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

The Random class methods are not thread safe.

http://msdn.microsoft.com/en-us/library/system.random.next(v=vs.90).aspx#2

So the first piece of code is just demonstrating some undefined behavior.

EDIT:

As for a little speculation, from what little I know about operating systems I believe random number generation is a pretty low level operation and hence might even require a context switch. While this is happening you may end up grabbing the same random number multiple times before it's had a chance to update. This would account for the lopsided distribution.