[__DynamicallyInvokable]
    public static int Count<TSource>(this IEnumerable<TSource> source)
    {
      if (source == null)
        throw Error.ArgumentNull("source");
      ICollection<TSource> collection1 = source as ICollection<TSource>;
      if (collection1 != null)
        return collection1.Count;
      ICollection collection2 = source as ICollection;
      if (collection2 != null)
        return collection2.Count;
      int num = 0;
      using (IEnumerator<TSource> enumerator = source.GetEnumerator())
      {
        while (enumerator.MoveNext())
          checked { ++num; }
      }
      return num;
    }