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Consolidate optimisation paths for Count #135270

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925118e
Consolidate optimisation paths for Count
MikeBarker-MSFT Oct 6, 2026
9c1a99a
Remove explicit tests to run the selector on Count.
MikeBarker-MSFT Oct 6, 2026
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26 changes: 2 additions & 24 deletions src/libraries/System.Linq/src/System/Linq/AnyAll.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -10,31 +10,9 @@ public static partial class Enumerable
{
public static bool Any<TSource>(this IEnumerable<TSource> source)
{
if (source is null)
{
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
}

if (source is ICollection<TSource> gc)
{
return gc.Count != 0;
}

if (!IsSizeOptimized && source is Iterator<TSource> iterator)
{
int count = iterator.GetCount(onlyIfCheap: true);
if (count >= 0)
{
return count != 0;
}

iterator.TryGetFirst(out bool found);
return found;
}

if (source is ICollection ngc)
if (TryGetNonEnumeratedCount(source, out int nonEnumeratedCount))
{
return ngc.Count != 0;
return nonEnumeratedCount != 0;
}

using IEnumerator<TSource> e = source.GetEnumerator();
Expand Down
24 changes: 16 additions & 8 deletions src/libraries/System.Linq/src/System/Linq/AppendPrepend.SpeedOpt.cs
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Expand Up @@ -121,13 +121,17 @@ public override List<TSource> ToList()

public override int GetCount(bool onlyIfCheap)
{
if (_source is Iterator<TSource> iterator)
if (!TryGetNonEnumeratedCount(_source, out int count))
{
int count = iterator.GetCount(onlyIfCheap);
return count == -1 ? -1 : checked(count + 1);
if (onlyIfCheap)
{
return -1;
}

count = _source.Count();
}

return !onlyIfCheap || _source is ICollection<TSource> ? checked(_source.Count() + 1) : -1;
return checked(count + 1);
}

public override TSource? TryGetFirst(out bool found)
Expand Down Expand Up @@ -274,13 +278,17 @@ public override List<TSource> ToList()

public override int GetCount(bool onlyIfCheap)
{
if (_source is Iterator<TSource> iterator)
if (!TryGetNonEnumeratedCount(_source, out int count))
{
int count = iterator.GetCount(onlyIfCheap);
return count == -1 ? -1 : checked(count + _appendCount + _prependCount);
if (onlyIfCheap)
{
return -1;
}

count = _source.Count();
}

return !onlyIfCheap || _source is ICollection<TSource> ? checked(_source.Count() + _appendCount + _prependCount) : -1;
return checked(count + _appendCount + _prependCount);
}

public override bool Contains(TSource value)
Expand Down
29 changes: 7 additions & 22 deletions src/libraries/System.Linq/src/System/Linq/Count.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -10,24 +10,9 @@ public static partial class Enumerable
{
public static int Count<TSource>(this IEnumerable<TSource> source)
{
if (source is null)
{
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
}

if (source is ICollection<TSource> collectionoft)
{
return collectionoft.Count;
}

if (source is Iterator<TSource> iterator)
{
return iterator.GetCount(onlyIfCheap: false);

@skyoxZ skyoxZ Oct 7, 2026 •
edited
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Sometimes it is possible to avoid iterating over all elements even when TryGetNonEnumeratedCount fails:

int count = new[] { 1, 2, 3 }.Distinct().Concat(Enumerable.Repeat(1, 999999)).Count();

The PR introduces a regression for such case.

}

if (source is ICollection collection)
if (TryGetNonEnumeratedCount(source, out int nonEnumeratedCount))
{
return collection.Count;
return nonEnumeratedCount;
}

int count = 0;
Expand Down Expand Up @@ -102,11 +87,6 @@ public static int Count<TSource>(this IEnumerable<TSource> source, Func<TSource,
/// </remarks>
public static bool TryGetNonEnumeratedCount<TSource>(this IEnumerable<TSource> source, out int count)
{
if (source is null)
{
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
}

if (source is ICollection<TSource> collectionoft)
{
count = collectionoft.Count;
Expand All @@ -129,6 +109,11 @@ public static bool TryGetNonEnumeratedCount<TSource>(this IEnumerable<TSource> s
return true;
}

if (source is null)
{
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
}
Comment on lines +112 to +115

@MikeBarker-MSFT MikeBarker-MSFT Oct 6, 2026

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Moving this null check below the type checks is a very minor optimisation for the case where any of the optimised types are provided.

@MihaZupan MihaZupan Oct 6, 2026

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How does moving it improve such cases? The null check has to be performed regardless, no?


count = 0;
return false;
}
Expand Down
12 changes: 6 additions & 6 deletions src/libraries/System.Linq/src/System/Linq/DefaultIfEmpty.SpeedOpt.cs
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Expand Up @@ -30,15 +30,15 @@ public override List<TSource> ToList()

public override int GetCount(bool onlyIfCheap)
{
int count;
if (!onlyIfCheap || _source is ICollection<TSource> || _source is ICollection)
if (!TryGetNonEnumeratedCount(_source, out int count))
{
if (onlyIfCheap)
{
return -1;
}

count = _source.Count();
}
else
{
count = _source is Iterator<TSource> iterator ? iterator.GetCount(onlyIfCheap: true) : -1;
}

return count == 0 ? 1 : count;
}
Expand Down
11 changes: 8 additions & 3 deletions src/libraries/System.Linq/src/System/Linq/OrderedEnumerable.SpeedOpt.cs
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Expand Up @@ -53,12 +53,17 @@ private void Fill(TElement[] buffer, Span<TElement> destination)

public override int GetCount(bool onlyIfCheap)
{
if (_source is Iterator<TElement> iterator)
if (!TryGetNonEnumeratedCount(_source, out int count))
{
return iterator.GetCount(onlyIfCheap);
if (onlyIfCheap)
{
return -1;
}

count = _source.Count();
}

return !onlyIfCheap || _source is ICollection<TElement> || _source is ICollection ? _source.Count() : -1;
return count;
}

internal TElement[] ToArray(int minIdx, int maxIdx)
Expand Down
68 changes: 0 additions & 68 deletions src/libraries/System.Linq/tests/SelectTests.cs
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Expand Up @@ -12,32 +12,6 @@ namespace System.Linq.Tests
{
public class SelectTests : EnumerableTests
{
[Fact]
public void SelectSideEffectsExecutedOnCount()
{
int i = 0;
// If we made no promises about side effects, i could be 0, but in practice users have
// taken a dependency on side effects executing on Count.
var count = Enumerable.Range(1, 10).Select(x => i++).Count();
Assert.Equal(10, count);
Assert.Equal(10, i);

i = 0;
count = Enumerable.Range(1, 10).Skip(5).Select(x => i++).Count();
Assert.Equal(5, count);
Assert.Equal(5, i);

i = 0;
count = Enumerable.Range(1, 10).Take(5).Select(x => i++).Count();
Assert.Equal(5, count);
Assert.Equal(5, i);

i = 0;
count = Enumerable.Range(1, 10).Skip(2).Take(3).Select(x => i++).Count();
Assert.Equal(3, count);
Assert.Equal(3, i);
}

[Fact]
public void SameResultsRepeatCallsStringQuery()
{
Expand Down Expand Up @@ -1220,48 +1194,6 @@ public static IEnumerable<object[]> MoveNextAfterDisposeData()
yield return [Enumerable.Range(1, 30)];
}

[Theory]
[MemberData(nameof(RunSelectorDuringCountData))]
public void RunSelectorDuringCount(IEnumerable<int> source)
{
int timesRun = 0;
var selected = source.Select(i => timesRun++);
selected.Count();

Assert.Equal(source.Count(), timesRun);
}

public static IEnumerable<object[]> RunSelectorDuringCountData()
{
var transforms = new Func<IEnumerable<int>, IEnumerable<int>>[]
{
e => e,
e => ForceNotCollection(e),
e => ForceNotCollection(e).Skip(1),
e => ForceNotCollection(e).Where(i => true),
e => e.ToArray().Where(i => true),
e => e.ToList().Where(i => true),
e => new LinkedList<int>(e).Where(i => true),
e => e.Select(i => i),
e => e.Take(e.Count()),
e => e.ToArray(),
e => e.ToList(),
e => new LinkedList<int>(e) // Implements IList<T>.
};

var r = new Random(42);

for (int i = 0; i <= 5; i++)
{
var enumerable = Enumerable.Range(1, i).Select(_ => r.Next());

foreach (var transform in transforms)
{
yield return [transform(enumerable)];
}
}
}

[Fact]
public void Select_SourceIsIList_EnumeratorDisposedOnComplete()
{
Expand Down
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