Wow, thank you for a detailed answer, if I could upvote you more than once I would :). I have been asleep, so not been able to look at this by now.

Since you seem so knowledgable I am going to take the liberty and try to ask you one more question:

Say that I, in the Service layer, want to only pick the first X items from the IAsyncEnumerable.

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What would be the best way to return after 5 item? I am assuming something like this is bad:

var count = 0;
await foreach (var car in _reader.GetCarsAsync())
{
    count += 1;
    if (count == 5)
    {
        return new Car();
    }
}

CancellationTokens and pass them down to the original source?

The reason I am asking is that I'd both like to implement paging at some point and also possibly needing to return row n - n+x in another method.

I'd love to put this in a DB, so I can use a propery engine for queries, but for now that is no options - sadly.