Detecting when you're in/out of the main thread in Xamarin.iOS

Question

Is there a way in Xamarin/MonoTouch to detect whether code is being called in the main thread?

I'm looking for something like the equivalent of Java's EventQueue.isEventDispatchThread() -- I've found in Swing programming it's handy to assert that from time to time (or sometimes to assert that it's not) -- making sure that models are consistently updated and read from the EDT, and that long-running calls don't block the UI.

I'd like to do the same in my MonoTouch app, to ensure that various bits of code are/aren't called from the UI, or wrapped in InvokeOnMainThread.

Updated: For anyone coming along later: Obj-C answer from JP below . The Xamarin/MonoTouch equivalent is NSThread.Current.IsMainThread.

possible duplicate of [Check whether or not the current thread is the main thread](http://stackoverflow.com/questions/3546539/check-whether-or-not-the-current-thread-is-the-main-thread) — bobobobo, May 07 '13 at 20:56
@bobobobo That question doesn't directly address the Monotouch syntax. — David Moles, May 07 '13 at 21:01
I didn't notice the [tag:monotouch]. Well, the answer ended up being the same. — bobobobo, May 07 '13 at 21:02
@bobobobo I can de-accept the answer and answer it myself with the note I added at the end of the question, if that would make you happier. — David Moles, May 07 '13 at 21:04

score 41 · Accepted Answer · answered Oct 19 '10 at 22:10

I don't know much about Monotouch, but in iOS +[NSThread isMainThread] might be what you're looking for.

Occasionally when writing multithreaded code, I will put in an assert like this:

NSAssert([NSThread isMainThread], @"Method called using a thread other than main!");

score 1 · Answer 2 · answered Mar 07 '11 at 16:35

1

The only problem with NSAssert([NSThread isMainThread], errorDesc) is that you BETTER be in the main thread when making this call. If you happen to be in a secondary thread and make the call then your application crashes! So it's kind of pointless.

It's better just to simply use [NSThread isMainThread] then assess the BOOL value it returns.

answered Mar 07 '11 at 16:35

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You're correct that the NSAssert in my answer will crash the app if it isn't run from the main thread. That's the purpose of using an NSAssert. During development, I use that assert to alert me if my code ever gets executed in the a non-main thread. You don't have to use the NSAssert from my answer -- you can do anything with that function. The NSAssert code is just an example of a possible use for the +[NSThread isMainThread] function. As a side note, your answer probably could have been a comment on my answer instead of a whole separate answer. – Jay Peyer Mar 07 '11 at 17:11

Daniele D. · Answer 3 · 2016-03-04T11:28:42.503

You can do like this in Monotouch / Xamarin.ios

        if (NSThread.Current.IsMainThread)
        {
            //You are in the MainThread
        }

This check is very useful to avoid the error that might occur when trying to modify the UI from a background Thread. Something Like this can be done:

if (NSThread.Current.IsMainThread)
{
    DoSomething();
}
else
{
    BeginInvokeOnMainThread(() => DoSomething());
}

Detecting when you're in/out of the main thread in Xamarin.iOS

3 Answers3

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