Comments on: Generic Variance in C# 4.0

By: InterKnowlogy Blogs » Blog Archive » The Evolution of Generics in C# 4.0

Wed, 02 Feb 2011 21:19:48 +0000

[...] there are already some great posts that already explain this in much greater depth and detail: (Generic Variance in C# 4.0 by Discord & Rhyme, What’s the difference between covariance and assignment [...]

By: Oidon

Oidon — Tue, 18 Nov 2008 15:18:58 +0000

As far as I understand, variance support is only being added for generic interfaces and delegates. (Actually, version 2.0 of the CLI has always supported this; C# is just catching up now.) Thus, your “class Foo” should not compile.

By: Buu Nguyen

Buu Nguyen — Fri, 14 Nov 2008 12:57:23 +0000

>>IList{string} strings = new List{string}();
>>IList{object} objects = strings;
In IList, T appears in both output and input positions (i.e. add, T []), therefore IList can’t be covariant on T. In Java, this would be possible with wildcards.

By: Nate Kohari

Nate Kohari — Thu, 06 Nov 2008 14:46:40 +0000

@Sebastian: That’s a good point. The list example comes directly from the official explanation of the new features, but I haven’t tried it myself in the VS2010 CTP. I’d be interested to know…

By: Sebastian Mueller

Sebastian Mueller — Fri, 31 Oct 2008 08:40:56 +0000

Damn, the comment system removed my brackets – replace {} in the below code with the corresponding brackets

If you say that this works:

IList{string} strings = new List{string}();
IList{object} objects = strings;

how is this goingo to work then:

objects.Add(new object());

?
this should work for IList{object} but not for IList{string} !

so I don’t think that IList is going to have an “out” type parameter, rather the GetEnumerable method of IList{T} might become IEnumerable{out T} – though I haven’t read the spec yet and I don’t know if you can actually add the out or in keyword to certain methods/properties to the class parameter.
(BTW: That’s the way Java Generics do it: out T and in T is like “? extends T” and “? super T”)

Other than that, great article and thanks for the link!

/Sebastian

By: Sebastian Mueller

Sebastian Mueller — Fri, 31 Oct 2008 08:37:57 +0000

If you say that this works:

IList strings = new List();
IList objects = strings;

how is this goingo to work then:

objects.Add(new object());

?
this should work for IList but not for IList !

so I don’t think that IList is going to have an “out” type parameter, rather the GetEnumerable method of IList might become IEnumerable – though I haven’t read the spec yet and I don’t know if you can actually add the out or in keyword to certain methods/properties to the class parameter.
(BTW: That’s the way Java Generics do it: out T and in T is like “? extends T” and “? super T”)

Other than that, great article and thanks for the link!

/Sebastian

By: Reflective Perspective - Chris Alcock » The Morning Brew #212

Reflective Perspective - Chris Alcock » The Morning Brew #212 — Thu, 30 Oct 2008 08:32:25 +0000

[...] Generic Variance in C# 4.0 – Nate Kohari looks at the variance of generic types in C#4 [...]