public static TSource ElementAt <TSource>(this IEnumerable <TSource> source, int index)
{
IList <TSource> list = source as IList <TSource>;
if (list != null)
{
return(list[index]);
}
if (index < 0)
{
throw new ArgumentOutOfRangeException(nameof(index));
}
using (IEnumerator <TSource> iterator = source.GetEnumerator())
{
Tuple <bool, TSource> result = iterator.ElementAt(index, -1);
if (result.Item1)
{
return(result.Item2);
}
}
throw new ArgumentOutOfRangeException(nameof(index));
}
private static Tuple <bool, TSource> ElementAt <TSource>(
this IEnumerator <TSource> iterator,
int targetIndex,
int lastIndex)
{
if (iterator.MoveNext())
{
int currentIndex = lastIndex + 1;
return(currentIndex == targetIndex
? Tuple.Create(true, iterator.Current)
: iterator.ElementAt(targetIndex, currentIndex));
}
return(Tuple.Create(false, default(TSource)));
}
public static TSource ElementAtOrDefault <TSource>(this IEnumerable <TSource> source, int index)
{
if (index >= 0)
{
IList <TSource> list = source as IList <TSource>;
if (list != null)
{
if (index < list.Count)
{
return(list[index]);
}
}
else
{
using (IEnumerator <TSource> iterator = source.GetEnumerator())
{
return(iterator.ElementAt(index, -1).Item2);
}
}
}
return(default(TSource));
}
