explicitly cast List<List<object>>

[colson]

Hi,

If I have a class A, and a List<List<object>> containing instances of
A. How do I explicitly cast List<List<object>> as List<List<A>>?

[Joanna Carter [TeamB]]

"colson" <ch******@gmail.com> a écrit dans le message de news:
11**********************@p79g2000cwp.googlegroups. com...

| If I have a class A, and a List<List<object>> containing instances of
| A. How do I explicitly cast List<List<object>> as List<List<A>>?

You can't. Do a Google search for covariance, this has been discussed many
times n these groups.

List<A> is not derived from List<object>, even though A is derived from
object; otherwise you could also add untyped objects to a List<A>, thus
defeating the type safety of generic classes.

BTW, why would you use a generic list to hold "object" instances, this
defeats the need for a generic list anyway, you might as well use an
ArrayList. Generic lists are for typesafe lists of instances of the same
type, not heterogeneous lists.

Joanna

--
Joanna Carter [TeamB]
Consultant Software Engineer

[Jon Skeet [C# MVP]]

colson <ch******@gmail.com> wrote:

If I have a class A, and a List<List<object>> containing instances of
A. How do I explicitly cast List<List<object>> as List<List<A>>?

You can't - they're not compatible. They wouldn't even be compatible if
you were dealing with single lists.

--
Jon Skeet - <sk***@pobox.com>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too

[Barry Kelly]

"colson" <ch******@gmail.com> wrote:

If I have a class A, and a List<List<object>> containing instances of
A. How do I explicitly cast List<List<object>> as List<List<A>>?

You cannot. Instantiated generic types have no inheritance relationship.
In particular, where type D descends from base class B, a type
parameterized with D is not a subtype of the same type parameterised
with B.

It doesn't make sense anyway. Consider:

---8<---
class B {}
class D1 : B {}
class D2 : B {}

// ...

List<D1> d = new List<D1>();

List<B> b = d;

b.Add(new D2());
--->8---

Notice how the call to "b.Add(new D2())" breaks type safety: an instance
of D2 was added to a list of D1, but D2 is *not* a D1! This would break
the whole point of the guarantees behind generics: improving development
experience by removing typecasting and improving runtime performance by
removing runtime checks.

-- Barry

--
http://barrkel.blogspot.com/

[Göran Andersson]

colson wrote:

Hi,

If I have a class A, and a List<List<object>> containing instances of
A. How do I explicitly cast List<List<object>> as List<List<A>>?

As already have been made clear by previous posters, you can't.

If you want to access an A object in the list, you have to cast that
specific instance to A:

A a = theList[outerIndex][innerIndex] as A;
if (a != null) {
...
}

Or if you are sure that the specific instance is of the correct class:

A a = (A)(theList[outerIndex][innerIndex]);