类型参数继承的通用类型转换

Question

Given the following code:

namespace sample
{
    class a { }

    class b : a { }

    public class wrapper<T> { }

    class test
    {
        void test1()
        {
            wrapper<a> y = new wrapper<b>();
            //Error 11  Cannot implicitly convert type 'sample.wrapper<sample.b>' to 'sample.wrapper<sample.a>' 
        }
    }
}

Logically speaking, a since b is a , a wrapper<b> is a wrapper<a> . Then why I can't make this conversion, or how can I make it?

Thanks.

Answer 1

since b is a, a wrapper<b> is a wrapper<a>

Well, this is not true for .NET generic classes, they can't be co-variant. You can achieve something similar using interface covariance:

class a { }
class b : a { }

public interface Iwrapper<out T> { }
public class wrapper<T> : Iwrapper<T> {}

class test
{
    void test1()
    {
        Iwrapper<a> y = new wrapper<b>();
    }
}

Answer 2

This is a matter of covariance.

Class b is an a , but wrapper<b> is not a wrapper<a> .

You can use C# 4's covariance syntax to allow it like so:

public interface IWrapper<out T> { ... }

public class Wrapper<T> : IWrapper<T> { ... }

This will instruct the CLR to see Wrapper<B> as a Wrapper<A> .

(For the record: C# has capitalization conventions; class names are Pascal-cased).

Answer 3

Lets do a scenario. Lets call the class a Mammal , the class b Dog , and lets say that the wrapper<T> class is List<T>

See what happens in this code

List<Dog> dogs = new List<Dog>();  //create a list of dogs
List<Mammal> mammals = dogs;   //reference it as a list of mammals

Cat tabby = new Cat();
mammals.Add(tabby)   // adds a cat to a list of dogs (!!?!)

Dog woofer = dogs.First(); //returns our tabby
woofer.Bark();  // and now we have a cat that speaks foreign languages

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(Paraphrase of my answer on How to store base class children in a dictionary? )