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#?

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

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();

原因是，在大多数情况下，类似的泛型类List<>在外部被视为普通类。例如，当您说List<string>()编译器说ListString()（其中包含字符串）时。[技术人员：这是正在发生的事情的非常简单的英语版本]

因此，显然编译器不能足够聪明地通过转换其内部集合的项目来将 ListString 转换为 ListObject。

这就是为什么有像 Convert() 这样的 IEnumerable 扩展方法，它允许您轻松地为存储在集合中的项目提供转换，这可以像从一个转换到另一个一样简单。

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.

Yes, you can, from .NET 3.5:

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

Mike - I believe contravariance isn't allowed in C# either

See Generic type parameter variance in the CLR for some more info.

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

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...

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.

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)

I have a:

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

And I was going to fill it with data collected in an List<object> 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.

Mm, thanks to previous comments I found two ways to find it out. The first one is getting the string list of elements and then casting it to IEnumerable object list:

IEnumerable<object> ob;
List<string> st = new List<string>();
ob = st.Cast<object>();

And the second one is avoiding the IEnumerable object type, just casting the string to object type and then using the function "toList()" in the same sentence:

List<string> st = new List<string>();
List<object> ob = st.Cast<object>().ToList();

I like more the second way. I hope this helps.

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