Preserving order with LINQ

Question

I use LINQ to Objects instructions on an ordered array. Which operations shouldn't I do to be sure the order of the array is not changed?

Answer 1

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.

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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;
    }