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Can't convert interface to concrete interface

Why i can't convert implementation of interface which concrete implement generic interface? I need for Cat, Dog etc own interface realisation.
public interface IMarker { }
public class ResultA : IMarker
{
}
public class ResultB : IMarker
{ }
public interface IService<T> where T : IMarker
{
public List<T> DoStuff();
}
public interface ICatService : IService<ResultA>
{ }
public interface IDogService : IService<ResultB>
{ }
public class CatService : ICatService
{
public List<ResultA> DoStuff()
{
return new List<ResultA>();
}
}
public class DogService : IDogService
{
public List<ResultB> DoStuff()
{
return new List<ResultB>();
}
}
public abstract class Animal
{
protected readonly IService<IMarker> _svc;
protected Animal(IService<IMarker> svc)
{
_svc = svc;
}
}
public class Cat : Animal
{
public Cat(ICatService svc) : base(svc)
{
}
}
public class Dog : Animal
{
public Dog(ICatService svc) : base(svc)
{
}
}
CS1503 Argument 2: cannot convert from 'ICatService' to 'IService'
I have DI for services i.e. :
services.AddTransient<ICatService, CatService>();

The reason for such behaviour is that in general case IService<ResultA> is not IService<IMarker> (basically I would argue the same cause for C# classes does not supporting variance which is for a pretty good reason - see more here and here).
In this concrete case everything can be fixed by making the interface covariant and leveraging the covariance of IEnumerable<T>:
public interface IService<out T> where T : IMarker
{
public IEnumerable<T> DoStuff();
}
public class CatService : ICatService
{
public IEnumerable<ResultA> DoStuff() => return new List<ResultA>();
}
public class Cat : Animal
{
public Cat(CatService svc) : base(svc)
{
}
}
But not sure that in your actual code you will be able to.
Or just make the base class generic (if this suits your use case):
public abstract class Animal<T> where T : IMarker
{
protected readonly IService<T> _svc;
protected Animal(IService<T> svc)
{
_svc = svc;
}
}
Original answer
CatService does not implement ICatService, i.e. the fact that ICatService inherits only IService<ResultA> does not mean that they are the same, C# is strongly-typed (mostly :-) language and compiler will consider those two interfaces being different ones (though related). You need either to make CatService to implement ICatService:
public class CatService : ICatService
{
// ...
}
Or register and resolve the IService<ResultA> interface (basically skipping intermediate interface at all):
services.AddTransient<IService<ResultA>, CatService>();
// ...
public Cat(IService<ResultA> svc) : base(svc){}

Generic interface and covariance - fighting the InvalidCastException

This is probably a classic covariance/contravariance question, it looks like it should work but I'm probably missing a trick.
I'm attempting to return a less derived type from a factory method, but I find that I cannot cast the more specialized concrete instance to a less derived base type.
public class AnimalSettings { ... }
public class CatSettings : AnimalSettings { ... }
public interface IAnimalService<TSettings> { ... }
public abstract AnimalService<TSettings> : IAnimalService<TSettings> where TSettings : AnimalSettings { ... }
public class CatService : AnimalService<CatSettings> { ... }
Then, in a factory method I have:
public static IAnimalService<AnimalSettings> GetAnimalService(AnimalType selector)
{
switch (selector)
{
case AnimalType.Cat:
return (IAnimalService<AnimalSettings>) new CatService();
break;
}
}
and the intention is to be able to do the following:
var service = MyServiceFactory.GetAnimalService(AnimalType.Cat);
service.DoAnimalBehavior();
This compiles fine, but at runtime my code is failing in the attempted cast return (IAnimalService<AnimalSettings>) new CatService();, with an InvalidCastException.
How should I be casting my more derived type to a less derived type so that callers can use that interfaced base type to invoke functionality?
Changing the cast to (IAnimalservice<CatSettings>) new CatService() does work, but it's intended that the caller receives a IAnimalservice<AnimalSettings> so that it can handle any sort of animal (In other words, the caller should not be using any of the more specialized types). Should I be specifying an in or out as part of the generic definition somewhere?

By giving a complete example it would be much easier to help. ;-)
So here is the working code. And as Sweeper already mentioned, you need to add the out parameter at the interface to make it work.
using System;
public class Program
{
public static void Main()
{
var catService = new CatService(new CatSettings());
var genericService = (IAnimalService<AnimalSettings>)catService;
genericService.DoAnimalBehavior();
}
}
public abstract class AnimalSettings
{
public abstract void DoAnimalBehavior();
}
public class CatSettings : AnimalSettings
{
public override void DoAnimalBehavior()
{
Console.WriteLine("Meeoh");
}
}
public interface IAnimalService<out TSettings>
{
void DoAnimalBehavior();
}
public abstract class AnimalService<TSettings> : IAnimalService<TSettings> where TSettings : AnimalSettings
{
private readonly TSettings _settings;
public AnimalService(TSettings settings)
{
_settings = settings;
}
public void DoAnimalBehavior()
{
_settings.DoAnimalBehavior();
}
}
public class CatService : AnimalService<CatSettings>
{
private readonly CatSettings _catSettings;
public CatService(CatSettings catSettings)
: base(catSettings)
{
_catSettings = catSettings;
}
}

Forcing a derived class to have a constructor with a signature

How do I force all derived classes of an interface to have a constructor with a signature? This doesn't work:
public interface Constructor<T> where T : Constructor<T>, new()
{
Constructor(T);
}
public interface IParameters
{
}
public interface IRule : Constructor<IParameters>
{
//IRule (IParameters); must exist
}

You can't, not via an interface. But you can sort of get at it with an abstract class. Similar to what the accepted answer here describes, try:
public abstract class MustInitialize<T>
{
public MustInitialize(T parameters)
{
}
}
public class Rule : MustInitialize<IParameters>, IRule
{
IParameters _parameters;
public Rule(IParameters parameters)
: base (parameters)
{
_parameters= parameters;
}
}

You can't force a specific constructor signature.
Even with an abstract class as demonstrated in Mark's answer, you can only force the constructor of the abstract class, but nothing is stopping the author of the derived class to do something like this:
public class Rule : MustInitialize<IParameters>, IRule
{
public Rule()
: base (new Parameters())
{
// Assuming Parameters is a class that implements the IParameters interface
}
}
However, you can force dependency injection by using method (setter) injection:
public interface IMethodInjection<T>
{
void Method(T injected);
}

I think you can design your base class like the following example:
public abstract class MyBase
{
private MyBase()
{
}
public MyBase(string a)
{
}
}
public class MyDerived : MyBase
{
public MyDerived(string a) : base(a)
{
}
}
You can even delete the private constructor if its not needed

Generic interfaces and inheritance

I have a base interface that looks like this
public interface IBaseWidget<T> where T: IBaseConfiguration
{
void Configure(T configuration);
}
I then have child interfaces that look like this:
public interface ISpecificWidget : IBaseWidget<IChildSpecificConfiguration>
{
}
IChildSpecificConfiguration implements IBaseConfiguration I then have classes that look like this
public class SomeClass : ISpecificWidget
{
public void Configure(IChildSpecificConfiguration configuration)
{
//do stuff
}
}
This all works fine and we will come back to this.
Where it starts to break down is that there are several different extensions of IBaseWidget<T> which means that there are several different SomeClass. Additionally SomeClass is only ever accessed via a facade so, based on a bunch of rules, I have a base facade that actually creates an instance of the necessary SomeClass. Prior to making IBaseWidget generic as shown above, my base facade looked like the following:
public abstract class BaseFacade<T> where T IBaseWidget
{
T Widget {get;set;}
private void Init()
{
Widget = (T)Activator.CreateInstance("type");
}
}
Facades for each implementation of IBaseFoo look like so:
public sealed class SomeFacade : BaseFacade<ISpecificWidget>
{
private void DoSomething()
{
Widget.DoSomething();
}
}
Again, this works fine. My problem is this: after making IBaseWidget generic I modified the base facade like so:
public abstract class BaseFacade<T> where T : IBaseWidget<IBaseConfiguration>
{
T Widget {get;set;}
IBaseConfiguration configuration; //IChildSpecificConfiguration passed in via constructor and assigned to variable
private void Init()
{
Widget = (T)Activator.CreateInstance("type");
Widget.Configure(configuration)
}
}
This breaks the above implmentation of SomeFacade with the message:
ISpecificWidget must be convertible to IBaseWidget<IBaseConfiguration>
My expectation is that when I defined ISpecificWidget like so:
public interface ISpecificWidget : IBaseWidget<IChildSpecificConfiguration>
{
}
it would be convertible to IBaseWidget but that is not the case. I know that I could solve this by having bases for each specific implementation of a widget, but I am trying to keep this as generic as possible. Any feedback would be greatly appreciated.

Maybe this could answer to your problem.
Add this interface:
public interface IBaseWidget
{
void Configure(IBaseConfiguration configuration);
}
Then inherits from it:
public interface IBaseWidget<T> : IBaseWidget
where T : IBaseConfiguration
{
void Configure(T configuration);
}
And finally:
public abstract class BaseFacade<T> where T : IBaseWidget
{
T Widget { get; set; }
IBaseConfiguration configuration; //IChildSpecificConfiguration passed in via constructor and assigned to variable
private void Init()
{
Widget = (T)Activator.CreateInstance(typeof());
Widget.Configure(configuration);
}
}
and:
public class SomeClass : ISpecificWidget
{
void IBaseWidget.Configure(IBaseConfiguration configuration)
{
throw new NotImplementedException();
}
public void Configure(IChildSpecificConfiguration configuration)
{
//do stuff
}
}

Problem with Abstract class, Interface, Container and methods

I've the following scenario
I've an Interface
public interface ImyInterface
{
void myInterfaceMethod(string param);
}
I've an Abstract Class
public abstract class myAbstractClass
{
public myAbstractClass()
{
//something valid for each inherited class
}
public void myAbstractMethod<T>(T param)
{
//something with T param
}
}
I've a class that inherits from myAbstractClass and implements ImyInterface
public class myClass : myAbstractClass, ImyInterface
{
public myClass():base()
{}
public void ThisMethodWillNeverCall()
{
// nothing to do
}
}
And, finally, I've a class where I'll create a ImyInterface object. At this point I would call myAbstractMethod, but...
public class myFinalClass
{
public void myFinalMethod()
{
ImyInterface myObj = _myContainer<ImyInterface>();
myObj.???
}
}
Obviously there isn't this method because it isn't declared into the interface.
My solution is the following
public interface ImyInterface
{
void myInterfaceMethod(string param);
void myFakeMethod<T>(T param);
}
public class myClass : myAbstractClass, ImyInterface
{
public myClass():base()
{}
public void ThisMethodWillNeverCall()
{
// nothing to do
}
//--- a fake method
public void myFakeMethod<T>(T param)
{
base.myAbstractMethod<T>(param);
}
}
Is there any other solution better than mine?
Thank you!

First of all, your naming convention is a mess. Read up on the guidelines that Microsoft have made.
It's also hard to tell what you are trying to achieve based on your example.
Back to your question:
You should only access an interface to work with that interface. Don't try to make any magic stuff with classes/interfaces to get them working together. That usually means that the class shouldn't try to implement the interface.
It's better that you create a new interface which have the features that you want and let your class implement both.