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Improve List performance in IEnumerable.SequenceEqual #67722

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Improve List performance in IEnumerable.SequenceEqual #67722

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8 changes: 5 additions & 3 deletions src/libraries/System.Linq/src/System/Linq/Enumerable.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,7 @@
// The .NET Foundation licenses this file to you under the MIT license.

using System.Collections.Generic;
using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

Expand All @@ -14,12 +15,13 @@ public static partial class Enumerable
/// <summary>Validates that source is not null and then tries to extract a span from the source.</summary>
[MethodImpl(MethodImplOptions.AggressiveInlining)] // fast type checks that don't add a lot of overhead
private static bool TryGetSpan<TSource>(this IEnumerable<TSource> source, out ReadOnlySpan<TSource> span)
{
// This constraint isn't required, but the overheads involved here can be more substantial when TSource
// is a reference type and generic implementations are shared. So for now we're protecting ourselves
// is a reference type and generic implementations are shared. So for now we're protecting ourselves
// and forcing a conscious choice to remove this in the future, at which point it should be paired with
// sufficient performance testing.
where TSource : struct
{
Debug.Assert(typeof(TSource).IsValueType);

if (source is null)
{
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
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4 changes: 2 additions & 2 deletions src/libraries/System.Linq/src/System/Linq/SequenceEqual.cs
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Expand Up @@ -24,9 +24,9 @@ public static bool SequenceEqual<TSource>(this IEnumerable<TSource> first, IEnum

if (first is ICollection<TSource> firstCol && second is ICollection<TSource> secondCol)
{
if (first is TSource[] firstArray && second is TSource[] secondArray)
if (typeof(TSource).IsValueType && first.TryGetSpan(out ReadOnlySpan<TSource> firstSpan) && second.TryGetSpan(out ReadOnlySpan<TSource> secondSpan))
{
return ((ReadOnlySpan<TSource>)firstArray).SequenceEqual(secondArray, comparer);
return firstSpan.SequenceEqual(secondSpan, comparer);
}

if (firstCol.Count != secondCol.Count)
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