Dim loopCount As Integer = 500
Dim regularSum = Enumerable.Range(0, loopCount).Select(Function(n) Clng(n)).Sum

regularSum.Dump("Correct value")

Dim results = Enumerable.Range(0, 10).Select(Function(d)
                                                 Dim total As Long = 0
                                                 Parallel.For(0, loopCount, New ParallelOptions() With {.MaxDegreeOfParallelism = 8},
                                                 Sub(n)
                                                    'this code is running in multiple tasks and not running in the same thread.
                                                     total += CLng(n)
                                                 End Sub)
                                                 Return total
                                             End Function).ToList


results.Dump("Bad way.  Don't do this ever")