Follow up on enabling a faster foreach

Wow – several hours of performance
training for the CLR team today… One thing that came up was how to enabling
writing fast foreach code by doing a better job of implementing IEnumerable…
Apparently a while back there was
some code that need to be super fast (think default winproc here) that did
nested foreach’s on a couple of ArrayLists… "urn:schemas-microsoft-com:office:office" />

Some thing
like:

ArrayList
l1 = … //put n items in

ArrayList
l2 =…

foreach
(object o in l1){

   foreach (object o in l2)
{

   }

}

As you know, the way foreach works
is that it allocates an Enumerator… so that means the inner loop is causes n
enumerators to be allocated. Not a
big problem for a normal app, but for this often accessed part of the system it
meant over a 1/3 of the memory usage for a sample app was Enumerators. Wow.

We certainly learned our lesson in
the ArrayList class. If we had it
to do over we’d follow the guidelines I
outlined earlier and in addition make sure the Current property is
inlineable. Oh wait, we DO have it
to do over again.. sort of… In Whidbey will be adding Generic versions of the
collections, and you can bet we addressed this problem
there!