Cast generic list form DerivedClass to BaseClass

Question

I've these two classes

public class A {}
public class B : A {}

And the cast from class A to class B works fine.

B testB1 = new B();
A testA1 = testB1;
B testB2 = (B)testA1; //this works

But: Why is this cast not working?

List<B> testB1List = new List<B>();
List<A> testA1List = ((IEnumerable<A>)testB1List).ToList();
List<B> testB2List = ((IEnumerable<B>)testA1List).ToList(); //not working

The solution is:

List<B> testB1List = new List<B>();
List<A> testA1List = ((IEnumerable<A>)testB1List).ToList();
List<B> testB2List = testA1List.Cast<B>().ToList();

But why is it like this?

@jason.kaisersmith not really. I don't understand the different between IEnumerable cast (not working) and testA1ListCast(). — lwanda, Mar 29 '20 at 14:45
As always, it would help if you'd specify exactly what happened, instead of just "not working". But fundamentally, a `List` *is not* an `IEnumerable`, even if it happens to contain references to `B` elements. — Jon Skeet, Mar 29 '20 at 15:27

JMP · Accepted Answer · 2020-03-29T15:40:37.413

3

Well the Cast extension method casts each member of the list to the specified type so in your example it can then be assigned as a List<B> because all list members have been cast to B.

But in the first example, you are casting the IEnumerable itself, not the members of the list:

(IEnumerable<B>)testA1List

So it fails because it is trying to cast List<A> to IEnumerable<B>.

edited Mar 29 '20 at 15:40

answered Mar 29 '20 at 15:24

JMP

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Thanks for the explanation. Now I get it. – lwanda Mar 29 '20 at 15:38

Cast generic list form DerivedClass to BaseClass

1 Answers1