Is C# Random Number Generator thread safe?

Question

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

Answer 1

Per documentation

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

http://msdn.microsoft.com/en-us/library/system.random.aspx

Answer 2

The traditional thread local storage approach can be improved upon by using a lock-less algorithm for the seed. The following was shamelessly stolen from Java's algorithm (possibly even improving on it):

public static class RandomGen2 
{
    private static readonly ThreadLocal<Random> _rng = 
                       new ThreadLocal<Random>(() => new Random(GetUniqueSeed()));

    public static int Next() 
    { 
        return _rng.Value.Next(); 
    } 

    private const long SeedFactor = 1181783497276652981L;
    private static long _seed = 8682522807148012L;

    public static int GetUniqueSeed()
    {
        long next, current;
        do
        {
            current = Interlocked.Read(ref _seed);
            next = current * SeedFactor;
        } while (Interlocked.CompareExchange(ref _seed, next, current) != current);
        return (int)next ^ Environment.TickCount;
   } 
}

Answer 3

Another thread safe way is to use ThreadLocal<T> as follows:

new ThreadLocal<Random>(() => new Random(GenerateSeed()));

The GenerateSeed() method will need to return a unique value each time it is called to assure that the random number sequences are unique in each thread.

static int SeedCount = 0;
static int GenerateSeed() { 
    return (int) ((DateTime.Now.Ticks << 4) + 
                   (Interlocked.Increment(ref SeedCount))); 
}

Will work for small numbers of threads.

Answer 4

For what its worth, here is a thread-safe, cryptographically strong RNG that inherits Random.

The implementation includes static entry points for ease of use, they have the same names as the public instance methods but are prefixed with "Get".

A call to the RNGCryptoServiceProvider.GetBytes is a relatively expensive operation. This is mitigated through the use of an internal buffer or "Pool" to make less frequent, and more efficient use of RNGCryptoServiceProvider. If there are few generations in an application domain then this could be viewed as overhead.

using System;
using System.Security.Cryptography;

public class SafeRandom : Random
{
    private const int PoolSize = 2048;

    private static readonly Lazy<RandomNumberGenerator> Rng =
        new Lazy<RandomNumberGenerator>(() => new RNGCryptoServiceProvider());

    private static readonly Lazy<object> PositionLock =
        new Lazy<object>(() => new object());

    private static readonly Lazy<byte[]> Pool =
        new Lazy<byte[]>(() => GeneratePool(new byte[PoolSize]));

    private static int bufferPosition;

    public static int GetNext()
    {
        while (true)
        {
            var result = (int)(GetRandomUInt32() & int.MaxValue);

            if (result != int.MaxValue)
            {
                return result;
            }
        }
    }

    public static int GetNext(int maxValue)
    {
        if (maxValue < 1)
        {
            throw new ArgumentException(
                "Must be greater than zero.",
                "maxValue");
        }
        return GetNext(0, maxValue);
    }

    public static int GetNext(int minValue, int maxValue)
    {
        const long Max = 1 + (long)uint.MaxValue;

        if (minValue >= maxValue)
        {
            throw new ArgumentException(
                "minValue is greater than or equal to maxValue");
        }

        long diff = maxValue - minValue;
        var limit = Max - (Max % diff);

        while (true)
        {
            var rand = GetRandomUInt32();
            if (rand < limit)
            {
                return (int)(minValue + (rand % diff));
            }
        }
    }

    public static void GetNextBytes(byte[] buffer)
    {
        if (buffer == null)
        {
            throw new ArgumentNullException("buffer");
        }

        if (buffer.Length < PoolSize)
        {
            lock (PositionLock.Value)
            {
                if ((PoolSize - bufferPosition) < buffer.Length)
                {
                    GeneratePool(Pool.Value);
                }

                Buffer.BlockCopy(
                    Pool.Value,
                    bufferPosition,
                    buffer,
                    0,
                    buffer.Length);
                bufferPosition += buffer.Length;
            }
        }
        else
        {
            Rng.Value.GetBytes(buffer);
        }
    }

    public static double GetNextDouble()
    {
        return GetRandomUInt32() / (1.0 + uint.MaxValue);
    }

    public override int Next()
    {
        return GetNext();
    }

    public override int Next(int maxValue)
    {
        return GetNext(0, maxValue);
    }

    public override int Next(int minValue, int maxValue)
    {
        return GetNext(minValue, maxValue);
    }

    public override void NextBytes(byte[] buffer)
    {
        GetNextBytes(buffer);
    }

    public override double NextDouble()
    {
        return GetNextDouble();
    }

    private static byte[] GeneratePool(byte[] buffer)
    {
        bufferPosition = 0;
        Rng.Value.GetBytes(buffer);
        return buffer;
    }

    private static uint GetRandomUInt32()
    {
        uint result;
        lock (PositionLock.Value)
        {
            if ((PoolSize - bufferPosition) < sizeof(uint))
            {
                GeneratePool(Pool.Value)
            }

            result = BitConverter.ToUInt32(
                Pool.Value,
                bufferPosition);
            bufferPosition+= sizeof(uint);
        }

        return result;
    }
}

Answer 5

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

    public int Next()
    {
        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.

Answer 6

There's nothing special done in the Next method to achieve thread safety. However, it's an instance method. If you don't share instances of Random across different threads, you don't have to worry about state corruption within an instance. Do not use a single instance of Random across different threads without holding an exclusive lock of some sort.

Jon Skeet has a couple nice posts on this subject:

StaticRandom
Revisiting randomness

As noted by some commentators, there is another potential problem in using different instances of Random that are thread-exclusive, but are seeded identically, and therefore induce the identical sequences of pseudorandom numbers, because they may be created at the same time or within close temporal proximity of each other. One way to alleviate that issue is to use a master Random instance (which is locked by a single thread) to generate some random seeds and initialize new Random instances for every other thread to use.