asynchronous - F#: Synchronously start Async within a SynchronizationContext

Question

Async.SwitchSynchronizationContext allows an Async action to switch to running within a given SynchronizationContext. I would like to synchronously begin an Async computation within a SynchronizationContext, rather than switching inside the Async.

This would ensure that Async actions are run in the desired order, and that they are not run concurrently.

Is this possible? The documentation for Async.SwitchSynchronizationContext mentions using SynchronizationContext.Post, but no such functionality is exposed for Async.

question from:https://stackoverflow.com/questions/65890049/f-synchronously-start-async-within-a-synchronizationcontext

Answer 1

I have not tested this, but I think the easiest way to achieve what you want is to combine the SynchronizationContext.Send method (mentioned in the comments) with the Async.StartImmediate operation. The former lets you start some work synchronously in the synchronization context. The latter lets you start an async workflow in the current context.

If you combine the two, you can define a helper that starts a function in a given synchronization context and, in this function, immediately starts an async workflow:

let startInContext (sync:SynchronizationContext) work = 
  SynchronizationContext.Current.Send((fun _ -> 
    Async.StartImmediate(work)), null)

For example:

async { 
  printfn "Running on the given sync context"
  do! Async.Sleep(1000)
  printfn "Should be back on the original sync context" }
|> startInContext SynchronizationContext.Current