I have the following generic interface structure:
public interface IBase {}
public interface IBase<TBase> : IBase
where TBase : class, IBase
{
IEnumerable<IA<TBase>> Derived { get; }
}
public interface IA {}
public interface IA<TBase> : IA
where TBase : class, IBase
{
IEnumerable<IB<TBase, IA<TBase>>> DerivedOfA { get; }
}
public interface IB {}
public interface IB<TBase, TA> : IB
where TBase : class, IBase
where TA : class, IA<TBase>
{
IEnumerable<IC<TBase, TA, IB<TBase, TA>>> DerivedOfB { get; }
}
public interface IC {}
public interface IC<TBase, TA, TB> : IC
where TBase : class, IBase
where TA : class, IA<TBase>
where TB : class, IB<TBase, TA>
{
}
The purpose of this interface structure is to define a contract of type objects in the heirarcy structure.
When I try to define the following:
public class Base : IBase
{
public IEnumerable<IA<Base>> Derived { get; }
public class DerivedA : IA<Base>
{
public IEnumerable<IB<Base , IA<Base>>> DerivedOfA { get; }
public class DerivedB : IB<Base, DerivedA>
{
public IEnumerable<IC<Base, IA<Base>, IB<Base, IA<Base>>>> DerivedOfB { get; }
}
}
}
Everything compiles.
But when I define the 3rd nesting level (IC) compilation fails.
public class Base : IBase
{
public IEnumerable<IA<Base>> Derived { get; }
public class DerivedA : IA<Base>
{
public IEnumerable<IB<Base, IA<Base>>> DerivedOfA { get; }
public class DerivedB : IB<Base, DerivedA>
{
public IEnumerable<IC<Base, IA<Base>, IB<Base, IA<Base>>>> DerivedOfB { get; }
public class DerviedC: IC<Base, DerivedA, DerivedB>
{
}
}
}
(this creates a strict inheritance between the objects with the fact that the classes are nested).
When I write the code, the compiler complains on the following line
public class DerviedC: IC<Base, DerivedA, DerivedB> <-----
with the following error message
Error CS0311 The type 'Base.DerivedA.DerivedB' cannot be used as type parameter 'TB' in the
generic type or method 'IC<TBase, TA, TB>'. There is no implicit reference conversion from
'Base.DerivedA.DerivedB' to 'IB<Base, Base.DerivedA>'.
It seems the 2nd inheritance level of the interfaces is not supported. Why?
As Base.DerivedB inherits IB<Base, DerivedA> I would assume the compiler will no how to up-cast the object to its reference.
How can I resolve this?
Related
I am encountering a problem with interfaces. I wish to chain a method which derives its chainable method from an abstract class, which implements an interface.
public interface IBaseInterface {
public IBaseInterface ChainableMethod()
}
public abstract AbstractClassThatHelps<T> where T: IBaseInterface n {
public T ChainableMethod() {
return (T) this;
}
}
public interface IDerived : IBaseInterface { }
public class DerivedClass : AbstractClassThatHelps<IDerived>, IDerived { }
IDerived derived = new DerivedClass();
derived.ChainableMethod().ChainableMethod();
The problem I face here: why can't T be returned when it is shown to implement the contract IModel?
How would I solve this differently? I wish to have type safety but I am forced for all derived classes to return IBaseInterface instead of their own interface.
Actual implementation:
We have multiple models (DerivedClass's) which implement their respective IDerived for dependency injection. These need helpers because i dont want to repeat myself.. So we use AbstractClassThatHelps as a base, but because we are dealing with chainable methods we need this base class to know what to return, so therefore generics. IBaseInterface can be seen as IModel. Where ChainableMethod can be seen as GetAll() for example.
In order for the following code to work AbstractClassThatHelps<T> must implement IBaseInterface. How can you return this, if this is not IBaseInterface
public abstract AbstractClassThatHelps<T> where T: IBaseInterface n{
public T ChainableMethod(){
return this;
}
}
Edit: I am not user what this design solves but here is my attempt at what you are trying to achieve→
public interface IBaseInterface
{
IBaseInterface ChainableMethod();
}
public abstract class AbstractClassThatHelps<T>:IBaseInterface where T : IBaseInterface{
public T ChainableMethod()
{
IBaseInterface i = this;
return (T)i.ChainableMethod();
}
IBaseInterface IBaseInterface.ChainableMethod()
{
return this;
}
}
public class Concrete : AbstractClassThatHelps<Concrete>
{
}
You can return an instance of T, but the return type cannot be T but must be IBaseInterface because that is what the interface requires.
This is working, your code was full of syntax errors:
public interface IBaseInterface
{
IBaseInterface ChainableMethod();
}
public abstract class AbstractClassThatHelps : IBaseInterface
{
public IBaseInterface ChainableMethod()
{
return this;
}
}
public interface IDerived : IBaseInterface
{
}
public class DerivedClass : AbstractClassThatHelps, IDerived
{
}
internal static class Program
{
public static void Main()
{
IDerived derived = new DerivedClass();
derived.ChainableMethod().ChainableMethod();
}
}
you could also try this:
public interface IBaseInterface
{
IBaseInterface ChainableMethod();
}
public abstract class AbstractClassThatHelps<T> : IBaseInterface where T : class, IBaseInterface
{
public T ChainableMethod()
{
return this as T;
}
IBaseInterface IBaseInterface.ChainableMethod()
{
return ChainableMethod();
}
}
public interface IDerived : IBaseInterface
{
IDerived Hello();
}
public class DerivedClass : AbstractClassThatHelps<IDerived>, IDerived
{
public IDerived Hello()
{
Console.WriteLine("Hello");
return this;
}
}
internal static class Program
{
public static void Main()
{
AbstractClassThatHelps<IDerived> derived = new DerivedClass();
derived.ChainableMethod().Hello().ChainableMethod();
}
}
I defined 3 interfaces:
public interface IManufacturerInput
{
}
public interface IManufacturerOutput
{
}
public interface IManufacturerApi<in S, out T>
where S : IManufacturerInput
where T : IManufacturerOutput
{
T Calculate(S);
}
And I defined a specific Manufacturer:
public class ManufacturerAInput : IManufacturerInput
{
}
public class ManufacturerAOutput : IManufacturerOutput
{
}
public class ManufacturerAApi : IManufacturerApi<ManufacturerAInput, ManufacturerAOutput>
{
public ManufacturerAOutput Calculate(ManufacturerAInput)
{
return null;
}
}
And In Main() I created a ManufacturerAApi, and try assign it to IManufacturerApi.
IManufacturerApi<IManufacturerInput, IManufacturerOutput> api = new ManufacturerAApi();
But it failed. The error message said (just abstract meaning):
Can't convert from ManufacturerAApi to IManufacturerApi<IManufacturerInput, IManufacturerOutput>
So is there any way I can make the assignment work? Thanks in advance.
What you are proposing isn't type safe. Let's change the names of your types to make the issue clearer:
public interface IPetFood { }
public interface IPetSound { }
public interface IPetCage<in S, out T>
where S : IPetFood
where T : IPetSound
{
T Feed(S s);
}
public class DogFood : IPetFood { }
public class CatFood : IPetFood { }
public class Bark : IPetSound { }
public class DogCage : IPetCage<DogFood, Bark>
{
public Bark Feed(DogFood input)
{
return new Bark();
}
}
And now suppose this is legal:
IPetCage<IPetFood, IPetSound> api = new DogCage();
Then we could do the following:
api.Feed(new CatFood()); //oops we've just given the dog some catfood.
The assignment will not work because S is contravariant, which means that any possible IPetFood passed into api.Feed would need to be a subtype of DogFood and you have the opposite; IPetFood is a superset of DogFood.
I am getting error on DerivedFileInfo class declaration.
Error : The type 'StorageManager.Test.DerivedStorageProvider' cannot
be used as type parameter 'TStorageProvider' in the generic type or
method 'StorageManager.Test.BaseFileInfo'. There is no implicit
reference conversion from 'StorageManager.Test.DerivedStorageProvider'
to 'StorageManager.Test.IStorageProvider'.
Is there is something missing for generic constrains or I will require generic covariant?
public interface ICacheProvider
{
}
public class BaseCacheProvider : ICacheProvider
{
}
public class DerivedCacheProvider : BaseCacheProvider
{
}
public interface IStorageProvider<TCacheProvider> where TCacheProvider : ICacheProvider
{
}
public abstract class BaseStorageProvider<TCacheProvider> : IStorageProvider<TCacheProvider> where TCacheProvider : ICacheProvider
{
}
public class DerivedStorageProvider : BaseStorageProvider<DerivedCacheProvider>
{
}
public interface IResourceInfo<TStorageProvider> where TStorageProvider : IStorageProvider<ICacheProvider>
{
}
public abstract class ResourceInfo<TStorageProvider> : IResourceInfo<TStorageProvider>
where TStorageProvider : IStorageProvider<ICacheProvider>
{
}
public abstract class BaseFileInfo<TStorageProvider> : ResourceInfo<TStorageProvider> where TStorageProvider : IStorageProvider<ICacheProvider>
{
}
public class DerivedFileInfo : BaseFileInfo<DerivedStorageProvider>
{
}
I have an application that is structured as an service layer wich uses a repository layer for persistence.
I'm trying to create a generic controller class to reuse shared behavior but I'm having trouble trying to set the generic parameters. The following code:
public class BusinessEntity
{ }
public class Person : BusinessEntity
{ }
public interface IRepository<T> where T : BusinessEntity
{ }
public interface IService<T, R>
where T : BusinessEntity
where R : IRepository<T>
{ }
public partial interface IPersonRepository : IRepository<Person>
{ }
public interface IPersonService : IService<Person, IPersonRepository>
{ }
public abstract class BaseController<X, Y>
where X : BusinessEntity
where Y : IService<X, IRepository<X>>
{ }
public class PersonController : BaseController<Person, IPersonService>
{ }
fails at compilation with
The type ConsoleApplication.IPersonService cannot be used as type parameter Y in the generic type or method ConsoleApplication.BaseController<X,Y>. There is no implicit reference conversion from ConsoleApplication.IPersonService to ConsoleApplication.IService<ConsoleApplication.Person,ConsoleApplication.IRepository<ConsoleApplication.Person>>
this works
public interface IPersonService : IService<Person, IRepository<Person>>
but I lose the custom repository
There is a way to make the compiler realize IPersonRepository is an IRepository<Person>?
public class BusinessEntity
{ }
public class Person : BusinessEntity
{ }
public interface IRepository<T> where T : BusinessEntity
{ }
public interface IService<T, R>
where T : BusinessEntity
where R : IRepository<T>
{ }
public partial interface IPersonRepository : IRepository<Person>
{ }
public interface IPersonService : IService<Person, IPersonRepository>
{ }
public abstract class BaseController<X, Y, Z>
where X : BusinessEntity
where Y : IService<X, Z>
where Z : IRepository<X>
{ }
public class PersonController : BaseController<Person, IPersonService, IPersonRepository>
{ }
To address your comment:
IPersonService can extend the base service class to add custom facilities, like FindPersonsUnderAge(). For this it requires a custom repository. Actually LINQ avoids a lot of custom repository code, but sometimes they are required.
Couldn't IPersonService do that without requiring the repository type to be a type parameter? For example:
public interface IService<T> where T : BusinessEntity { }
public interface IPersonService : IService<Person>
{
IEnumerable<Person> FindPersonsByAge(double minAge, double maxAge);
}
public class Service<T, R> : IService<T>
where T : BusinessEntity
where R : IRepository<T>
{ }
public class PersonService : Service<Person, IPersonRepository>, IPersonService
{ }
Thanks to sll for pointing me in the right direction
public interface IService<T, out R>
where T : BusinessEntity
where R : IRepository<T>
{ }
does the trick
I trying to inherit a class Blah2, but after adding a method it says BlahA doesn't implement that method.
How can I add a method to my new class?
public class Blah2 : BlahA
{
}
public class Blah3 : Blah2
{
public List<int> MyNewMethod()
{
}
}
Note: BlahA is an abstract class.
Update
public abstract class BlahA : IBlah
{
}
Update II - the error
Error 3 'Blah.Components.BlahA' does not contain a definition for 'Blah3' and no extension method 'Blah3' accepting a first argument of type 'Blah.Components.BlahA' could be found (are you missing a using directive or an assembly reference?)
Well if it's implementing an interface as you posted in your comments, then the problem is that your BlahA class doesn't satisfy the requirements of the interface. There must be some method in the interface (I'm assuming its the MyNewMethod) that you're not implementing in your abstract BlahA class.
If my assumption is correct, add this to your base class:
public abstract List<int> MyNewMethod();
and in your sub class, add the word override to your method declaration.
Some code:
public interface MyInterface
{
void MyMethod();
}
public abstract class Base : MyInterface
{
public abstract void MyMethod();
}
public class SubA : Base
{
public override void MyMethod()
{
throw new NotImplementedException();
}
}
public class SubB : SubA
{
public void Foo() { }
}
Wrting this code and compiling works fine
public abstract class BlahA
{
}
public class Blah2 : BlahA
{
}
public class Blah3 : Blah2
{
public List<int> MyList()
{
return new List<int>();
}
}
We will need a bit more of the code that isnt working
EDIT:
from comments you need to implement the method from interface in abstract class.
public interface IBlah
{
int GetVal();
}
public abstract class BlahA : IBlah
{
public int GetVal()
{
return 1;
}
}
public class Blah2 : BlahA
{
}
public class Blah3 : Blah2
{
public List<int> MyList()
{
int i = GetVal();
return new List<int>();
}
}