Concurrent Programming in Xamarin.iOS – Manipulating UI from background threads

Background threads cannot modify the UI; almost all UIKit methods must be called on the main thread.

From a subclass of NSObject (including any UIViewController or UIView):

InvokeOnMainThread(() =>
{
    // Call UI methods here
});

From a standard C# class:

UIApplication.SharedApplication.InvokeOnMainThread(() =>
{
    // Call UI methods here
});

InvokeOnMainThread waits for your code running on the main thread to execute before continuing. If you don’t need to wait, use BeginInvokeOnMainThread.

if you want to reproduce, please indicate the source:
Concurrent Programming in Xamarin.iOS – Manipulating UI from background threads - CodeDay