埃里克·利珀特(Eric Lippert)访谈中的一个难题:继承和通用类型设置 [英] Puzzle from an Interview with Eric Lippert: Inheritance and Generic Type Setting

问题描述

有人可以向我解释以下代码为何输出其功能吗? 为什么T在第一个字符串中是String而不是Int32，为什么在下一个输出中是相反的情况?

这个难题来自对埃里克·利珀特的采访

浏览代码时，我真的不知道它是Int32还是String:

public class A<T>
    {
        public class B : A<int>
        {
            public void M() { System.Console.WriteLine(typeof(T)); }
            public class C : B { }
        }
    }

    public class P
    {
        public static void Main()
        {            
            (new A<string>.B()).M(); //Outputs System.String
            (new A<string>.B.C()).M(); //Outputs System.Int32
            Console.Read();
        }
    }

解决方案

略微更改代码:

public class A<T>
{
    public class B : A<int>
    {
        public void M() { System.Console.WriteLine(typeof(T)); }
        public class C : A<T>.B { }
    }
}

public class P
{
    public static void Main()
    {            
        (new A<string>.B.C()).M(); //Outputs System.String
    }
}

请注意如何将C的基类从B更改为A<T>.B.这会将输出从System.Int32更改为System.String.

否则，A<string>.B.C不是从A<string>.B派生的，而是从A<int>.B派生的，从而导致您看到的行为.这是因为一般而言，基类中定义的名称可以通过不限定条件的查找获得，而名称B是在基类A<int>中定义的.

Can someone explain to me why the below code outputs what it does? Why is T a String in the first one, not an Int32, and why is it the opposite case in the next output?

This puzzle is from an interview with Eric Lippert

When I look through the code, I really have no idea if it's going to be an Int32 or a String:

public class A<T>
    {
        public class B : A<int>
        {
            public void M() { System.Console.WriteLine(typeof(T)); }
            public class C : B { }
        }
    }

    public class P
    {
        public static void Main()
        {            
            (new A<string>.B()).M(); //Outputs System.String
            (new A<string>.B.C()).M(); //Outputs System.Int32
            Console.Read();
        }
    }

解决方案

Changing the code slightly:

public class A<T>
{
    public class B : A<int>
    {
        public void M() { System.Console.WriteLine(typeof(T)); }
        public class C : A<T>.B { }
    }
}

public class P
{
    public static void Main()
    {            
        (new A<string>.B.C()).M(); //Outputs System.String
    }
}

Note how I changed C's base class from B to A<T>.B. This changes the output from System.Int32 to System.String.

Without that, A<string>.B.C derives not from A<string>.B, but from A<int>.B, causing the behaviour you've seen. That's because in general, names defined in base classes are available by unqualified lookup, and the name B is defined in the base class A<int>.

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