Random.Next() sometimes returns same number in separate threads
c#, multithreading, random
Solution
Note: The most appropriate approach as of .NET 6 would be to use Random.Shared which is thread-safe.
Random is not thread-safe - you shouldn't be using the same instance from multiple threads. It can get much worse than just returning the same data - by using it from multiple threads, you can get it "stuck" in a state where it will always return 0, IIRC.
Obviously you don't just want to create a new instance for each thread at roughly the same time, as they'll end up with the same seeds...
I have an article which goes into the details of this and provides an implementation which lazily instantiates one instance of `Random` per thread using an incrementing seed.
Problem
I have the following class ``` class Program { static Random _Random = new Random(); static void Main(string[] args) { ... for (int i = 0; i < no_threads; ++i) { var thread = new Thread(new ThreadStart(Send)); thread.Start(); } ... } static void Send() { ... int device_id = _Random.Next(999999); ... } } ``` The code creates the specified number of threads, starts each one, and assigns each thread a random `device_id`. For some reason, the first two threads that are created often have the same `device_id`. I can't figure out why this happens.