Bug 29645 - Random.NextDouble stops producing random number and returns zero after a while
Summary: Random.NextDouble stops producing random number and returns zero after a while
Alias: None
Product: iOS
Classification: Xamarin
Component: XI runtime ()
Version: XI 8.9.x (iOS 8.3)
Hardware: Macintosh Mac OS
: --- normal
Target Milestone: Untriaged
Assignee: Bugzilla
Depends on:
Reported: 2015-05-01 08:53 UTC by Ed
Modified: 2015-05-01 10:16 UTC (History)
2 users (show)

Is this bug a regression?: ---
Last known good build:

Demo project (161.37 KB, application/zip)
2015-05-01 08:53 UTC, Ed

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Description Ed 2015-05-01 08:53:11 UTC
Created attachment 11020 [details]
Demo project

I was stress testing ThreadPool to check if it causes issues to my socket connection, and after a while i noticed that Random.NextDouble() in my test just stops producing random numbers. This happens on both iPhone and Simulator. Check the attached project. And see this video: http://www.screencast.com/t/mcC69TXWl

PS: For the ThreadPool, using it for my internal purposes indeed somehow caused lags to my socket connection, as a workaround i simply wrote my own implementation and it worked.
Comment 1 Rolf Bjarne Kvinge [MSFT] 2015-05-01 09:02:55 UTC
You're accessing the Random instance from multiple threads at the same time, and Random isn't thread-safe.

In fact MSDN [1] says that you'll get 0 if you do it: "If you don't ensure that the Random object is accessed in a thread-safe way, calls to methods that return random numbers return 0."

If you change your sample to have one Random per thread, it works fine:

	static Random rnd;// = new Random();

	static void OnNSTimerTick(Foundation.NSTimer Timer)

	static void ThreadPoolCallback(object State)
		if (rnd == null)
			rnd = new Random ();

		for (int i = 0; i < 1000000; i++)

		double rndValue = rnd.NextDouble();


[1] https://msdn.microsoft.com/en-us/library/system.random(v=vs.110).aspx
Comment 2 Ed 2015-05-01 10:16:41 UTC
Thanks for pointing this out, my mistake.