通用基类的隐式引用转换完成错误

Question

I would like to be able to create the following class structure:

    public class Person
    {
        public Person() { }
        public string Name { get; set; }
    }

    class Student : Person { }
    class FireFighter : Person { }
    class PoliceOfficer : Person { }

    class Building<T> where T : Person, new()
    {
        public IEnumerable<T> Members { get; set; }
    }

    class School : Building<Student> { }
    class FireStation : Building<FireFighter> { }
    class PoliceStation : Building<PoliceOfficer> { }

    class Container<T> where T : Building<Person>, new() { }

    class SchoolDistrict : Container<School> { }

However this gives me the following error:

The type 'School' cannot be used as type parameter 'T' in the generic type or method 'Container'. There is no implicit reference conversion from 'School' to 'Building'

What am I doing wrong here?

Answer 1

Generic typing won't go as far as you need. As stated; Building<Person> is not Building<Student> . However, this update to the last two classes will allow compilation:

    class Container<T, U> 
        where U : Person, new()
        where T : Building<U>, new() { }

    class SchoolDistrict : Container<School, Student> { }

Answer 2

School is a Building<Student> . Building<Student> is not a Building<Person> even though Student is a Person .

Answer 3

Introducing an interface will allow you to take advantage of Covariance in the generic declaration. This will allow Building to be treated as Building which is ultimately what you are wanting to achieve. Note that only interfaces support this via the 'out' modifier:

    public class Person
    {
        public Person() { }
        public string Name { get; set; }
    }

    class Student : Person { }

    class Building<T> : IBuilding<T>
        where T : Person, new()
    {
        public IEnumerable<T> Members { get; set; }
    }

    internal interface IBuilding<out TPerson> where TPerson : Person { }
    class School : Building<Student> { }

    class Container<T>
        where T : IBuilding<Person>, new() { }

    class SchoolDistrict : Container<School> { }