System.Linq.Enumerable has special treatment of IList<T>, which should also apply to IReadOnlyList<T>

[AlexVallat]

In several places in System.Linq.Enumerable, it checks for a more performant path by testing if the source is IList<T> (for example

runtime/src/libraries/System.Linq/src/System/Linq/Last.cs

Line 76 in 0f5b753

if (source is IList<TSource> list)

)

This should ideally also test for IReadOnlyList<T>, as IList<T> does not inherit from IReadOnlyList<T>, and an implementer of IReadOnlyList<T> is equally valid for the fast paths here.

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Issue Details

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In several places in System.Linq.Enumerable, it checks for a more performant path by testing if the source is IList<T> (for example

runtime/src/libraries/System.Linq/src/System/Linq/Last.cs

Line 76 in 0f5b753

if (source is IList<TSource> list)

)

This should ideally also test for IReadOnlyList<T>, as IList<T> does not inherit from IReadOnlyList<T>, and an implementer of IReadOnlyList<T> is equally valid for the fast paths here.

Author:	AlexVallat
Assignees:	-
Labels:	area-System.Linq, untriaged
Milestone:	-

[huoyaoyuan]

#42254

[Wraith2]

from @EgorBo in #42254 (comment)

I found quite a few complains or rejected attempts to optimize LINQ for IReadOnlyCollection:

#28651 - LINQ results implicit support for IReadOnlyCollection
#27517 - Performance: Make constructor List(IEnumerable collection) know about IReadOnlyCollection
#26679 - Linq ToDictionary() should presize for IReadOnlyCollection
#14366 - System.Linq performance improvement suggestions (mentions IROC<>)
#24793 - Respect IReadOnlyList in the BCL
#23910 - Add optimized path for IReadOnlyCollection/IReadOnlyList in System.Linq
#18714 - Consider checking for IReadOnlyCollection in Enumerable.ToArray
#27516 - Performance of LINQ .Any() - type check to leverage .Count property? (mentions IROC)
#27517 - Performance: Make constructor List(IEnumerable collection) know about IReadOnlyCollection
dotnet/corefx#28472 - Check for IReadOnlyCollection
#43001 - LINQ IEnumerable extension methods should add special case IReadOnlyCollection

[eiriktsarpalis]

Duplicate of #24793. Closing in favor of #42254.