An Inheritance Puzzle, Part One

C#小测试（二）：嵌套子类带来的困惑

Anders Cui — Mon, 04 Aug 2008 06:53:19 GMT

如果我们的代码中同时出现泛型、继承、嵌套类这三种语言元素，那么在根据名称解析类型的时候可能就会有歧义了。本文中的问题及其结论是非常有意思的，其分析过程也非常的绕，大家一起来讨论下吧:)

C#小测试（二）：嵌套子类带来的困惑

Anders Cui — Sun, 03 Aug 2008 14:25:48 GMT

re: An Inheritance Puzzle, Part One

Gautam — Fri, 28 Sep 2007 00:15:57 GMT

Hi Eric,

If we modify the code like this ..

public class A<T>

{

public void N()

{

Console.WriteLine(typeof(T).ToString());

}

public class B : A<int>

{

public void M()

{

Console.WriteLine(typeof(T).ToString());

}

public class C : B

{

}

static void Main()

{

A<string>.B b = new A<string>.B();

b.M();

b.N();

Here, b.M() shows String but b.N() shows Int32. I am confused how could it happen.....

re: An Inheritance Puzzle, Part One

Gautam — Thu, 27 Sep 2007 23:50:57 GMT

Hi Eric, I have a confusion, you mentioned that ** the question hinges solely upon what "B" means in "class C : B", and in this case it means A<int>.B. **

So, if you consider the following code

A<string>.B b = new A<string>.B();

b.M();

Why in this case b.M() prints String not INT32 , here also "B" should be considered as A<int>.B ... what's your comment ....

An Inheritance Puzzle, Part Two

Fabulous Adventures In Coding — Fri, 07 Sep 2007 01:26:52 GMT

Today, the answer to Friday's puzzle . It prints "Int32". But why? Some readers hypothesized that M would

re: An Inheritance Puzzle, Part One

Eric Lippert — Sat, 28 Jul 2007 18:13:47 GMT

Steve:

Excellent example! Why is it that this DOESN'T reproduce the odd behaviour? The germane difference between the two is that in your example, A<T> does not contain anything called B.

When I post what is really happening on Monday it'll be clear why this does what it does.

You need the "new" because your method M shadows the in-scope type variable M from A<M>. We try to make it either illegal to shadow like this, or make you put the keyword "new" on one of them as a big waving red flag that says "there is a name ambiguity involving this method, so keep that in mind when you are reading this code".

re: An Inheritance Puzzle, Part One

Steve — Sat, 28 Jul 2007 16:45:22 GMT

Hi,

public class A<M> {

public class X<T> {

public class B : A<int> {

public new void M() {

System.Console.WriteLine(typeof(T).ToString());

System.Console.WriteLine(typeof(M).ToString());

}

public class C : B { }

}

...

A<string>.X<double>.B.C c = new A<string>.X<double>.B.C();

c.M();

Prints string and double.

And why do I need the new keyword in M here?

Your postings are way shorter than Chris Brumme's when he was still posting, but I am finding myself equally confused after reading them (I love that). So I guess you can consider yourself much more efficient:-)

re: An Inheritance Puzzle, Part One

Andrew Ducker — Sat, 28 Jul 2007 13:13:46 GMT

Took me a few minutes to grasp this one. The <T> is clearly referring to the containing class. Which means that b.M() shows a string. But the container class for c is b - which is A<Int>.

So yes, that makes sense - just took a moment to get there.

re: An Inheritance Puzzle, Part One

WaterBreath — Sat, 28 Jul 2007 07:45:26 GMT

Sorry, I prefer to err on the side of politeness...

As far as B inheriting T.... I was trying to infer the inner workings here, to explain why in the world the example would yield int instead of string. Some sort of weird inheritance of T was the only way I could figure that this would happen.

Obviously there's some assumption I'm making that is wrong. Either I don't understand the scoping rules, or I've got the order in which things happen all mixed up.

I still think it _should_ output string. =P

Sigh.... I'll wait for Monday.

re: An Inheritance Puzzle, Part One

Eric Lippert — Sat, 28 Jul 2007 07:11:41 GMT

First off, Mr. Lippert is my father, dude.

Second, I do not know what you mean by "the inheritance of the generic parameter T". T isn't inherited. B doesn't have a T. Only A<T> has a T.

Hopefully my explanation on Monday will clear it all up.