where T : IStatistic
{
}
...
public abstract class Statistic : Entity, IStatistic
{
protected abstract IStatisticsHelper Repository { get; }
...
public class AverageCheckTime : Statistic
{
protected override IStatisticsHelper Repository
{
get { return RepositoryFactory.AverageCheckTimeRepository; }
}
No, C# 3 does not support generic variance. C# 4 does, but you would have to declare that IStatisticsRepository is covariant in T:
public interface IStatististicsRepository<out T> : IRepository<T>
where T : IStastistic
Variance isn't safe in general - it depends on how the generic type parameter is used. C# 4 supports both covariance and contravariance for type arguments which are reference types, but only when the generic type involved is an interface or a delegate, and only when the type parameter is used in the appropriate way within the interface/delegate.
Without seeing the declaration for IRepository<T>, we can't tell whether or not it's safe. For example, if IRepository<T> contains a method like this:
void Save(string id, T value);
then it wouldn't be safe, because you'd be able to write:
IStatisticsRepository<IStatistic> repo = RepositoryFactory.AverageCheckTimeRepository;
IStatistic foo = new SomeOtherStastisticType();
repo.Save("Foo", foo);
That would be trying to save a SomeOtherStatisticType value in an AverageCheckTimeRepository, which violates type safety. It's only safe to make the interface covariant in T if values of type T only come "out" of the interface. (There are some wrinkles around exactly what that means, mind you...)
For a lot more information on this, see Eric Lippert's blog series on the topic.
Is the code below the only way to implement covariant return types?
public abstract class BaseApplication<T> {
public T Employee{ get; set; }
}
public class Application : BaseApplication<ExistingEmployee> {}
public class NewApplication : BaseApplication<NewEmployee> {}
I want to be able to construct an Application or a NewApplication and have it return the appropriate Employee type from the Employee property.
var app = new Application();
var employee = app.Employee; // this should be of type ExistingEmployee
I believe this code works fine, but it gets really nasty when I have several properties that require the same behavior.
Are there any other ways to implement this behavior? Generics or otherwise?
UPDATE: This answer was written in 2010. After two decades of people proposing return type covariance for C#, it looks like it will finally be implemented; I am rather surprised. See the bottom of https://devblogs.microsoft.com/dotnet/welcome-to-c-9-0/ for the announcement; I'm sure details will follow. The portions of the answer below which speculate on the possibility of the feature being implemented should be considered of historical interest only going forwards.
First off, the answer to your question is no, C# does not support any form of return type covariance on virtual overrides.
A number of answerers and commenters have said "there is no covariance in this question". This is incorrect; the original poster was entirely correct to pose the question as they did.
Recall that a covariant mapping is a mapping which preserves the existence and direction of some other relation. For example, the mapping from a type T to a type IEnumerable<T> is covariant because it preserves the assignment compatibility relation. If Tiger is assignment compatible with Animal, then the transformation under the map is also preserved: IEnumerable<Tiger> is assignment compatible with IEnumerable<Animal>.
The covariant mapping here is a little bit harder to see, but it is still there. The question essentially is this: should this be legal?
class B
{
public virtual Animal M() {...}
}
class D : B
{
public override Tiger M() {...}
}
Tiger is assignment-compatible with Animal. Now make a mapping from a type T to a method "public T M()". Does that mapping preserve compatibility? That is, if Tiger is compatible with Animal for the purposes of assignment, then is public Tiger M() compatible with public Animal M() for the purposes of virtual overriding?
The answer in C# is "no". C# does not support this kind of covariance.
Now that we have established that the question has been asked using the correct type algebra jargon, a few more thoughts on the actual question. The obvious first problem is that the property has not even been declared as virtual, so questions of virtual compatibilty are moot. The obvious second problem is that a "get; set;" property could not be covariant even if C# did support return type covariance because the type of a property with a setter is not just its return type, it is also its formal parameter type. You need contravariance on formal parameter types to achieve type safety. If we allowed return type covariance on properties with setters then you'd have:
class B
{
public virtual Animal Animal{ get; set;}
}
class D : B
{
public override Tiger Animal { ... }
}
B b = new D();
b.Animal = new Giraffe();
and hey, we just passed a Giraffe to a setter that is expecting a Tiger. If we supported this feature we would have to restrict it to return types (as we do with assignment-compatibility covariance on generic interfaces.)
The third problem is that the CLR does not support this kind of variance; if we wanted to support it in the language (as I believe managed C++ does) then we would have to do some reasonably heroic measures to work around signature matching restrictions in the CLR.
You can do those heroic measures yourself by carefully defining "new" methods that have the appropriate return types that shadow their base class types:
abstract class B
{
protected abstract Animal ProtectedM();
public Animal Animal { get { return this.ProtectedM(); } }
}
class D : B
{
protected override Animal ProtectedM() { return new Tiger(); }
public new Tiger Animal { get { return (Tiger)this.ProtectedM(); } }
}
Now if you have an instance of D, you see the Tiger-typed property. If you cast it to B then you see the Animal-typed property. In either case, you still get the virtual behaviour via the protected member.
In short, we have no plans to ever do this feature, sorry.
There might be multiple problems with what you try to achieve.
First of all, as somebody already noticed, there is no covarianace in your example. You can find a short description of covariance and generics here, new features in C# 2.0 - Variance, covariance on generics.
Secondly it seems that you try to solve with generics what should be solved with polymorphism. If both ExistingEmployee and NewEmployee inherit from a base class Employee, your problem would be solved:
public class Application {
public ExistingEmployee Employee { get; }
}
public class NewApplication {
public NewEmployee Employee { get; }
}
...
Application app = new Application;
var emp = app.Employee; // this will be of type ExistingEmployee!
Please note that the below is also true:
Employee emp = app.Employee; // this will be of type ExistingEmployee even if
// declared as Employee because of polymorphism
The one thing that would be different between polymorphism and generics, would be that if you return the variable to the specific type you would need a cast in the later case:
ExistingEmployee emp = (ExistingEmployee)app.Employee; // would have not been needed
// if working with generics
Hope this helps.
You could code against an Employee Interface to get what you want I think.
public interface IEmployee
{}
public abstract class BaseApplication<T> where T:IEmployee{
public T IEmployee{ get; set; }
}
public class ExistingEmployee : IEmployee {}
public class NewEmployee : IEmployee {}
public class Application : BaseApplication<ExistingEmployee> {}
public class NewApplication : BaseApplication<NewEmployee> {}
The code you posted will not compile but I get the basic idea of what you want to do. In short the answer is yes, that is the only way. If you want a property to return different types and be typed differently in extended classes then you have to use generics in the way that you already are.
If you can encapsulate the public contract of an employee object, new or existing, into an interface then you don't need to use generics at all. Instead you can just return the interface and let polymorphism take over.
public interface IEmployee
{ }
public class Employee1 : IEmployee
{ }
public class Employee2 : IEmployee
{ }
public abstract class ApplicationBase
{
public abstract IEmployee Employee { get; set; }
}
public class App1 : ApplicationBase
{
public override IEmployee Employee
{
get { return new Employee1(); }
set;
}
}
public class App2 : ApplicationBase
{
public override IEmployee Employee
{
get { return new Employee2(); }
set;
}
}
You can achieve a somewhat neat looking version of this using generics.
Covariant return types are not supported by c#. So this is not a solution, however, my feeling is that syntactically speaking this reads well. It does achieve a similar result.
I find it useful when creating fluent API's where the base class needs to perform some actions, but I need the derived implementation back. All it really achieves is to hide the cast.
public class Base
{
public virtual T Foo<T>() where T : Base
{
//... // do stuff
return (T)this;
}
}
public class A : Base
{
public A Bar() { "Bar".Dump(); return this; }
public A Baz() { "Baz".Dump(); return this; }
// optionally override the base...
public override T Foo<T>() { "Foo".Dump(); return base.Foo<T>(); }
}
var x = new A()
.Bar()
.Foo<A>() // cast back to A
.Baz();
Opinions will vary, and it's not 100% pretty. It's probably not appropriate for an API that will be published, but for internal use, for instance in unit tests, I find it useful.
YES!! Like this. There is more boiler plate than you would hope for, but it does work. The trick is done with extension methods.
It dose some nasty casting internally, but presents a covariant interface.
See also: http://richarddelorenzi.wordpress.com/2011/03/25/return-type-co-variance-in-c/
using System;
namespace return_type_covariance
{
public interface A1{}
public class A2 : A1{}
public class A3 : A1{}
public interface B1
{
A1 theA();
}
public class B2 : B1
{
public A1 theA()
{
return new A2();
}
}
public static class B2_ReturnTypeCovariance
{
public static A2 theA_safe(this B2 b)
{
return b.theA() as A2;
}
}
public class B3 : B1
{
public A1 theA()
{
return new A3();
}
}
public static class B3_ReturnTypeCovariance
{
public static A3 theA_safe(this B3 b)
{
return b.theA() as A3;
}
}
public class C2
{
public void doSomething(A2 a){}
}
class MainClass
{
public static void Main (string[] args)
{
var c2 = new C2();
var b2 = new B2();
var a2=b2.theA_safe();
c2.doSomething(a2);
}
}
}
One idea without generics, but it has other downsides:
public abstract class BaseApplication {
public Employee Employee{ get; protected set; }
}
public class Application : BaseApplication
{
public new ExistingEmployee Employee{ get{return (ExistingEmployee)base.Employee;} set{base.Employee=value; }}
}
public class NewApplication : BaseApplication
{
public new NewEmployee Employee{ get{return (NewEmployee)base.Employee;} set{base.Employee=value; }}
}
In particular with this code you can cast to the base class and assign an employee of an undesirable type. So you need to add checks against that in the setter of the base-class. Or remove the setter, which I usually prefer anyways. one way to do that is making the setter protected.
Another is adding a virtual function EmployeeType() which you override in derived classes and return a derived type. Then you check in the setter if EmployeeType().IsInstanceOf(value) and else throw an exception.
And IMO simulating covariant return types is one of the few good applications of the new marker. It returns the same thing as the base-class and just adds additional guarantees to the function contract.