Alex Rivera | Logout

Why does C#/CLR not support method override co/contra-variance?

Asked 2009-05-07T21:29:57.700
13

There are quite a few questions & answers about hacking around the limitation of C# not allowing method return (and argument) types to be changed to compatible types on overrides, but why does this limitation exist, either in the C# compiler or in the CLR? As I an see, there is nothing that could break if co/contra-variance was allowed, so what is the reasoning behind it?

A similar question could be asked for widening access parameters - eg overriding a protected internal method with a public method (something which Java supports, IIRC)

c# .net clr overriding covariance
Edit
Report

1 Answer

-1

To expand on Joel's answer - the CLR has supported limited variance for a long time, but the C# compiler doesn't use them until 4.0 with the new "in" and "out" modifiers on generic interfaces and delegates. The reasons are complicated, and I would get into a mess trying to explain, but it isn't as simple as it seems.

Re making a "protected internal" method into a "public" method; you can do this with method hiding:

public new void Foo(...) { base.Foo(...); }

(as long as the arguments etc are all also public) - any use?

answered 2009-05-07T21:40:36.597

Your Answer