Why the method cannot declare the return type of `List <IMyInterface>` and return a specific class that implements it

Question

I have the following interface

public interface IMyInterface
{
    // Some method and property signatures
}

I have the following class implementing the above interface

public class MyClass : IMyInterface
{
    // Some methods and properties as declared by IMyInterface
}

Then I have this method in some random class that I want to return a list of objects that implement IMyInterface. In this particular implementation, these objects are instances MyClass.

public List<IMyInterface> getItems(int id)
{
    return new List<MyClass>();
}

This will lead to a compilation error (apparently, in real time also in Visual Studio)

Cannot implicitly convert the type 'System.Collections.Generic.List <MyClass>' to 'System.Collections.Generic.List <IMyInterface>'

, , # .

public List<IMyInterface> getItems(int id)
{
    return new List<MyClass>().Cast<IMyInterface>().ToList();
}

, , -, ; . # Aequitarum Custos , .

- ?

Answer 1

, T List<T> . IEnumerable<out T>:

out T

.

. , , .

, , getItems:

public IEnumerable<IMyInterface> getItems(int id)
{
    return new List<MyClass>() as IEnumerable<IMyInterface>;
}

.

Answer 2

, Base Parent, , List<Base> List<Parent>. IList<T> , List<T> (# , ).

, : http://blogs.msdn.com/b/csharpfaq/archive/2010/02/16/covariance-and-contravariance-faq.aspx

List<T> , :

List<Parent> list = new List<Base>();

, ?

list.Add(new OtherBase());

, , , .

Answer 3

Consider the following example:

public interface IAnimal
{
}

public class Cat : IAnimal
{
}

public class Dog : IAnimal
{
}

And you are the equivalent method:

public List<IAnimal> getAnimals()
{
    return new List<Dog>();
}


// You just Dog'd a list of Cats
IEnumerable<Cat> cats = getAnimals();