Obvious Extension Methods: IEnumerable Cache

So you don’t want to do a ToList(), but you don’t want to enumerate your IEnumerable<T> more than once? How about an extension method called “Cache” that Caches the output as you go.

13. public static class Extensions
14. {
15.     public static CachedEnumerable<T> Cache<T>(this IEnumerable<T> enumerable)
16.     {
17.         return new CachedEnumerable<T>(enumerable);
18.     }
19. }
21. public class CachedEnumerable<T> : IEnumerable<T>
22. {
23.     IEnumerator<T> _originalEnumerator;
24.     readonly IEnumerable<T> _originalEnumerable;
25.     readonly List<T> _cache = new List<T>();
27.     public CachedEnumerable(IEnumerable<T> enumerable)
28.     {
29.         _originalEnumerable = enumerable;
30.     }
31.     public IEnumerator<T> GetEnumerator()
32.     {
33.         foreach (var t in _cache)
34.         {
35.             yield return t;
36.         }
37.         if (_originalEnumerator == null)
38.         {
39.             _originalEnumerator = _originalEnumerable.GetEnumerator();
40.         }
41.         while (_originalEnumerator.MoveNext())
42.         {
43.             _cache.Add(_originalEnumerator.Current);
44.             yield return _originalEnumerator.Current;
45.         }
46.     }
48.     IEnumerator IEnumerable.GetEnumerator()
49.     {
50.         return GetEnumerator();
51.     }
52. }

That’s it. If you want to see it in action, open LINQPad, select Language: “C# Program”, delete everything, paste in the above, then finally, paste in the below:

void Main()
{
  var x = GetNumbers().Cache();

  "TAKE 2.".Dump();
  x.Take(2).Dump("TWO:");

  "Get them all.".Dump();
  x.Dump("ALL:");

  "Get them all again.".Dump();
  x.Dump("ALL:");
}

public IEnumerable<T> GetNumbers()
{
  yield return 1.Dump("Numbers is hard");
  Thread.Sleep(500);
  yield return 2.Dump("Numbers is hard");
  Thread.Sleep(500);
  yield return 3.Dump("Numbers is hard");
  Thread.Sleep(500);
  yield return 4.Dump("Numbers is hard");
  Thread.Sleep(500);
  yield return 5.Dump("Numbers is hard");
  Thread.Sleep(500);
  yield return 6.Dump("Numbers is hard");
}

There.