Co- and contra-variance (Lucian Wischik)

re: Co- and contra-variance (Lucian Wischik)

Xenon Headlights — Wed, 08 Jun 2011 13:43:48 GMT

ok i will check the blog link, thanks

Nouveautés de Visual Basic 2010 [VB10] : Générique Variance, Covariance, Contravariance

RedoBlog - The .NET Gentleman !!! — Sat, 07 Feb 2009 12:33:13 GMT

Nous voici quasiment au terme des nouveautés de Visual Basic 2010 (VB10). Abordons cette fois-ci la notion

VB 2010 Unveiled at PDC 2008! (Lisa Feigenbaum)

VB Feeds — Mon, 03 Nov 2008 05:13:12 GMT

Last week at the Professional Developers Conference , we made a number of very exciting announcements

VB 2010 Unveiled at PDC 2008! (Lisa Feigenbaum)

The Visual Basic Team — Mon, 03 Nov 2008 03:19:21 GMT

Last week at the Professional Developers Conference , we made a number of very exciting announcements

Generic Variance Part1 : Do you really need it ?

VB Feeds — Tue, 21 Oct 2008 10:10:56 GMT

Lucian has kicked off the conversation on generic variance in VB , so I thought I’d write a few posts

re: Co- and contra-variance (Lucian Wischik)

int19h — Mon, 11 Aug 2008 11:09:58 GMT

It seems to me that declaration-site variance (as implemented in the CLR, and now proposed for C# and VB.NET) is less useful than call-site variance (as implemented in Java using wildcards). Here's why: stop for a moment and think, how many of BCL generic classes could actually be converted to use this feature? There's IEnumerable<T>, alright - but what else? ICollection<T> is right out, because it is IEnumerable<T> (and hence covariant), but it also takes T as an argument for Add() (which is contravariant) - the intersection of those is invariant. The same goes for all interfaces further extending ICollection<T>, such as IList<T>.

On the other hand, with wildcards, I can deal with ICollection<T> either in covariant or contravariant fashion: for example, only using GetEnumerator() and Count - a typical real-world scenario - is covariant; only using Add() is contravariant. You can do this with declaration-site variance too, but you have to split every interface into its covariant and contravariant parts, and I doubt the BCL team would do it at this point; besides, it relies upon interface designers to do the right thing, and gives the user no recourse if they don't.

Generic Variance Part1 : Do you really need it ?

@ Head — Mon, 11 Aug 2008 06:21:28 GMT

Lucian has kicked off the conversation on generic variance in VB , so I thought I’d write a few posts

re: Co- and contra-variance (Lucian Wischik)

Rik Hemsley — Fri, 08 Aug 2008 11:14:00 GMT

Damien Guard came up with a solution to this:

http://damieng.com/blog/2006/05/24/ienumerable_t_to_ienumerable_subclassoft

re: Co- and contra-variance (Lucian Wischik)

Jeff Burt — Fri, 08 Aug 2008 08:51:25 GMT

The easy way to do this at the moment is the following:

<System.Runtime.CompilerServices.Extension()> _

Public Function ToIEnumerable(Of T)(ByVal args As IEnumerable) As IEnumerable(Of T)

Return (From item As Object In args Select DirectCast(item, T))

End Function

Quick and simple. Your original code would just change to the following:

Dim y = Expression.Call(instance, method, args.ToIEnumerable(Of Expression))

car insurance » Co- and contra-variance (Lucian Wischik)

car insurance » Co- and contra-variance (Lucian Wischik) — Fri, 08 Aug 2008 00:55:23 GMT

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