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Fix tiny regression in Enumerable.ElementAt #99437

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merged 1 commit into from
Mar 8, 2024
Merged

Fix tiny regression in Enumerable.ElementAt #99437

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4 changes: 1 addition & 3 deletions src/libraries/System.Linq/src/System/Linq/AppendPrepend.SpeedOpt.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -171,9 +171,7 @@ public override int GetCount(bool onlyIfCheap)
}

index--;
return
_source is Iterator<TSource> iterator ? iterator.TryGetElementAt(index, out found) :
TryGetElementAtNonIterator(_source, index, out found);
return _source.TryGetElementAt(index, out found);
}

return base.TryGetElementAt(index, out found);
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47 changes: 27 additions & 20 deletions src/libraries/System.Linq/src/System/Linq/ElementAt.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -16,7 +16,18 @@ public static TSource ElementAt<TSource>(this IEnumerable<TSource> source, int i
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
}

TSource? element = TryGetElementAt(source, index, out bool found, guardIListLength: false);
if (source is IList<TSource> list)
{
return list[index];
}

bool found;
TSource? element =
#if !OPTIMIZE_FOR_SIZE
source is Iterator<TSource> iterator ? iterator.TryGetElementAt(index, out found) :
#endif
TryGetElementAtNonIterator(source, index, out found);

if (!found)
{
ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index);
Expand Down Expand Up @@ -102,31 +113,27 @@ public static TSource ElementAt<TSource>(this IEnumerable<TSource> source, Index
return element;
}

private static TSource? TryGetElementAt<TSource>(this IEnumerable<TSource> source, int index, out bool found, bool guardIListLength = true) =>
private static TSource? TryGetElementAt<TSource>(this IEnumerable<TSource> source, int index, out bool found)
{
if (source is IList<TSource> list)
{
return (found = (uint)index < (uint)list.Count) ?
list[index] :
default;
}

return
#if !OPTIMIZE_FOR_SIZE
source is Iterator<TSource> iterator ? iterator.TryGetElementAt(index, out found) :
source is Iterator<TSource> iterator ? iterator.TryGetElementAt(index, out found) :
#endif
TryGetElementAtNonIterator(source, index, out found, guardIListLength);
TryGetElementAtNonIterator(source, index, out found);
}

private static TSource? TryGetElementAtNonIterator<TSource>(IEnumerable<TSource> source, int index, out bool found, bool guardIListLength = true)
private static TSource? TryGetElementAtNonIterator<TSource>(IEnumerable<TSource> source, int index, out bool found)
{
Debug.Assert(source is not null);

if (source is IList<TSource> list)
{
// Historically, ElementAt would simply delegate to IList[int] without first checking the bounds.
// That in turn meant that whatever exception the IList[int] throws for out-of-bounds access would
// propagate, e.g. ImmutableArray throws IndexOutOfRangeException whereas List throws ArgumentOutOfRangeException.
// Other uses of this, though, do need to guard, such as ElementAtOrDefault and all the various
// internal TryGetElementAt helpers. So, we have a guardIListLength parameter to allow the caller
// to specify whether to guard or not.
if (!guardIListLength || (uint)index < (uint)list.Count)
{
found = true;
return list[index];
}
}
else if (index >= 0)
if (index >= 0)
{
using IEnumerator<TSource> e = source.GetEnumerator();
while (e.MoveNext())
Expand Down