inheritance and container in C#

Question

I work in C# here, let's say i have:

class A
{}

class B : A
{}

List<B> myList;

I would like, in a part of the code cast this myList as List< A>, but when I try to, I get an error:

List<A> myUpcastedList = (List<A>)myList; //not working

is it possible to do it ? if yes, what is the syntax ?

Why do you need to cast it to the base class? You can use it without casting. And I don't think what you're trying to do is possible because the List or List are not the type itself but the containers of the types A and B. I might be mistaken but I think what you're trying to do is not possible this way. — UnTraDe, Mar 07 '13 at 21:29
I have a function taking a list, a position and a range and returning a new list with every object in range, I wanted it to be generic, but i guess i'll juste duplicate my code — Titan, Mar 07 '13 at 23:22

MarcinJuraszek · Answer 1 · 2013-03-07T21:37:15.340

5

List<B> cannot be casted to List<A>. You have to create new List<A> and fill it with items from source commection. You can use LINQ to Objects for that:

var aList= bList.Cast<A>().ToList();

You should also read a bit about covariance and contravariance, e.g. on Eric Lippert’s Blog

edited Mar 07 '13 at 21:37

answered Mar 07 '13 at 21:31

MarcinJuraszek

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Eric Lippert's current series on Monads is also relevant (though more advanced): http://ericlippert.com/2013/02/21/monads-part-one/ – Pieter Geerkens Mar 07 '13 at 21:39

score 3 · Accepted Answer · answered Mar 07 '13 at 21:56

3

A list of tigers cannot be used as a list of animals, because you can put a turtle into a list of animals but not into a list of tigers.

In C# 4 and 5 this is legal if you use IEnumerable<T> instead of List<T> because there is no way to put a turtle into a sequence of animals. So you can say:

List<B> myList = new List<B>();
IEnumerable<A> myUpcastedList = myList; // legal

answered Mar 07 '13 at 21:56

Eric Lippert

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I used IEnumerable once when creating a custom container and wanted to be able to "foreach" it. I dont understand what it's suposed to do here – Titan Mar 07 '13 at 23:43
@Titan: An "enumerable" is a sequence of items; that's why you can "foreach" it. If you want to treat a list of tigers as a sequence of animals, that's legal. If you want to treat a list of tigers as a list of animals, that's illegal because if it were legal then you could insert a turtle into the list of animals, but it is really a list of tigers. Since there is no "insert" method on `IEnumerable`, we can make it legal. – Eric Lippert Mar 08 '13 at 00:03

score 1 · Answer 3 · answered Mar 07 '13 at 21:30

1

As far as I'm aware this is not possible. An alternative would be to do the following.

using System.Linq;

var myUpcastedList = myList.Cast<A>().ToList();

answered Mar 07 '13 at 21:30

Peter Karlsson

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1

`Cast` return `IEnumerable `, not `List ` – MarcinJuraszek Mar 07 '13 at 21:31

score 1 · Answer 4 · answered Mar 07 '13 at 21:32

1

I was able to:

List<A> myUpcastedList = myList.ToList<A>();

this makes a copy of the list though... not sure that's what you intended

answered Mar 07 '13 at 21:32

David Hope

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score 1 · Answer 5 · edited May 23 '17 at 10:32

1

You can use this question maybe useful for you

List<A> testme = new List<B>().OfType<A>().ToList();

edited May 23 '17 at 10:32

Community

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1

answered Mar 07 '13 at 21:34

So you make a new list based on the first list, without using the first list? – antonijn Mar 07 '13 at 21:35

inheritance and container in C#

5 Answers5