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In System.Threading.Channels, there's a ChannelOptions.SingleReader property that selects an optimized implementation if there can only be one read issued at a time.

ChannelOptions is inherited by BoundedChannelOptions which has the FullMode property.

Two of the FullMode values are able to remove items from the Channel's internal storage/queue:

BoundedChannelFullMode.DropNewest Removes and ignores the newest item in the channel in order to make room for the item being written.

BoundedChannelFullMode.DropOldest Removes and ignores the oldest item in the channel in order to make room for the item being written.

Does this removal of the newest/oldest item, which occurs during a write, count as a reader for the purposes of the SingleReader property?

(That wouldn't make these two values mutually exclusive with SingleReader = true, but it would mean that when used in combination, not only can two reads not overlap each other, but a read also can't overlap a write.)

Theodor Zoulias
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Ben Voigt
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1 Answers1

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Configuring a bounded channel with the SingleWriter and SingleReader options has only a decorative effect. Currently (.NET 7) there is only one bounded Channel<T> implementation, which supports multiple writers and readers. This implementation is based on a Deque<T> (internal .NET type similar to a Queue<T>) synchronized with a lock. You can set the SingleWriter/SingleReader options to any combination of values, and it will make absolutely no difference. The SingleReader option has effect only on unbounded channels, and the SingleWriter hasn't any effect anywhere.

So does the removal of the newest/oldest item, which occurs during a write, count as a reader? The docs don't specify it explicitly. The documentation of the Channels library is somewhat lacking in general. My opinion is that it shouldn't count as a reader. In a mechanism where the roles of writing and reading are so carefully separated, It would be highly surprising if a simple act of writing was actually a writing+reading hybrid. But since the SingleWriter/SingleReader options have no effect on bounded channels, debating about this is currently academic.

Theodor Zoulias
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    Thanks for the comprehensive explanation of the .NET 7 implementation. It's sad to have to rely on implementation details though, since future versions could make those properties functional rather than purely decorative. – Ben Voigt Feb 01 '23 at 21:35
  • @BenVoigt to be honest I don't expect anything revolutionary to happen in the Channels library the next 5 years, if ever. There are not many incentives to make any major changes at the moment. Most of the developers who are using it, seem to be satisfied with the existing feature set. You can take a look at the [area-System.Threading.Channels](https://github.com/dotnet/runtime/issues?q=label%3Aarea-System.Threading.Channels), to get an idea of what kind of stuff people are asking for. – Theodor Zoulias Feb 01 '23 at 22:16