If you wanted to implement your own "Count" you would implement ICollection<T> which the IEnumerable<T> extension method would call into.
IEnumerator GetEnumerator()
method most of the real "meat" of ienumerable is in the static System.Linq.Enumerable class. You see c# doesn't have multiple inheritence or scala like traits or type classes but it does have a cool compiler hack called Extension methods(http://msdn.microsoft.com/en-us/library/vstudio/bb383977.asp...). You take a static class with static helper functions like you might make in java and you add a this keyword to the first argument then you can call it with a syntax that makes it look like it was a method on that class. For example something like
public static class Enumerable
{
public static int Count<TSource>(this IEnumerable<TSource> source
{
int i=0;
for(TSource t in source)
{
i++;
}
return i;
)
}
I believe in scala you can use traits in a similar manner for default implementation and specialize them for collections but traits are more complicated/powerfull feature.
As for your Scala remark, extension methods in Scala are done via implicits.
I should have mentioned that I know even less about scala then c#. I/m not quite sure how implicits are implemented/optimized but I'm pretty sure you can have inheritence/overiding traits as implicits.