Co- and contra-variance (Lucian Wischik)

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# car insurance » Co- and contra-variance (Lucian Wischik) said on August 7, 2008 5:55 PM:

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# Jeff Burt said on August 8, 2008 1:51 AM:

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))

# Rik Hemsley said on August 8, 2008 4:14 AM:

Damien Guard came up with a solution to this:

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

# @ Head said on August 10, 2008 11:21 PM:

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

# int19h said on August 11, 2008 4:09 AM:

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.

# VB Feeds said on October 21, 2008 3:10 AM:

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

# The Visual Basic Team said on November 2, 2008 7:19 PM:

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

# VB Feeds said on November 2, 2008 9:13 PM:

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

# RedoBlog - The .NET Gentleman !!! said on February 7, 2009 4:33 AM:

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

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