Regarding the collections implementing this[int] and assuming the collection won't change during the enumeration, does the foreach (var item in list) loop produce the same sequence as for (var i = 0; i < list.Count; ++i) anytime?
This means, when I need the ascending order by index, could I use foreach or is just simply safer to use for? Or it just depends on the curren collection implementation and migh vary or change in time?
Generally speaking, yes, but strictly spoken: no. It really depends on the implementation.
Usually with for you would use the this indexer properties. foreach uses GetEnumerator() to get the enumerator that iterates over the collection. Depending on the implementation the enumerator might yield another result than the for.
The implied logic of a list is that is has a specific order, and when implementing IList you may state is it save to assume that the order of both the indexer properties as the enumerator are the same.