I examined the methods of System.Linq.Enumerable, discarding any that returned non-IEnumerable results. I checked the remarks of each to determine how the order of the result would differ from order of the source.
Preserves Order Absolutely. You can map a source element by index to a result element
- AsEnumerable
- Cast
- Concat
- Select
- ToArray
- ToList
Preserves Order. Elements are filtered or added, but not re-ordered.
- Distinct
- Except
- Intersect
- OfType
- Prepend (new in .net 4.7.1)
- Skip
- SkipWhile
- Take
- TakeWhile
- Where
- Zip (new in .net 4)
Destroys Order – we don’t know what order to expect results in.
- ToDictionary
- ToLookup
Redefines Order Explicitly – use these to change the order of the result
- OrderBy
- OrderByDescending
- Reverse
- ThenBy
- ThenByDescending
Redefines Order according to some rules.
- GroupBy – The IGrouping objects are yielded in an order based on the order of the elements in source that produced the first key of each IGrouping. Elements in a grouping are yielded in the order they appear in source.
- GroupJoin – GroupJoin preserves the order of the elements of outer, and for each element of outer, the order of the matching elements from inner.
- Join – preserves the order of the elements of outer, and for each of these elements, the order of the matching elements of inner.
- SelectMany – for each element of source, selector is invoked and a sequence of values is returned.
- Union – When the object returned by this method is enumerated, Union enumerates first and second in that order and yields each element that has not already been yielded.
Edit: I’ve moved Distinct to Preserving order based on this implementation.
private static IEnumerable<TSource> DistinctIterator<TSource>
(IEnumerable<TSource> source, IEqualityComparer<TSource> comparer)
{
Set<TSource> set = new Set<TSource>(comparer);
foreach (TSource element in source)
if (set.Add(element)) yield return element;
}