Generic Variance in C# 4.0
Generic Variance in C# 4.0 has been implemented in such a way that it\'s possible to write the following without an exception (which is what would happen in C# 3.0):
-
No, your example wouldn't work for three reasons:
- Classes (such as
List) are invariant; only delegates and interfaces are variant - For variance to work, the interface has to only use the type parameter in one direction (in for contravariance, out for covariance)
- Value types aren't supported as type arguments for variance - so there's no converstion from
IEnumerabletoIEnumerablefor example
(The code fails to compile in both C# 3.0 and 4.0 - there's no exception.)
So this would work:
IEnumerablestrings = new List (); IEnumerable The CLR just uses the reference, unchanged - no new objects are created. So if you called
objects.GetType()you'd still getList.I believe it wasn't introduced earlier because the language designers still had to work out the details of how to expose it - it's been in the CLR since v2.
The benefits are the same as other times where you want to be able to use one type as another. To use the same example I used last Saturday, if you've got something implements
IComparerto compare shapes by area, it's crazy that you can't use that to sort aList- if it can compare any two shapes, it can certainly compare any two circles. As of C# 4, there'd be a contravariant conversion fromIComparertoIComparerso you could callcircles.Sort(areaComparer). - Classes (such as
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