[Mono-list] Generics Conversions

Ricky Clarkson ricky.clarkson at gmail.com
Mon Feb 23 08:10:36 EST 2009

I'll explain this by jumping to Java for a moment.  Java's arrays (but not
its generics) are broken in that they do what you want, which is broken:

Number[] numbers = new Double[1];
numbers[0] = 5;

This causes an ArrayStoreException at runtime.  C#'s arrays and generics are
not broken in this manner, and will give a compile error instead.

A list of strings is not a list of objects, as long as a list is mutable.
C# 4 adds covariance and contravariance.

IEnumerable<object> objects = new List<string>(); will be valid.

List<object> more = new List<string>(); will not be valid.  List will still
be declared as List<T>, and IEnumerable will be interface IEnumerable<out T>
{ stuff }.

Right now I'd use IEnumerable<object> objects = stringList.Select(x =>
(object)x); or if you must have List<>, List<object> objects =
stringList.Select(x => (object)x).ToList();

The advantage of the IEnumerable is that no actual copying is done.

Have fun,

2009/2/23 Sascha Leib <sascha.leib at gmail.com>

> Hello all,
> I noticed one behaviour when using Generics:
>            List<String> StringList = new List<String>();
>            List<Object> ObjectList = (StringList as List<Object>);  //
> Error
> The error message is:
>    Program.cs(14,51): error CS0039: Cannot convert type
> `System.Collections.Generic.List<string>' to
> `System.Collections.Generic.List<object>' via a built-in conversion
> Is that a wanted behaviour? Or am I missing something? This really stops
> me from using more Generics at the moment :-/
> /sascha
> PS: At least it is consistent with how VS handles it...
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