C# – Ordered Enumeration: IEnumerable or Array?

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Typical context: I make an extension method for a collection which considers that the elements are ordered. The function starts at the beginning, at index 0, and the order has significance. Examples: Grouping by sequence or Indexes of an item.

However, I'm always stumped as to what to extend: IEnumerable<T> or T[]. My rationale was clarity of purpose: An array has a notion of ordering, whereas an IEnumerable is implemented by many generic collections, not all of which have a notion of order:

Dictionary – unordered
HashSet – unordered
LinkedList – ordered
List – ordered
Queue – ordered
SortedDictionary – sorted (not original order)
SortedList – sorted (not original order)
SortedSet – sorted (not original order)
Stack – reversed

As well as any other implementation which might or might not be ordered.

Also, I'm not certain if the enumerator is reset if an enumeration is not completed, so if that's a worry, then who knows at what point the enumeration would start? Enumerating an array would always start at the beginning.

So to me, it makes more sense to extend a T[]. But am I correct in thinking that? Am I worrying too much? What's the proper approach for ensuring "ordered" enumeration?

Best Answer

I'm not certain if the enumerator is reset if an enumeration is not completed, so if that's a worry, then who knows at what point the enumeration would start? Enumerating an array would always start at the beginning.

These two sentences make me think that you have deep misunderstandings about how the enumerable pattern works. Can you explain why you think that an abandoned enumerator has anything whatsoever to do with a later enumeration?

An enumerable is a device which produces enumerators. If an enumerator is abandoned, that does not in any way affect the enumerable. If I sell books, and Bob buys a book and only reads it halfway, that doesn't mean that when I sell a different copy of the book to Alice, she has to start reading where Bob left off.

I'm always stumped as to what to extend: IEnumerable<T> or T[]. My rationale was clarity of purpose: An array has a notion of ordering, whereas an IEnumerable is implemented by many generic collections, not all of which have a notion of order:

Does your extension method need to (1) access the collection out of order? (2) write to the collection?

If so, then extend IList<T> If not, extend IEnumerable<T>.

What about: (3) pass the collection on to a method that expects an array?

Then extend array.

Your question is basically "I don't know whether to extend Animal or Giraffe". If your extension method is not specific to Giraffes, extend Animal. If your extension method is not specific to arrays, extend all lists. If it is not specific to lists, extend all sequences. If it is not specific to sequences, odds are good that an extension method is the wrong mechanism for you; I recommend against extending all objects.

Related Solutions

C# – Detecting IEnumerable State Machines

Your question, as I understand it, seems to be based on an incorrect premise. Let me see if I can reconstruct the reasoning:

The linked-to article describes how automatically-generated sequences exhibit a "lazy" behaviour, and shows how this can lead to a counter-intuitive result.
Therefore I can detect whether a given instance of IEnumerable is going to exhibit this lazy behaviour by checking to see if it is automatically generated.
How do I do that?

The problem is that the second premise is false. Even if you could detect whether or not a given IEnumerable was the result of an iterator block transformation (and yes, there are ways to do that) it wouldn't help because the assumption is wrong. Let's illustrate why.

class M { public int P { get; set; } }
class C
{
  public static IEnumerable<M> S1()
  {
    for (int i = 0; i < 3; ++i) 
      yield return new M { P = i };
  }

  private static M[] ems = new M[] 
  { new M { P = 0 }, new M { P = 1 }, new M { P = 2 } };
  public static IEnumerable<M> S2()
  {
    for (int i = 0; i < 3; ++i)
      yield return ems[i];
  }

  public static IEnumerable<M> S3()
  {
    return new M[] 
    { new M { P = 0 }, new M { P = 1 }, new M { P = 2 } };
  }

  private class X : IEnumerable<M>
  {
    public IEnumerator<X> GetEnumerator()
    {
      return new XEnum();
    }
    // Omitted: non generic version
    private class XEnum : IEnumerator<X>
    {
      int i = 0;
      M current;
      public bool MoveNext()
      {
        current = new M() { P = i; }
        i += 1;
        return true;
      }
      public M Current { get { return current; } }
      // Omitted: other stuff.
    }
  }

  public static IEnumerable<M> S4()
  {
    return new X();
  }

  public static void Add100(IEnumerable<M> items)
  {
    foreach(M item in items) item.P += 100;
  }
}

All right, we have four methods. S1 and S2 are automatically generated sequences; S3 and S4 are manually generated sequences. Now suppose we have:

var items = C.Sn(); // S1, S2, S3, S4
S.Add100(items);
Console.WriteLine(items.First().P);

The result for S1 and S4 will be 0; every time you enumerate the sequence, you get a fresh reference to an M created. The result for S2 and S3 will be 100; every time you enumerate the sequence, you get the same reference to M you got the last time. Whether the sequence code is automatically generated or not is orthogonal to the question of whether the objects enumerated have referential identity or not. Those two properties -- automatic generation and referential identity -- actually have nothing to do with each other. The article you linked to conflates them somewhat.

Unless a sequence provider is documented as always proffering up objects that have referential identity, it is unwise to assume that it does so.

Design – Count true array elements

This seems like a natural use for a fold to me

i.e. something like this in pseudo functional code

let countiftrue bs = fold (fun acc b -> if b then acc + 1 else acc) 0 bs

better yet have it as a higher order function that accepts a predicate to make it even more useful

let countif p xs = fold (fun acc x -> if p x then acc + 1 else acc) 0 xs

Best Answer

Related Solutions

C# – Detecting IEnumerable State Machines

Design – Count true array elements

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