CODEWITHSUNDEEP
c#random
Gener4tor
Gener4tor

Reputation: 388

Is the C# random number generator guaranteed to be stable?

I am writing a program which uses a lot of random numbers to generate stuff. But I want to assure that when starting with the same seed the generated content is the same. So when I create my random number like this:

Random r = new Random(seed);
int num = r.Next();

How is it guaranteed that num is always the same over time (means: several releases of .NET)? Is there a standardized list in the background? Or should I use my own "random list" to be sure that this will never change?

Upvotes: 4

Views: 1751

Answers (2)

stuartd
stuartd

Reputation: 73253

No, the result from the same seed can vary across versions of the framework, and it's in the documentation here:

The implementation of the random number generator in the Random class isn't guaranteed to remain the same across major versions of the .NET Framework. As a result, you shouldn't assume that the same seed will result in the same pseudo-random sequence in different versions of the .NET Framework.

That documentation also contains variations of this advice in other sections, e.g. in Retrieve the same sequence of random values:

The following example uses 100100 as an arbitrary seed value to instantiate the Random object, displays 20 random floating-point values, and persists the seed value. It then restores the seed value, instantiates a new random number generator, and displays the same 20 random floating-point values. Note that the example may produce different sequences of random numbers if run on different versions of the .NET Framework.

And:

However, note that Random objects in processes running under different versions of the .NET Framework may return different series of random numbers even if they're instantiated with identical seed values.

Upvotes: 11

Marc Gravell
Marc Gravell

Reputation: 1063058

To be explicit: no, there isn't a "standardized list" - it is a PRNG, so it just does the same mathematical operations over and over and over - which does make it predictable (deterministic) from the same seed, as long as no-one changes the PRNG algorithm being used. Given that people often use Random for things like seeding test data, I think it would cause an outcry if it suddenly changed (without it being opt-in in some way, like a new constructor parameter), so I would find it unlikely to do so (and they haven't to date), but: if "unlikely" isn't good enough, you could always just implement any of a number of PRNG algorithms in your own code.

Upvotes: 4

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