Does .GroupBy() guarantee order in its groupings?

Question

Say I have an (ordered) sequence of animals:

Eagle
Elephant
Tarantula
Terrapin
Tiger

and I group by first letter:

Animals.GroupBy(animal => animal.First())

will the elements of the IGroupings in the resulting sequence be in the same order as the input sequence?

Answer 1

Yes, they will be: GroupBy (MSDN).

The IGrouping<TKey, TElement> objects are yielded in an order based on the order of the elements in source that produced the first key of each IGrouping<TKey, TElement>. Elements in a grouping are yielded in the order they appear in source.

Answer 2

Quote from the MSDN page for GroupBy:

The IGrouping<TKey, TElement> objects are yielded in an order based on the order of the elements in source that produced the first key of each IGrouping<TKey, TElement>. Elements in a grouping are yielded in the order they appear in source.

So your example will result in:

1. Group 1

   • Eagle
   • Elephant

2. Group 2

   • Tarantula
   • Terrapin
   • Tiger

Of course that only applies to the IEnumerable<T> implementation. The IQueryable<T> implementation has no such guarantee.