In C#, why can't a List<string> object be stored in a List<object> variable

Question

It seems that a List object cannot be stored in a List variable in C#, and can't even be explicitly cast that way.

List<string> sl = new List<string>();
List<object> ol;
ol = sl;

results in Cannot implicitly convert type ‘System.Collections.Generic.List’ to ‘System.Collections.Generic.List’

And then...

List<string> sl = new List<string>();
List<object> ol;
ol = (List<object>)sl;

results in Cannot convert type ‘System.Collections.Generic.List’ to ‘System.Collections.Generic.List’

Of course, you can do it by pulling everything out of the string list and putting it back in one at a time, but it is a rather convoluted solution.

Answer 1

The reason is that a generic class like List<> is, for most purposes, treated externally as a normal class. e.g. when you say List<string>() the compiler says ListString() (which contains strings). [Technical folk: this is an extremely plain-English-ified version of what's going on]

Consequently, obviously the compiler can't be smart enough to convert a ListString to a ListObject by casting the items of its internal collection.

That's why there's extension methods for IEnumerable like Convert() that allow you to easily supply conversion for the items stored inside a collection, which could be as simple as casting from one to another.

Answer 2

Think of it this way, if you were to do such a cast, and then add an object of type Foo to the list, the list of strings is no longer consistent. If you were to iterate the first reference, you would get a class cast exception because once you hit the Foo instance, the Foo could not be converted to string!

As a side note, I think it would be more significant whether or not you can do the reverse cast:

List<object> ol = new List<object>();
List<string> sl;
sl = (List<string>)ol;

I haven't used C# in a while, so I don't know if that is legal, but that sort of cast is actually (potentially) useful. In this case, you are going from a more general class (object) to a more specific class (string) that extends from the general one. In this way, if you add to the list of strings, you are not violating the list of objects.

Does anybody know or can test if such a cast is legal in C#?

Answer 3

Mike - I believe contravariance isn't allowed in C# either - See http://blogs.msdn.com/rmbyers/archive/2005/02/16/375079.aspx for some more info.

Answer 4

This has a lot to do with covariance, e.g., generic types are considered as parameters, and if the parameters do not resolve properly to a more specific type then the operation fails. The implication of such is that you really cannot cast to a more general type like object. And as stated by Rex, the List object won't convert each object for you.

You might want to try the ff code instead:

List<string> sl = new List<string>();
//populate sl
List<object> ol = new List<object>(sl);

or:

List<object> ol = new List<object>();
ol.AddRange(sl);

ol will (theoretically) copy all the contents of sl without problems.

Answer 5

If you're using .NET 3.5 have a look at the Enumerable.Cast method. It's an extension method so you can call it directly on the List.

List<string> sl = new List<string>();
IEnumerable<object> ol;
ol = sl.Cast<object>();

It's not exactly what you asked for but should do the trick.

Edit: As noted by Zooba, you can then call ol.ToList() to get a List

Answer 6

You cannot cast between generic types with different type parameters. Specialized generic types don't form part of the same inheritance tree and so are unrelated types.

To do this pre-NET 3.5:

List<string> sl = new List<string>();
// Add strings to sl

List<object> ol = new List<object>();

foreach(string s in sl)
{
    ol.Add((object)s);  // The cast is performed implicitly even if omitted
}

Using Linq:

var sl = new List<string>();
// Add strings to sl

var ol = new List<object>(sl.Cast<object>());

// OR
var ol = sl.Cast<object>().ToList();

// OR (note that the cast to object here is required)
var ol = sl.Select(s => (object)s).ToList();

Answer 7

That's actually so that you don't try to put any odd "object" in your "ol" list variant (as List<object> would seem to allow) - because your code would crash then (because the list really is List<string> and will only accept String type objects). That's why you can't cast your variable to a more general specification.

On Java it's the other way around, you don't have generics, and instead everything is List of object at runtime, and you really can stuff any strange object in your supposedly-strictly typed List. Search for "Reified generics" to see a wider discussion of java's problem...

Answer 8

Such covariance on generics is not supported, but you can actually do this with arrays:

object[] a = new string[] {"spam", "eggs"};

C# performs runtime checks to prevent you from putting, say, an int into a.

Answer 9

Yes, you can, from .NET 3.5:

List<string> sl = new List<string>();
List<object> ol = sl.Cast<object>().ToList();

Answer 10

I think that this (contravariance) will actually be supported in C# 4.0. http://blogs.msdn.com/charlie/archive/2008/10/27/linq-farm-covariance-and-contravariance-in-visual-studio-2010.aspx

Answer 11

Here is another pre-.NET 3.5 solution for any IList whose contents can be cast implicitly.

public IList<B> ConvertIList<D, B>(IList<D> list) where D : B
{
    List<B> newList = new List<B>();

    foreach (D item in list)
    {
        newList.Add(item);
    }

    return newList;
}

(Based on Zooba's example)

Answer 12

I have a:

private List<Leerling> Leerlingen = new List<Leerling>();

And i was going to fill it with data collected in an List What finally worked for me was this one:

Leerlingen = (List<Leerling>)_DeserialiseerLeerlingen._TeSerialiserenObjecten.Cast<Leerling>();

.Cast it to the type you want to get an IEnumerable from that type, then typecast the IEnemuerable to the List<> you want.