Find out if I'm on the unity thread

Question

How can I check if the thread I'm on is the Unity thread?

I tried capturing the threadId at constructor time, but somewhere along the lifetime of the program, the threadId moves.

In my project, some secondary thread processes need access to a newly created object.
I use the producer-consumer pattern so they can be created on the Unity thread. An object factory queues a request and on Update() the objects I requested are instantiated on the correct thread. Between Queued and Instantiated the factory method waits for an ObjectCreated event with an AutoResetEvent.

Now sometimes this factory will be called from the main thread and the AutoResetEvent will block its own thread. I also tried it the dirty way with

// First try on this thread
try
{
    return action();
}
catch (ArgumentException ex)
{
    Debug.Log("Tried on same thread, but failed. "+ex.Message);
}
PushToQueueAndWait(action);

But when unity throws the exception, caught or not, the program halts.

If I could check whether I'm on the correct thread, I could switch between queueing and just executing.

Blanchblancha

I've never seen the main threadID change, how are you getting the value? System.Threading.Thread.CurrentThread.ManagedThreadId works for me and doesn't change insofar as my testing goes. Also, running threaded code in the Unity Editor's preview mode can result in unexpected behavior. – Gwenni

System.Threading.Thread.CurrentThread.ManagedThreadId seems to change from what it is at constructor time and what it is somewhere halfway running. Don't know any details. I have read somewhere that it could have something to do with the editor. Trying to be less vague, but haven't found a pattern yet. – Blanchblancha

I'm skeptical that Unity's main thread could ever change. That would suggest it freezes the main event loop, copies that state to a new thread, and starts the loop anew. Unity doesn't support multithreading so it's hard to imagine why they would engineer such a thing. If you develop a working example of the problem it'd be great to see. – Gwenni

answers.unity3d.com/questions/62631/… – Blanchblancha

Answer 1

I solved it by capturing the entire thread and then equalling it like so:

public void Start(){
    mainThread = System.Threading.Thread.CurrentThread;
}

bool isMainThread(){
    return mainThread.Equals(System.Threading.Thread.CurrentThread);
}

Related: https://mcmap.net/q/31443/unity-i-wonder-about-thread-id-plz-answer-me

Blanchblancha

Notice that Update() is always called on the main thread, so checking whether you're on the main thread within Update() will always result in true. I'll edit the answer to check for it in a function that can be called from any thread. – Tufthunter

Answer 2

I Agree with Boris Callens but on one point, Should use mainThread.ManagedThreadId == System.Threading.Thread.CurrentThread.ManagedThreadId when checking. Equals is defaulting to Object and is not working properly.

Inserted

"is not working properly" <== could you be more specific? What's the problem with mainThread.Equals(System.Threading.Thread.CurrentThread)? – Alcus

I can say, that Unity devs use exactly this way to determine main thread. You can see this in this - code github.com/Unity-Technologies/UnityCsReference/blob/master/… – Leakage