CODEWITHSUNDEEP
c#.netrandomparallel-processing
Dave J
Dave J

Reputation: 485

C# Random not changing

I already searched for this question but none of the solutions I found helped. Maybe somebody can help me.

I have the following loop:

private static readonly Random RANDOM = new Random();
...
int[] array; // is initialized, when used. ;)
if (array.Sum() != 0)
{
    int j = 0;
    do {
        j = RANDOM.Next(8);
    } while (array[j] == 0);
}

This whole loop is in another loop which, again, is in a Parallel.Foreach-loop. It seems, that j is always 0. Most times it is not noticeable, but if array[0] == 0, then it won't get out of the loop. I got the suspicion that the do-while-loop might be to fast. But also after some seconds (~30) it does not leave the loop. So the Random does not seem to return a new or different value (even in the same thread).

I also tried this solution but with no effect.

Upvotes: 3

Views: 616

Answers (3)

qJake
qJake

Reputation: 17139

Try this instead, since Random is not thread safe:

private static Random RANDOM = new Random();
private static object _randomLock = new object();

...

int[] array; // is initialized, when used. ;)
if (array.Sum() != 0)
{
    int j = 0;
    do {
        lock(_randomLock)
        {
            j = RANDOM.Next(8);
        }
    } while (array[j] == 0);
}

Upvotes: 4

Dave J
Dave J

Reputation: 485

Thanks for the quick answers. Matthew's answer already gave me what i needed.
I used

public static class RandomGen2 
{ 
    private static Random _global = new Random(); 
    [ThreadStatic] 
    private static Random _local; 

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

for my solution. It works very well and quite fast.

Upvotes: 3

Haney
Haney

Reputation: 34922

Random is not threadsafe and isn't sufficiently random for parallel operations. Consider using a thread safe RNG such as RNGCryptoServiceProvider, but know that it will be a fair bit slower than Random as the algorithm to generate numbers is much more complex.

Upvotes: 2

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