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Unfortunately, there’s an expensive lock on every access, and the more threads that want random numbers, the more contention there will be for this lock. This has the benefit of being easy to write, simple to explain, and obviously does the right thing.
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Private static Random _inst = new Random() A shared Random instance is created, and every access to the Random instance is protected by a lock, e.g. There are several approaches then one could take in using Random from multiple threads to avoid these kinds of issues. The point of this example is that, in a multithreaded context, you may be creating a Random instance for each iteration, but many iterations across threads are likely to end up with the same value. When I run this app, I consistently see each number showing up thousands of times before the number changes again, not very random. This program continually creates new Random instances and gets a random value from each, counting the number of “random” values in a row that are the same. Otherwise, we have a different number than If it’s the same as the previous number, This means that Random will suffer a similar fate, as is exemplified in the following program: TickCount returns a value that’s heavily influenced by the resolution of the system timer, and thus multiple calls to TickCount in a row are likely to yield the same value (on the system on which I’m writing this post, TickCount changes in value approximately once every 15 milliseconds). When a seed isn’t explicitly provided to Random through its constructor, it internally uses Environment.TickCount (at least in the current release) as the seed. This can also affect the quality of the random numbers. NET Framework).Īnother common mistake is to fix a problem like that in the previous example by instantiating a new Random instance each time a random number is needed. Not very random, is it? This is due to an implementation detail of Random that does not tolerate multithreaded access (it wasn’t designed for it, and the docs explicitly call out that Random should not be used from multiple threads, as they do for most types in the.
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While it won’t happen every time you run the app, on a multi-core machine it’s likely that once the Parallel.For in this example exits, rand.Next() will always return 0. For an example of this, consider the following program: This is a bad idea, and can have some drastic consequences on the quality of the random numbers (degrading them well below the “good enough” bar). The first is to assume that Random is thread-safe and is ok to be used concurrently from multiple threads.
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There are several common mistakes I’ve seen developers make. However, effectively utilizing Random in a parallel app can be challenging in a high-performance manner. For situations where random numbers don’t need to be cryptographically-strong, the System.Random class is typically a fast-enough mechanism for generating values that are good-enough. It’s very common in a parallel application to need random numbers for this or that operation.
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