Random Number Generator: Class level or Method level?

Question

When using a Random Number Generator, which is the better way to use it for greater randomness of the new value:

1. Have a method that instantiates a new instance of the RNG each time and then returns a value?

2. Have an instance of the RNG at the class level, which is instantiated once in the Constructor, and all subsequent calls for a new random value using the existing instance?

The issue is that there may be many calls for a random number, often in different scopes not connected with each other.

This is not a performance issue, so the fact that each call might instantiate a new instance makes no difference. This is all about the randomness of the returned value.

Answer 1

Option 1 does not work, actually.

Option 2 is the only choice. RNG's absolutely require that you generate the values in sequence from a single seed.

Your "create a new generator with a new seed" breaks the mathematical foundation. What you get then totally depends on your seeds, which -- sadly -- won't be very random.

Answer 2

I suggest option 3: have a single RNG used throughout the program. It requires locking or a thread-local if the RNG isn't thread-safe (e.g. in .NET), but it makes life a lot simpler and you don't need to worry about repetition.

See the relevant MiscUtil page for details of the .NET StaticRandom class I wrote for this very purpose. (It's incredibly simple - nothing clever at all.)