How can I properly inherit a class (of another class and interfaces) that have a generic type with a generic type constraint (where)?
class A { }
class B { }
interface I { }
class C<T> where T : A, B, I { }
In this example A, B and I is treated as base for T. Inhertiance of 2 classes is not possible in C#.
But I want that A is Baseclass of the generic type T and B/I is baseclass/interface of C. How to do this?
As soon as I use the where-constraint for the generic type I cannot derive my class C anymore
public class A
{
}
public class B
{
}
public interface I
{
}
public class C<T> : B, I where T : A
{
}
Related
I have a set up approximately like this:
public class A { ... }
public class A<T> : A { ... }
public class B<T> : A<T> { ... }
public class C : B<SomeType> { ... }
I am iterating over a list of type A, and I want to check if something is of type B, and if it is, convert it to... B... I can see how it already poses a problem of how am I gonna get the type... but if I cannot do that, what are my other alternatives?
EDIT:
To clarify, B contains a method I'd like to use that A does not contain as it simply has no need for it... I want to check which of the objects on that list are of type B so that I can run this method on them
EDIT 2:
I forgot to mention the A in the middle earlier... sorry for the confusion
I want to check which of the objects on that list are of type B
Then introduce a type B into your type hierarchy. Currently you don't have a type B, you've got B<SomeType> which is a different type. so
public class A { ... }
public abstract class B : A { ... }
public class B<T> : B { ... }
public class C : B<SomeType> { ... }
Or declare the method you want on an interface
public class A { ... }
public class A<T> : A { ... }
public interface IB
public class B<T> : A<T>, IB { ... }
public class C : B<SomeType> { ... }
Then for any A you can check if it is IB. This is probably the most natural solution since .NET doesn't have multiple inheritance.
I want to have a generic class which could call a method based on its generic type which is defined by a derived class. For that I implemented a base interface and a generic interface which has the base interface as generic and also derives from the base interface.
In the generic interface I want a method based on the type T of the base interface.
After that I wanted to implement a class based on the generic interface which should be able to call the generic method. This is the example code:
public interface BaseInterface
{ }
public interface GenericInterface<T> : BaseInterface where T : BaseInterface
{
void Foo(T t);
}
public class C<T> : GenericInterface<T> where T : BaseInterface
{
public C()
{
// None of these works
Foo(this);
Foo((T)this);
Foo((BaseInterface)this);
}
public void Foo(T t) { }
}
Is there a way to achieve my desired behavior here?
The error message here is:
cannot convert from 'C<T>' to 'T'
which in my eyes should be possible because C derives from BaseInterface which is T
While both C<T> and T need to derive from BaseInterface, that does not mean that then need to be the same. I might for example declare another type B : BaseInterface, and C<B>. So we would get the method Foo(B t) , it would obviously not be possible to call the method with this as the parameter, since C<B> is not B.
If you just need a method that needs a BaseInterface parameter, just declare it as Foo(BaseInterface t) instead. That way you could call it with this without any problem.
It looks like you're looking for something like the Curiously Recurring Template Pattern for C#. In a simplified form it looks like this:
class Base<T>
{
public void Foo(T t) { }
}
class C : Base<C>
{
C()
{
Foo(this);
}
}
In your case T is a type that derives from BaseInterface. Although C<T> also derives from BaseInterface it doesn't mean that you can bind a C<T> to T t. It's like trying to bind a string to a List, just because both implement IEnumerable. They are still different types.
For your full example it could look like this
public interface BaseInterface
{ }
public interface GenericInterface<T> : BaseInterface where T : BaseInterface
{
void Foo(T t);
}
public class C<T> : GenericInterface<C<T>> where T : BaseInterface
{
public C()
{
Foo(this);
}
public void Foo(C<T> t) { }
}
Here's what you need for the Curiously Recurring Template in C#.
public interface BaseInterface { }
public interface GenericInterface<T> : BaseInterface where T : GenericInterface<T>
{
void Foo(T t);
}
public abstract class C<T> : GenericInterface<T> where T : C<T>
{
public abstract void Foo(T t);
}
Now you can go ahead and implement a real class:
public class D : C<D>
{
public D()
{
Foo(this);
Foo((D)this);
}
public override void Foo(D t) { }
}
That works fine.
However, calling Foo((BaseInterface)this); will never work in this code. It just doesn't make sense.
I Tried this, maybe can help you
public class C<T> : GenericInterface<T> where T : BaseInterface
{
public C()
{
T t = default(T);
BaseInterface bi;
bi = t; // here can cast , bacuse T : BaseInterface , not BaseInterface : T
t = bi;//here cast error ,
}
public void Foo(T t) { }
}
It's same like this case:
public class B { }
public class A : B
{
public void CastTest()
{
A a = null;
B b = null;
b = a;
a = b;//here error
}
}
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 have the following code.
class Header<T> where T: IItem { }
class HeaderA : Header<ItemA> { }
class HeaderB : Header<ItemB> { }
interface IItem { }
class ItemA : IItem { }
class ItemB : IItem { }
Header<IItem> h = new HeaderA();
The last line cannot be compiled.
Cannot implicitly convert type 'UserQuery.HeaderA' to 'UserQuery.Header<UserQuery.IItem>'
HeaderA is a subtype of Header and ItemA is a subtype of IItem. Why it doesn't work?
In short, you're trying to use a concept called covariance, which is not supported in .NET generic classes, and not supported by default in interfaces.
If you want to allow the class to do this, you can specify it in C# 3 or later using the out contextual keyword on a generic interface:
interface IHeader<out T> where T : IItem { }
class Header<T>: IHeader<T> where T:IItem { }
class HeaderA : Header<ItemA> { }
class HeaderB : Header<ItemB> { }
interface IItem { }
class ItemA : IItem { }
class ItemB : IItem { }
public void Foo()
{
//now this works; notice the use of the interface instead of the base class.
IHeader<IItem> h = new HeaderA();
}
By using the interface with the keyword, you are basically telling the compiler that no usage of the interface will ever have to know more about the generic type than that it meets the constraints of the interface's generic type declaration (or that it's an object). As such, while you can now assign more derived generics to variables of the interface type, you can only deal with them as the interface type, never as any derived type.
The out keyword is not acceptable for class definitions; you cannot force usages of Header<T> to be covariant.
I have the following Classes / Interfaces:
// Model
public class A : IA { }
// ModelLogic
public class B : IB<A> { }
// Model Interface
public interface IA { }
// ModelLogic Interface
public interface IB<T> where T : IA { }
I try to create a new instance using the following code:
IB<IA> foo = new B();
I am getting the following error:
Cannot implicitly convert type 'B' to 'IB<IA>'. An explicit conversion exists (are you missing a cast?)
Can someone please explain why this is not possible?
OK, let's replace A with Fish, IA with IAnimal, B with Aquarium, and IB<T> with IContainer<T>. And we'll add a member to IContainer<T>, and a second implementation of IAnimal:
// Model
public class Fish : IAnimal { }
public class Tiger : IAnimal { }
// ModelLogic
public class Aquarium : IContainer<Fish>
{
public Fish Contents { get; set; }
}
// Model Interface
public interface IAnimal { }
// ModelLogic Interface
public interface IContainer<T> where T : IAnimal
{
T Contents { get; set; }
}
IContainer<IAnimal> foo = new Aquarium(); // Why is this illegal?
foo.Contents = new Tiger(); // Because this is legal!
You can put a Tiger into foo -- foo is typed as a container that can contain any animal. But you can only put a Fish into an Aquarium. Since the operations you can legally perform on an Aquarium are different than the operations you can perform on an IContainer<IAnimal>, the types are not compatible.
The feature you want is called generic interface covariance and it is supported by C# 4, but you have to prove to the compiler that you will never put a tiger into your fish tank. What you want to do is:
// Model
public class A : IA { }
// ModelLogic
public class B : IB<A> { }
// Model Interface
public interface IA { }
// ModelLogic Interface
public interface IB<out T> where T : IA { }
Notice the covariance annotation on IB. This out means that T can only be used as an output, not as an input. If T is only an output then there is no way for someone to put a tiger into that fish tank because there is no "put into" property or method possible.
I wrote a number of blog articles while we were adding that feature to C#; if you are interested in the design considerations that went into the feature, see:
http://blogs.msdn.com/b/ericlippert/archive/tags/covariance+and+contravariance/
To fix your code, just change
public interface IB<T> where T : IA { }
to
public interface IB<out T> where T : IA { }
It's not easy to see when you have empty interfaces. Consider you have one method M in interface IB:
public interface IB<T> where T : IA
{
void M(T t);
}
And here is implementation of B:
public class B : IB<A>
{
public void M(A t)
{
// only object of type A accepted
}
}
Then you have object C, which also implements IA:
public class C : IA { }
So, if your code would be possible, then you could call:
IB<IA> foo = new B();
foo.M(new C());
Problem is that class B accepts only objects of type A. Error!