Random.Next() sometimes returns same number in separate threads

Question

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.

Answer 1

Note: The most appropriate approach as of .NET 6 would be to use Random.Shared which is thread-safe.

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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.