It’s been said that OOP is essentially the combining of behavior and state. Asynchronous code has no problem expressing behavior; it is functional in nature, after all. When you design asynchronous objects, most of the problems that arise are due to state.

Exposed State

As we discussed last week, properties should represent the current state of an object. This can cause a problem regarding semantics: what is the “current state” of an object that has an asynchronous operation in progress?

One familiar example of this problem is Stream.Position, which represents the current offset of the stream “pointer”. When you call Read or Write, the actual reading/writing is done and Position is updated to reflect the new position, all before the Read or Write method returns.

Now, consider ReadAsync and WriteAsync: when is Position updated? When the reading/writing is complete, or before it actually happens? If it happens before, is it updated synchronously or could it happen after the actual ReadAsync or WriteAsync method returns?

As a side note, this problem affects all asynchronous code, not just async code. The same questions about ReadAsync can be asked about BeginRead.

This is a great example of how a property that exposes state has perfectly clear semantics for synchronous code, but no obviously correct semantics for asynchronous code. It’s not the end of the world - you just need to think about your entire API when async-enabling your types, and document the semantics you choose.

Hidden State

Hidden state is state that can impact asynchronous operations, even if that state isn’t exposed through properties. Stream.Position only causes problems if the code needs to use that property; hidden state impacts other asynchronous operations.

A prominent example of hidden state is in HttpWebRequest, which can only perform one HTTP request at a time. There is no indication that a request is in progress; if you attempt to start a second asynchronous request, you’ll receive an InvalidOperationException.

HttpClient improves the situation; a single HttpClient instance supports multiple simultaneous HTTP requests. There are still a few state-related problems (e.g., you can’t set HttpClient.Timeout once a request is in progress), but it does reduce the impact of hidden state when compared to HttpWebRequest.

I am generally against hidden state from an API design perspective, but implementation is often easier if you do allow hidden state. In these cases, I prefer to have both a lower-level type (like HttpWebRequest) with well-documented hidden state and then have a higher-level type (like HttpClient) without hidden state.

Output State

There’s another kind of “state” that should be addressed: when changes to state are treated as output and used to update other state (usually including the UI, if you’re using MVVM or a similar system). In this case, you have data-bound properties that notify listeners when they change.

I don’t have much to add to last week’s discussion of data-bound properties. Just keep in mind that updates should be done on the UI context, and that properties should always represent the current state. So you may need an “indeterminate” or “unknown” value to start with (e.g., null), which can be asynchronously updated.