I wanted to make the following inheritance with included generics, but the final cast
a as A<XBase>
always results in null, because the cast is not valid. Could anybody elaborate on why this cast would be invalid, as well as maybe a solution to this problem.
public class XBase {}
public interface A<T> where T : XBase
{
//Edited
void Method(T param);
}
public class Implementor : A<Implementor.ImplementorX >
{
public class ImplementorX : XBase {public int a;}
//Edited
void Method(ImplementorX param) {}
}
public class HelloWorld
{
public static void Main(string[] args)
{
var a = new Implementor();
var castRes = a as A<XBase>;
Console.WriteLine(castRes != null);
}
}
see live example https://rextester.com/BTNVT61833
EDITED: Added a method to interface A<T>
bc else it could be solved with @DavidG's response
CodePudding user response:
If you make an explicit cast:
var castRes = A<XBase>(a);
then you will see the following error:
Unable to cast object of type '' to type '`
Why? In my view, it is better to understand using real world example. I've renamed classes based on this explanation. There are comments which maps explanations to your classes in question.
Abstractions:
// XBase
public class Animal { }
// class ImplementorX : XBase {public int a;}
public class Bird : Animal
{
public string WingColor { get; set; }
}
// interface A<T> where T : XBase
public interface IHat<T> where T : Animal
{
void Hide(T param);
T Pull();
}
Concrete implementations:
// class Implementor : A<Implementor.ImplementorX >
public class Peacock : IHat<Bird>
{
// void Method(ImplementorX param) {}
void IHat<Bird>.Hide(Bird param)
{ }
public Bird Pull()
{ }
}
and how it can be called:
public static void Main(string[] args)
{
Peacock peacockHat = new Peacock();
IHat<Animal> animalHat = (IHat<Animal>) peacockHat; // runtime error 'Unable to cast
// object of type 'HelloWorld.Peacock' to type 'HelloWorld.IHat`1
// because
animalHat.Hide(new Dolphin()); // Hide a dolphin in a peacock hat?
}
So we cannot hide
hat of Peacock
from Dolphin
. It is not okay. CLR prevents us from making inappropriate actions.
In short:
In short, imagine you have two animals such as Wolf
and Sheep
. And these classes implements IAnimal interface:
public interface IAnimal
{ }
public class Wolf: IAnimal
{ }
public class Sheep : IAnimal
{ }
So Sheep
, Wolf
classes implement the inherited interface IAnimal
:
IAnimal
/ \
/ \
Sheep Wolf
And then these animals can be put in cage:
public class Cage<T> where T : IAnimal
{
public void Put(T animal)
{ }
}
Then you create a cage for Sheep
. After that somebody wants to cast Sheep
cage to IAnimal
:
Cage<Sheep> sheepCage = new Cage<Sheep>();
sheepCage.Put(new Sheep());
Cage<IAnimal> animalCage = (Cage<Wolf>)sheepCage; // compile error
// if there were no error, then you will be able to do:
animalCage.Put(new Wolf()); // it is not good idea