Optimize LINQ Count() > X [duplicate]

Question

Question: given IEnumerable<>, how to check what sequence contains more than x items?

---

MCVE:

static void Main(string[] args)
{
    var test = Test().Where(o => o > 2 && o < 6); // ToList()
    if (test.Count() > 1) // how to optimize this?
        foreach (var t in test) // consumer
            Console.WriteLine(t);
}

static IEnumerable<int> Test()
{
    for (int i = 0; i < 10; i++)
        yield return i;
}

The problem here is what Count() will run complete sequence and that's 1E6+ items (ToList() is also bad idea). I am also not allowed to change consumer code (it's a method accepting complete sequence).

Answer 1

In case of large test collection (when Count() is expensive) you can try a typical trick:

if (test.Skip(1).Any()) 

In general case test.Count() > x can be rewritten into

if (test.Skip(x).Any()) 

Edit: you may want to hide such a tricks in a method, say EnsureCount:

  public static partial class EnumerableExtensions {
    public static IEnumerable<T> EnsureCount<T>(this IEnumerable<T> source, int count) {
      if (null == source)
        throw new ArgumentNullException("source");

      if (count <= 0)
        foreach (var item in source)
          yield return item;
      else {
        List<T> buffer = new List<T>(count);

        foreach (var item in source) {
          if (buffer == null)
            yield return item;
          else {
            buffer.Add(item);

            if (buffer.Count >= count) {
              foreach (var x in buffer)
                yield return x;

              buffer = null;
            }
          }
        }
      }
    }
  } 

and so your code will be

  var test = Test()
    .Where(o => o > 2 && o < 6)
    .EnsureCount(2); // <- Count() > 1, so at least 2 items

  foreach (var t in test)
    Console.WriteLine(t);