泛型基的隐式引用转换编译错误 class

Question

我希望能够创建以下 class 结构：

    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> { }

但是这给了我以下错误：

类型 'School' 不能用作泛型类型或方法 'Container' 中的类型参数 'T'。没有从 'School' 到 'Building'

的隐式引用转换

我做错了什么？

Answer 1

School 是一个 Building<Student>。 Building<Student> 不是 Building<Person>，尽管 Student 是 Person。

Answer 2

通用类型无法满足您的需要。就像声明的那样; Building<Person> 不是 Building<Student>。但是，这次对最后两个类的更新将允许编译：

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

    class SchoolDistrict : Container<School, Student> { }

Answer 3

引入接口将使您能够在泛型声明中利用协变。这将使 Building 被视为 Building，这最终是您想要实现的目标。请注意，只有接口通过 'out' 修饰符支持此功能：

    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> { }

泛型基的隐式引用转换编译错误 class

Implicit Reference Conversion Compliation Error with Generic Base class

c#

generics

covariance