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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;
}
}

Declaring Method with generic type

I was using generic types in C# and I am new to using generic types. So, right now I am stuck with a problem. I have some classes like these:
public class MyModel1
{
}
public class MyModel2
{
}
public class BaseClass<T>
{
}
public class ChildClass1 : BaseClass<MyModel1>
{
}
public class ChildClass2 : BaseClass<MyModel2>
{
}
public class AnotherClass
{
//What will be the syntax of declaring this method
//The syntax of the following method is wrong and incomplete.
//It's there just to give an idea about whai i want to do.
public void MyMethod<T>()
where T : BaseClass<..what to write..>
{
}
}
My question is what will be the correct syntax of declaring MyMethod if I want to call MyMethod like this:
MyMethod<ChildClass1>();

If I understood correctly, you try to filter "MyMethod" so that T is a class of type "ChildClass ...".
You can add a generic parameter to your function like this:
public void MyMethod<T, U>()
where T : BaseClass<U>
{
}
But then you have to call MyMethod in that way.
MyMethod<ChildClass1, MyModel1>();
So it's quite complicated to use.
Another solution is to create a new "blank" class :
public abstract class Base // mark it as abstract if you don't need to use it in your code
{
}
public class MyModel1
{
}
public class MyModel2
{
}
public class BaseClass<T> : Base //The class inherits the new class
{
}
public class ChildClass1 : BaseClass<MyModel1>
{
}
public class ChildClass2 : BaseClass<MyModel2>
{
}
public class AnotherClass
{
public void MyMethod<T>()
where T : Base
{
}
}

You've forgotten to mention the return type and adding <T> after the class name. For example, if the return type is void, you could declare the method as:
public void MyMethod<T>()
where T : BaseClass<T>
{
}

This will work (by which I mean it compiles)
public void MyMethod<T>()
where T : BaseClass<MyModel1>
{ }
so does this:
public void MyMethod<T>()
where T : ChildClass1
{ }
Further edit after reading your comment...
You can do this:
public class AnotherClass<TBaseClass, TModel> where TBaseClass : BaseClass<TModel>
{
public void MyMethod(TBaseClass input)
{ }
}
I have a term for this, hopefully non-offensive. I call it The Generic Rabbit Hole of Madness. It's what happens when we try to combine generics and inheritance so that one set of classes can accomplish a broad set of goals that become increasingly confusing, and we solve it by adding more generic parameters and more generic classes.
You reach the bottom of the hole if you
- use <dynamic>
- check to see what the actual type is using GetType(), typeof, or is
- get it to compile but can't remember what it's supposed to do

Unity : Register and resolve class that implements generic interface

I am trying to use generics to subscribe handlers to different events. I have the following interfaces and classes:
public interface IEvent { }
public class CompletedEvent : IEvent { }
public interface IHandle<T> where T: IEvent { }
public class CompletedHandler : IHandle<CompletedEvent>
{
public void Handle(CompletedEvent args)
{
args.Completed.Path = "\...";
}
}
And I would like to resolve handler like this :
Container.ResolveAll<IHandle<T>>();
I have tried several approaches to register the above class in Unity but I did not succeed. Among which were
Container.RegisterType(typeof(IHandle<CompletedEvent>), typeof(CompletedHandler));
and
Container.RegisterType<IHandle<CompletedEvent>, CompletedHandler>();
Thank you!

I found the root of the problem and I am in a hurry to share about it and a solution, if some one is going the same erroneous steps I did. It turned out that my registration was okay, but the problem was in the handler factory where I violated the Liskov substitution principle. Here is the code.
public interface IEventFactory
{
IEvent Create(...);
}
public class CompletedEventFactory : IEventFactory
{
...
public IEvent Create(...)
{
return _container.Resolve<CompletedEvent>(new ParameterOverride(...));
}
}
public class Service
{
...
public void DoWork()
{
var completedEvent = _eventFactory.Create(...);
DomainEvents.Raise(completedEvent);
}
}
And my resolve was inside of the Raise method:
public static void Raise<T>(T args) where T : IDomainEvent
{
...
Container.ResolveAll<IHandle<T>>();
}
I was catching an InvalidOperationException - The current type, ...IHandle`1[...IEvent], is an interface and cannot be constructed. Are you missing a type mapping?
Here is the solution :
public interface IEventFactory
{
CompletedEvent Create(...);
}
Lessons that I learned:
Do not violate fundamental principles, however promising it may seems at the moment.
Include the exception in the post as it is a valuable input into the information about the problem.

Casting Generic to abstract base - covariance

The code below gives compile time error:
Error 170 Cannot convert type 'Tests.ChangeListener' to 'Tests.BaseListener'
How do I get this to compile?
namespace Tests
{
[TestClass]
public class UnitTest1
{
[TestMethod]
public void ShouldCompile()
{
BaseListener<IChange> listener = (BaseListener<IChange>)new ChangeListener();
}
}
public interface IChange
{
}
public interface ISpecificChange : IChange
{
}
public abstract class BaseListener<T>
where T : IChange
{
}
public class ChangeListener : BaseListener<ISpecificChange>
{
}
}

Since you can't do contravariance or covaraiance (ie in and out) on an abstract class you'll probably want an interface for your listener. Modifying the above to look like this allows it to compile (note entities not mentioned remain the same as the original code - attributes stripped to save me needing to import references while testing):
public class UnitTest1
{
public void ShouldCompile()
{
IListener<IChange> listener = new ChangeListener();
}
}
public interface IListener<out T> {}
public abstract class BaseListener<T> : IListener<T>
where T : IChange
{
}
This is obviously adding in a step that you don't currently have and may not be able to use for whatever reasons but it is the simplest way to get the code compiling and I think do what you want.

.net generic constraints and object inheritance compiling issue

I have the following situation below. This code will throw a compiler error for Test2
The type 'InheritedChild' cannot be used as type parameter 'T' in the generic type or method 'panelGenericIOGrid'. There is no implicit reference conversion from 'InheritedChild' to 'SerializerBase'.
public class SerializerBase<T>
{
}
public class DirectChild : SerializerBase<DirectChild>
{
}
public class InheritedChild : DirectChild
{
}
public class panelGenericIOGrid<T> : UserControl
where T: SerializerBase<T>, new()
{
}
...
panelGenericIOGrid<DirectChild> test;
panelGenericIOGrid<InheritedChild> test2;
...
I'm pretty convinced my implentation is funadmentally wrong. I want the following situation, both DirectChild and InheritedChild will give their appropriote type to the SerializerBase constuctor.
How do I get the code to work the way it needs to? Thanks!
Some info on the actual information. SerializerBase has a set of static functions that are implemented to automatically serialize and deserialize themselves based on their type.
DirectChild has a set of strings that are going to be stored on disk and recovered.
Inhertiedchild has all the members of DirectChild plus more.
Basically I'm going to need DirectChild.Serialize(filename), and IndirectChild.Serialize(filename), where the Serialize is a public member of SerializeBase

The problem is that InheritedChild doesn't implement SerializerBase<InheritedChild>, so it doesn't fulfil the constraints for T in panelGenericIOGrid<T>.
Unfortunately it's not clear that the solution is meant to be as we don't know what you're trying to achieve.
What are the members of SerializerBase<T> in real life? If you could give us more context, it would help us to help you.

It seems to me that you're missing an interface:
public interface ISerializerBase<T> { }
public class SerializerBase<T> : ISerializerBase<T> { }
public class DirectChild : SerializerBase<DirectChild> { }
public class InheritedChild : DirectChild, ISerializerBase<InheritedChild> { }
public class panelGenericIOGrid<T> where T: ISerializerBase<T>, new() { }
I don't know how that will change your design though. It might be that you'll need to reimplement some inherited methods or some interface methods in the InheritedChild.
But, maybe you can do this otherwise:
public interface MSerializable {}
public static class Serializable {
public static void Serialize(this MSerializable self, string fileName) {
// self will refer to the right type,
// no need to use generics if all you want is to serialize it ...
}
}
public class DirectChild : MSerializable { }
public class InheritedChild : DirectChild { }
public class panelGenericIOGrid<T> where T: MSerializable, new() { }
Will you do a binary serialization, or will you serialize it to XML?