Why was “SwitchTo” removed from Async CTP / Release?

Question

I tried to use the SwitchTo method today to switch to the GUI thread, and found that the example I lifted it from does not work, simply because the method is not there.

Answer 1

ConfigureAwait is actually more dangerous than SwitchTo. Mentally tracking the current context and the last SwitchTo call is no more difficult than tracking where a variable was last assigned. On the other hand, ConfigureAwait switches context if and only if the call actually ran asynchronously. If the task was already completed, the context is preserved. You have no control over this.

Answer 2

It's 2020 and it looks like SwitchTo is set to come back to CLR soon, according to David Fowler and Stephen Toub in this GitHub issue, as there's no more limitations for await inside try/catch.

IMO, using something like await TaskScheduler.Default.SwitchTo() explicitly is better than relying upon ConfigureAwait(false) in the 3rd party library code, especially if we want to make sure that code doesn't execute on any custom synchronization context. I have a blog post with more details on that, including an experimental implementation of SwitchTo.

In a nutshell, I believe the first option from the below clearly indicates the intent, with minimum boilerplate code:

// switch to the thread pool explicitly for the rest of the async method
await TaskScheduler.Default.SwitchTo();
await RunOneWorkflowAsync();
await RunAnotherWorkflowAsync();

// execute RunOneWorkflowAsync on the thread pool 
// and stay there for the rest of the async method
await Task.Run(RunOneWorkflowAsync).ConfigureAwait(false);
await RunAnotherWorkflowAsync();

await Task.Run(async () => 
{
  // start on the thread pool
  await RunOneWorkflowAsync();
  await RunAnotherWorkflowAsync();
}).ConfigureAwait(false);
// continue on the thread pool for the rest of the async method

// start on whatever the current synchronization context is
await RunOneWorkflowAsync().ConfigureAwait(false);
// continue on the thread pool for the rest of the async method,
// unless everything inside `RunOneWorkflowAsync` has completed synchronously
await RunAnotherWorkflowAsync();

Answer 3

Stephen Toub has some more information on the reasoning in this thread.

To summarize, it's not a good idea for two reasons:

1. It promotes unstructured code. If you have "heavy processing" that you need to do, it should be placed in a Task.Run. Even better, separate your business logic from your UI logic.
2. Error handling and (some) continuations run in an unknown context. catch/finally blocks in Test would need to handle running in a thread pool or UI context (and if they're running in the thread pool context, they can't use SwitchTo to jump on the UI context). Also, as long as you await the returned Task you should be OK (await will correct the continuation context if necessary), but if you have explicit ContinueWith continuations that use ExecuteSynchronously, then they'll have the same problem as the catch/finally blocks.

In short, the code is cleaner and more predictable without SwitchTo.