c# - 使用类型参数约束实现两次相同的通用接口(interface)

Question

拥有

public interface IGeneric<T>{}

public class Student{}

public class Teacher{}

这是可能的

public class ConcreateClass : IGeneric<Student>, IGeneric<Teacher>{}

这是不可能的

public class GenericClass<T1, T2> : IGeneric<T1>, IGeneric<T2> {}

因为GenericClass<String, String> instance;会导致两个接口(interface)实现的歧义。但为什么这是不可能的

public class GenericClassWithTypeConstraints<T1, T2> : IGeneric<T1>, IGeneric<T2>
    where T1 : Student
    where T2 : Teacher
{}

因为 T1 和 T2 不能属于同一类？ (编译错误和没有类型约束的情况一样)

编辑
Why does this result in CS0695 中的“Soner Gönül”提出了一种解决方法，使用两级类继承，如下所示:

public interface IGeneric<T> { String Name { get; } }

public class Student{}

public class Teacher{}

public class GenericClassBase<T1> : IGeneric<T1>
    where T1 : Student
{ String IGeneric<T1>.Name { get { return "Generic class of Student"; } } }

public class GenericClassDerived<T1, T2> : GenericClassBase<T1>, IGeneric<T2>
    where T1 : Student
    where T2 : Teacher
{ String IGeneric<T2>.Name { get { return "Generic class of Teacher"; } } }

像这样的代码然后产生预期的结果

GenericClassDerived<Student, Teacher> X = new GenericClassDerived<Student, Teacher>();
Console.WriteLine(((IGeneric<Student>)X).Name); //outputs "Generic class of Student"
Console.WriteLine(((IGeneric<Teacher>)X).Name); //outputs "Generic class of Teacher"

Answer 1

as T1 and T2 cannot be of the same class?

是的，他们可以是同一个类(class)。约束不能是 sealed classes¹ 所以 Teacher 可以从 Student 派生，反之亦然。这不合逻辑，但编译器对此一无所知。

例如，

using System;

public class Student{}

public class Teacher : Student{}

public class GenericClassWithTypeConstraints<T1, T2>
    where T1 : Student
    where T2 : Teacher
{}

class Test {
    static void Main() {
        var obj = new GenericClassWithTypeConstraints<Teacher, Teacher>();
    }
}

编译没有任何问题。 Demo

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1. Docs:

   Constraints can also be class types, such as abstract base classes. However, constraints cannot be value types or sealed classes.