Recipes

Performant pagination in C#

Pagination on IEnumerable<> is typically done using .Skip() and .Take(). But if you want more than one page this is going to iterate over the enumerable multiple times. I don't feel like figuring it out, but the performance will definitely be worse than O(n). So here's a way to turn an enumerable into a paged enumerable of enumerables that iterates the original only once.

public static class IEnumerableExtensions
{
  public static IEnumerable<IEnumerable<T>>(this IEnumerable<T> data, int pageSize)
  {
    var enumerator = data.GetEnumerator();

    while(enumerator.MoveNext())
    {
      yield return GetPage(enumerator, pageSize);
    }

    static IEnumerable<T> GetPage(IEnumerator<T> enumerator, int pageSize)
    {
      var count = 0;

      do
      {
        yield return enumerator.Current;
        count++;
      } while (count < pageSize && enumerator.MoveNext());
    }
  }
}