Fix Enumerable.Take/SkipLast on mutable collections #42506
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LINQ operators are lazy and can't factor in the current contents of a mutable list when making decisions about how to subsequently process that list.
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Tagging subscribers to this area: @eiriktsarpalis, @jeffhandley |
jkotas
reviewed
Sep 21, 2020
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Looks good to me, but wouldn't it in principle be possible to preserve the existing optimization by delaying it to enumerator initialization time? Granted, this might require nontrivial refactoring of the existing implementation. |
Right. That's a separate feature. The faulty optimization is being reverted here, and if someone wants to try again separately and validate it's useful, that's welcome. |
eiriktsarpalis
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Sep 21, 2020
timandy
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ghost
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LINQ operators are lazy and can't factor in the current contents of a mutable list when making decisions about how to subsequently process that list. The optimizations in dotnet/corefx#36051 and dotnet/corefx#41342 violated that and broke TakeLast and SkipLast when the underlying source changes after calling Take/SkipLast and before calling GetEnumerator.
Fixes #39864