List（Of clsClass）= List（Of clsSubClass） [英] List(Of clsClass) = List(Of clsSubClass)
问题描述
我有一个继承自clsClass的类clsSubClass。
当我实例化clsClass（MyClass）的对象时，我从clsSubclass（MySubClass）实例化一个
对象我可以做一个MyClass = MySubclass。
但是当我声明它们的两个通用列表时（Dim MyList1 as List（of
clsClass）" andDim MyList2 as List（Of clsSubClass）），我不能做
MyList1 = MyList2。
为什么这是完全正确的，有没有办法实现这种行为？
非常感谢，
Pieter
Hi,
I have a class clsSubClass which inherits from clsClass.
When I instantiate an object of clsClass (MyClass), and I instantiate an
object from clsSubclass (MySubClass) I can do an "MyClass = MySubclass".
But when I declare two generic list of them ("Dim MyList1 as List(Of
clsClass)" and "Dim MyList2 as List(Of clsSubClass)"), I can''t do an
"MyList1 = MyList2".
Why is this exactly, and is there a way to implement this behaviour?
Thanks a lot in advance,
Pieter
推荐答案
" Pieter" < pi **************** @ hotmail.comwrote：
"Pieter" <pi****************@hotmail.comwrote:
当我实例化clsClass（MyClass）的对象时，我从clsSubclass（MySubClass）实例化一个
对象我可以做一个MyClass = MySubclass。
但是当我声明它们的两个通用列表时（ Dim MyList1 as List（of
clsClass）"和Dim MyList2 as List（Of clsSubClass）"），我不能做一个
" MyList1 = MyList2"。
为什么这是确切的，有没有办法实现这种行为？
When I instantiate an object of clsClass (MyClass), and I instantiate an
object from clsSubclass (MySubClass) I can do an "MyClass = MySubclass".
But when I declare two generic list of them ("Dim MyList1 as List(Of
clsClass)" and "Dim MyList2 as List(Of clsSubClass)"), I can''t do an
"MyList1 = MyList2".
Why is this exactly, and is there a way to implement this behaviour?
你不能这样做因为它允许你将超类实例添加到
a列表中，根据它的定义，它只是应该包含子类
实例。
当然，你可以编写一个循环来将元素复制到单独的列表中
另一种类型。
Eq。
You can''t do that because it would allow you to add superclass instances to
a list that, by its definition, is only supposed to contain subclass
instances.
Of course, you can write a loop to copy the elements to a separate list of
another type.
Eq.
7月15日，9：34 * am，Pieter < pieterNOSPAMcou ... @ hotmail.comwrote：
On Jul 15, 9:34*am, "Pieter" <pieterNOSPAMcou...@hotmail.comwrote:
我有一个继承自clsClass的类clsSubClass。
我实例化一个clsClass（MyClass）的对象，并从clsSubclass（MySubClass）实例化一个
对象我可以做一个MyClass = MySubclass...
但是当我声明它们的两个通用列表（Dim MyList1 as List（Of
clsClass）和Dim MyList2 as List（Of clsSubClass））时，我可以'做一个
" MyList1 = MyList2"。
I have a class clsSubClass which inherits from clsClass.
When I instantiate an object of clsClass (MyClass), and I instantiate an
object from clsSubclass (MySubClass) I can do an "MyClass = MySubclass"..
But when I declare two generic list of them ("Dim MyList1 as List(Of
clsClass)" and "Dim MyList2 as List(Of clsSubClass)"), I can''t do an
"MyList1 = MyList2".
的确如此。这是因为泛型不会出现差异。
Indeed. That''s because generics don''t exhibit variance.
为什么这是完全正确的，有没有办法实现这种行为？
Why is this exactly, and is there a way to implement this behaviour?
简要回答：考虑这段代码。
List< BananabananaBunch = new List< Banana>（）;
列表< Fruitfruitbowl = bananaBunch;
fruitbowl.Add（新Apple（））;
假设这是合法的 - 那么bananaBunch将包含一个Apple，
显然是无效的。
有关详细信息，请参阅Eric Lippert的一系列博客文章：
http://blogs.msdn.com/ericlippert/ar ... e / default.aspx
Jon
Brief answer: consider this code.
List<BananabananaBunch = new List<Banana>();
List<Fruitfruitbowl = bananaBunch;
fruitbowl.Add(new Apple());
Suppose this were legal - then bananaBunch would contain an Apple,
which is clearly invalid.
For a lot of detail, see Eric Lippert''s series of blog articles:
http://blogs.msdn.com/ericlippert/ar...e/default.aspx
Jon
哦确实你是对的： - SI应该考虑到这一点:-)
虽然：导入也不起作用，这应该适用于我的操作吗？
因为clsSubClass是也是一个clsClass：它可以导入这些
项......
" Jon Skeet [C＃MVP]" < sk *** @ pobox.com写信息
新闻：e6 *************************** ******* @ l64g2000 hse.googlegroups.com ...
7月15日上午9：34，Pieter < pieterNOSPAMcou ... @ hotmail.comwrote：
Oh yes indeed you''re right :-S I should have thought about that :-)
Although: An Import doesn''t work neither, which should work in my opnion?
Because clsSubClass is also an clsClass: it shoudl be able to import these
items...
"Jon Skeet [C# MVP]" <sk***@pobox.comwrote in message
news:e6**********************************@l64g2000 hse.googlegroups.com...
On Jul 15, 9:34 am, "Pieter" <pieterNOSPAMcou...@hotmail.comwrote:
这篇关于List（Of clsClass）= List（Of clsSubClass）的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
