C＃，通用列表和继承

Question

I have a class called Foo that defines a list of objects of type A:

class Foo {
    List<A> Items = new List<A>();
}

I have a class called Bar that can save and load lists of objects of type B:

class Bar {
    void Save(List<B> ComplexItems);
    List<B> Load();
}

B is a subclass of A. Foo, Bar, A and B are in a library and the user can create children of any of the classes.

What I would like to do is something like the following:

Foo MyFoo = new Foo();
Bar MyBar = new Bar();

MyFoo.Items = MyBar.Load();
MyBar.Save(MyFoo.Items);

Obviously this won't work. Is there a clever way to do this that avoids creating intermediate lists?

thanks, Andy

Answer 1

使用.NET 4.0和IEnumerable<T>而不是List<T> （ http://blogs.msdn.com/b/csharpfaq/archive/2010/02/16/covariance-and-contravariance-faq.aspx ），否则在T上Save泛型（列表项类型）。

Answer 2

If you can change your save function to take an IEnumerable then you can :

void Save(IEnumerable<B> ComplexItems);
Save(MyFoo.Items.Cast<B>());