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>();
    }
}