How should I convert a function returning a non-generic Task to ValueTask?

Question

I'm working on some code which builds a buffer in memory and then empties it into a TextWriter when the buffer fills up. Most of the time, the character will go straight into the buffer (synchronously) but occasionally (once every 4kb) I need to call TextWriter.WriteAsync .

In the System.Threading.Tasks.Extensions package there only appears to be a ValueTask<T> struct, and no non-generic ValueTask (without a type parameter). Why is there no ValueTask , and what should I do if I need to convert a method returning a non-generic Task (that is, the async equivalent of a void method) to ValueTask ?

Answer 1

Shot in the dark, but I think it's because Task.CompletedTask is sufficient for most non-generic cases.

One way to think of ValueTask<T> is as a union of Task<T> and T (for asynchronous and synchronous cases respectively). Accordingly a non-generic ValueTask would be a union of Task and... nothing, so just a Task .

I can't think of a case where a non-generic ValueTask would be practically different than caching an already completed Task (which is what Task.CompletedTask is), though I'd love to learn about any.

Answer 2

Based on this article , you can change any asynchronous method directly from Task to ValueTask , but you must beware of its usage patterns mentioned in the article; specifically:

• Awaiting the instance multiple times.
• Calling AsTask multiple times.
• Using more than one of these techniques to consume the instance.

And if you need to get the result back as a Task / Task<TResult> , you should use .AsTask() .