Getting random numbers in a thread-safe way

Note: since writing this answer, I've become aware of issues creating multiple Random instances, even though it sounds like it should work. I've generally found that a better alternative is to have a single Random instance and just lock on that. Although that is a single potential bottleneck, in most applications it won't cause problems.

I suspect it's just to avoid the cost of reading the thread-static variable multiple times. It's relatively inefficient to do so, compared with reading a local variable.

Your suggestion would work, but it would be slightly less efficient, that's all. In other cases, there would be the possibility of the value changing between fetches - but of course that's not a problem in this case, as it's a thread-local variable.

With .NET 4 and higher, this would be simpler using ThreadLocal<T>:

public static class RandomGen2 {     private static Random _global = new Random();     private static ThreadLocal<Random> _local = new ThreadLocal<Random>(() =>    {        int seed;        lock (_global) seed = _global.Next();        return new Random(seed);    });    public static int Next()     {         return _local.Value.Next();    } }