Important for anyone researching this difficult topic in Unity specifically,
be sure to see another question I asked which raised related key issues:
In Unity specifically, "where" does an await literally return to?
For C# experts, Unity is single-threaded1
It's common to do calculations and such on another thread.
When you do something on another thread, you often use async/wait since, uh, all the good C# programmers say that's the easy way to do that!
void TankExplodes() {
ShowExplosion(); .. ordinary Unity thread
SoundEffects(); .. ordinary Unity thread
SendExplosionInfo(); .. it goes to another thread. let's use 'async/wait'
}
using System.Net.WebSockets;
async void SendExplosionInfo() {
cws = new ClientWebSocket();
try {
await cws.ConnectAsync(u, CancellationToken.None);
...
Scene.NewsFromServer("done!"); // class function to go back to main tread
}
catch (Exception e) { ... }
}
OK, so when you do this, you do everything "just as you normally do" when you launch a thread in a more conventional way in Unity/C# (so using Thread or whatever or letting a native plugin do it or the OS or whatever the case may be).
Everything works out great.
As a lame Unity programmer who only knows enough C# to get to the end of the day, I have always assumed that the async/await pattern above literally launches another thread.
In fact, does the code above literally launch another thread, or does c#/.Net use some other approach to achieve tasks when you use the natty async/wait pattern?
Maybe it works differently or specifically in the Unity engine from "using C# generally"? (IDK?)
Note that in Unity, whether or not it is a thread drastically affects how you have to handle the next steps. Hence the question.
Issue: I realize there's lots of discussion about "is await a thread", but, (1) I have never seen this discussed / answered in the Unity setting (does it make any difference? IDK?) (2) I simply have never seen a clear answer!
1 Some ancillary calculations (eg, physics etc) are done on other threads, but the actual "frame based game engine" is one pure thread. (It's impossible to "access" the main engine frame thread in any way whatsoever: when programming, say, a native plugin or some calculation on another thread, you just leave markers and values for the components on the engine frame thread to look at and use when they run each frame.)
I don't like answering my own question, but as it turns out none of the answers here is totally correct. (However many/all of the answers here are hugely useful in different ways).
In fact, the actual answer can be stated in a nutshell:
SynchronizationContext.Current.That's it.
Thus in any particular version of Unity (and note that, as of writing 2019, they are drastically changing Unity - https://unity.com/dots) - or indeed any C#/.Net environment at all - the question on this page can be answered properly.