declaration-site extension methods possibly dangerous

[Fatih Coskun]

Rémi Forax wrote:
> Fatih Coskun a écrit :
>> declaration-site extension methods can be easily confused with 
>> abstract methods of an interface. I can imagine many developers 
>> trying to override extension methods in their subclasses in order to 
>> provide different implementations. But this will not work. 
>> declaration-site extension methods cannot be overriden, they are 
>> hidden instead.
>
> Why ?, The proposal doesn't say anything about that.
 From the proposal:
"An extension method in an interface is hidden in a class implementing 
that interface if the class defines a static or instance method with the 
same signature."

The proposal states that extension methods can be hidden, i.e. they 
cannot be overriden. It couldn't be otherwise, because extension methods 
are in fact just static methods. Static methods are always statically 
bound (via the static type of a variable), this means it is never 
possible to override them in a dynamic manner like instance methods can be.
> I don't understand why use-site extension don't suffer from the same 
> disease.
use-site extension methods suffer from the same consequence in that they 
cannot be overriden in a class implementing the interface. But they do 
not suffer in the same way in that one can be easily confused about 
this. Because use-site extension methods are not defined in the 
interface, one will not be tempted to override them. The developer will 
not be tempted to do something that can't be done.

Fatih