Eager vs lazy

gistfile1.txt

    // Eagerly performs nullchecks.
    public static IEnumerable<TResult> ZipNew<TFirst, TSecond, TResult>(
        this IEnumerable<TFirst> first,
        IEnumerable<TSecond> second,
        Func<TFirst, TSecond, TResult> resultSelector) {
      if(first == null) throw new ArgumentNullException(nameof(first));
      if(second == null) throw new ArgumentNullException(nameof(second));
      if(resultSelector == null) throw new ArgumentNullException(nameof(resultSelector));
      return ZipNewInternal(first, second, resultSelector);
    }

    // Uses yield return, so entire method is only evaluated when iteration happens.
    private static IEnumerable<TResult> ZipNewInternal<TFirst, TSecond, TResult>(
        IEnumerable<TFirst> first,
        IEnumerable<TSecond> second,
        Func<TFirst, TSecond, TResult> resultSelector) {
      using(ZipEnumerator<TFirst, TSecond, TResult> zipEnumerator = new ZipEnumerator<TFirst, TSecond, TResult>(first, second, resultSelector)) {
        while(zipEnumerator.MoveNext()) {
          yield return zipEnumerator.Current;
        }
      }
    }