[soc88]

You pretty much point to solutions which don't solve the problem mentioned in the blog post at all and seeing how C# extension methods basically reintroduced instanceOf checks as an implementation necessity I'm pretty sure that extension methods are the wrong way to go.

[rwallace]

I don't remember C# extension methods being particularly prone to incur instanceOf checks, but I could be missing something; can you expand on this?

[soc88]

Have a look at how stuff like Count is implemented. It basically does instanceOf checks for ICollection to figure out if the underlying type supports a better way to compute the result (instead of iterating trough the whole collection).

This is completely non-extensible and people have to pay the price for this syntactic sugar (e. g. extension methods not discoverable with Reflection).

Other languages have a much cleaner approach: traits (in languages like Scala) and default methods (in Java) both solve the problem in a more straightforward and correct way.