I want to group an EntityCollection and do a AggregateFunction.Sum on a particular field. Is this possible or must I use a TypedViewList?

Either you loop inside the colection and calculate the sum yourself, or use a TypedList, TypedView or a dynamic List instead of the EntityCollection.

Walaa wrote:

Either you loop inside the colection and calculate the sum yourself

Looping through a collection with a size off a 1000 entities isn't very efficient. Is it? SQL is must better in aggregating decimals. So I'll use a typed list.

Walaa was referring to the fact that you can't fetch an entity collection with an aggregate. So you have to use different ways to get the aggregated value if you already have the collection / want to use the collection, e.g. manually traverse the list. A typed list is one way to get the value, you can also look into a scalar query or a dynamic list.