C#, Generic Lists and Inheritance

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

what does "B is a child of A" mean? In the code listing you've given, A and B are type parameters. — Adam Ralph
– Adam Ralph, Commented Mar 16, 2011 at 0:18
ok, but in your example, A and B are the type parameters used in the definitions of types Foo and Bar. This is quite confusing. — Adam Ralph
– Adam Ralph, Commented Mar 16, 2011 at 10:17
OK. Consider A = Shape and B = Circle. I want to load a list of circles into a list of shapes and save a list of shapes that I know are all circles. Circle is derived from Shape. — Andy
– Andy, Commented Mar 16, 2011 at 22:20

Mark Sowul · Accepted Answer · 2011-03-16 13:18:55Z

3

Use .NET 4.0 and IEnumerable<T> instead of List<T> (http://blogs.msdn.com/b/csharpfaq/archive/2010/02/16/covariance-and-contravariance-faq.aspx), otherwise make Save generic on T (the list item type) as well.

edited Mar 16, 2011 at 13:18

answered Mar 16, 2011 at 0:16

Mark Sowul

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2 Comments

Andy Over a year ago

I see that IEnumerable is what I need but sadly I can't use .NET 4.0. Bah!

Mark Sowul Over a year ago

Then I'd make class Bar generic: class Bar<T> where T : A { void Save(List<T> ComplexItems); List<T> Load(); } and instantiate it with Bar<B>. Alternatively make just the methods themselves generic (may be easier especially with type inference): void Save<T>(List<T> ComplexItems) where T : A; List<T> Load<T>(); where T : A }

J.N. · Accepted Answer · 2011-03-16 00:16:08Z

1

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

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

answered Mar 16, 2011 at 0:16

J.N.

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1 Comment

Andy Over a year ago

Thanks. What about the Load function?

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