FileStream.BeginRead/Write with useAsync no longer use overlapped I/O

[stephentoub]

On Windows when FileStream is opened for overlapped I/O, its BeginRead/Write methods would use overlapped I/O. Now in .NET 6 when no buffering is used, this is no longer the case: BeginRead/Write aren't overridden on the Windows strategy, and these operations end up using the base Stream implementation that queues work items (which will in turn block waiting on async I/O).

cc: @adamsitnik

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Issue Details

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On Windows when FileStream is opened for overlapped I/O, it's BeginRead/Write methods would use overlapped I/O. Now in .NET 6 when no buffering is used, this is no longer the case: BeginRead/Write aren't overridden, and these operations end up using the base Stream implementation that queues work items (which will in turn block waiting on async I/O).

cc: @adamsitnik

Author:	stephentoub
Assignees:	-
Labels:	area-System.IO, untriaged
Milestone:	6.0.0

[teo-tsirpanis]

I am working on it.