I have a memory stream that I'm writing to in a background task. I have a separate task that is reading from the stream.
The reader misses items that were written to the stream which implies that the reader and writer are both mutating the stream.
Is that explanation plausible? If so, do I need to synchronise these activities?
I have a service that returns a stream in process to another part of the code. It would seem, if this is the case, that streams are not an appropriate choice.
A MemoryStream only supports a single access at any time; it does not support concurrent read and write, concurrent reads, or concurrent writes. You could synchronize, but to be honest that sounds like you're using MemoryStream in a very unusual way - you would need to synchronize over entire chunks of operations, not just discreet reads/writes. In particular, note that MemoryStream only has a single position; it doesn't have a separate read position and write position, so you would need to exercise great care to make sure that the position is correct for the current operation.
I wonder whether Pipe might be what you're actually after, since that is both async-first and supports separate read / write operations; Pipe is basically a streaming buffer where a producer can push data in (with back-pressure if the backlog becomes too large), while a consumer can pull data out, all while using pooled memory.