c# - 使用协变/逆变实现的接口(interface)会产生编译时错误

Question

我实现了一个带有协变/逆变类型约束的接口(interface)，编译器告诉我 'Student' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method 'UserQuery.IMail<T,U>'

据我所知，我满足了这些要求。我做错了什么？

class Person { public Person(){} }
class Student : Person { public Student(){} }
class MatureStudent : Student {public MatureStudent(){}}

interface IMail<in T, out U> where T : new() where U : new() {
    void Receive(T t);
    U Return();
}

class Mail<Student,MatureStudent> : IMail<Student,MatureStudent> {
    public void Receive(Student s) {}
    public MatureStudent Return() { return new MatureStudent(); }
}

Answer 1

问题是您也将 Mail 声明为通用的 - 您已将 Student 和 MatureStudent 类型参数。你只想:

class Mail : IMail<Student,MatureStudent> {