Parallel.For and For yield different results

Question

If I run this test:

 var r = new Random();
 var ints = new int[13];
 Parallel.For(0, 2000000, i => {            
     var result = r.Next(1, 7) + r.Next(1, 7)         


        

Answer 1

In addition to @spencerruport's assertion that the Random class is not thread safe, your parallel code is also not threadsafe:

 Parallel.For(0, 2000000, i => {            
     //say two threads produce same total at same time
     var result = r.Next(1, 7) + r.Next(1, 7); 
     //what happens on the next line when a context-switch
     //occurs during this non-atomic operation?
     ints[result] += 1;
 });

It might be better to leverage PLINQ to do the collecting of results on your behalf:

Enumerable.Range(0, 2000000)
    .AsParallel()
    .Select(_ => SafeRandom(1, 7) + SafeRandom(1, 7))
    .GroupBy(x => x)
    .Select(g => new {value = g.Key, frequency = g.Count()})

instead of managing access to shared memory (your ints array above) yourself.

A reasonable implementation of SafeRandom might look something like this:

private static int seedUnique=0;
private static ThreadLocal tlRand=new ThreadLocal(() => {
    var x=Interlocked.Add(ref seedUnique, 93459872);
    var r=new Random((int)(DateTime.UtcNow.Ticks + x));
    return r;
});
public static int SafeRandom(int min, int max)
{
    return tlRand.Value.Next(min,max);
}