Is C# Random Number Generator thread safe?

Question

Is C#'s Random.Next() method thread safe?

Answer 1

No, using the same instance from multiple threads can cause it to break and return all 0's. However, creating a thread-safe version (without needing nasty locks on every call to Next()) is simple. Adapted from the idea in this article:

public class ThreadSafeRandom
{
    private static readonly Random _global = new Random();
    [ThreadStatic] private static Random _local;

    public int Next()
    {
        if (_local == null)
        {
            int seed;
            lock (_global)
            {
                seed = _global.Next();
            }
            _local = new Random(seed);
        }

        return _local.Next();
    }
}

The idea is to keep a separate static Random variable for each thread. Doing that in the obvious way fails, however, because of another issue with Random - if multiple instances are created at nearly the same time (within about 15ms), they will all return the same values! To fix this, we create a globally-static Random instance to generate the seeds used by each thread.

The above article, by the way, has code demonstrating both of these issues with Random.