[RESOLVED] Random Bug

[dbasnett]

This weekend I watched a video on YouTube(see comments for link) that said there is a bug in the Random class when used with threading. It was in C# so I thought I'd recreate it in VB. I was surprised to find it does fail. And after failing the first time it is like the Random is broken.

To use this code create a Form with three buttons and two labels with default names.

Button 1 is a sanity check and only fails after Button 2 fails.

    ' Is this  https://www.youtube.com/watch?v=WRB4OHpSXHs
    '   true?
    ' .Net 4.8
    '
    ' Strictly speaking zero values aren't errors.
    '  Documentation says this about .Next 
    '   A 32-bit signed integer that is greater than or equal to 0 and less than Int32.MaxValue.
    ' but ...

    Private Shared PRNG As New Random
    Private LRand As Concurrent.BlockingCollection(Of Integer)
    Const Tries As Integer = 100000
    Const Thrds As Integer = 25
    Private STPW As New Stopwatch
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        SetUp()

        For x As Integer = 1 To Tries * Thrds
            LRand.Add(PRNG.Next)
        Next
        STPW.Stop()
        Dim Nzro As Integer
        Nzro = (From i In LRand Where i > 0 Select i).Count
        Dim zct As Integer = LRand.Count
        If Nzro <> zct Then
            Label1.Text = "1 ERR  " & (Nzro / zct).ToString("p1") 'per cent of non-zero values
        Else
            Label1.Text = "Good  " & Nzro.ToString("n0")
        End If
        Label2.Text = STPW.Elapsed.TotalMilliseconds.ToString("n1")
    End Sub

    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
        SetUp()

        Parallel.For(1, Thrds + 1,
                     Sub(idx As Integer)
                         For zx As Integer = 1 To Tries
                             Do While Not LRand.TryAdd(PRNG.Next)
                                 Threading.Thread.Sleep(0)
                             Loop
                         Next
                     End Sub)
        STPW.Stop()
        Dim Nzro As Integer
        Nzro = (From i In LRand Where i > 0 Select i).Count
        Dim zct As Integer = LRand.Count
        If Nzro <> zct Then
            Label1.Text = "2 ERR  " & (Nzro / zct).ToString("p1") 'per cent of non-zero values
        Else
            Label1.Text = "Good  " & Nzro.ToString("n0")
        End If
        Label2.Text = STPW.Elapsed.TotalMilliseconds.ToString("n1")
    End Sub

    Private Sub SetUp()
        ' PRNG = New Random
        Label1.Text = ""
        LRand = New Concurrent.BlockingCollection(Of Integer)
        STPW.Restart()
    End Sub

Here is what I did to fix? this issue,

    Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click
        SetUp()

        Parallel.For(1, Thrds + 1,
                     Sub(idx As Integer)
                         For zx As Integer = 1 To Tries
                             Do While Not LRand.TryAdd(MyPRNG.NextI)
                                 Threading.Thread.Sleep(0)
                             Loop
                         Next
                     End Sub)
        STPW.Stop()
        Dim Nzro As Integer
        Nzro = (From i In LRand Where i > 0 Select i).Count
        Dim zct As Integer = LRand.Count
        If Nzro <> zct Then
            Label1.Text = "3 ERR  " & (Nzro / zct).ToString("p1") 'per cent of non-zero values
        Else
            Label1.Text = "Good  " & Nzro.ToString("n0")
        End If
        Label2.Text = STPW.Elapsed.TotalMilliseconds.ToString("n1")
    End Sub

    Private Class MyPRNG
        Private Shared Block As New Threading.SpinLock '(True)
        Private Shared _PRNG As New Random

        Public Shared Sub NextB(buffer() As Byte)
            Block.Enter(Nothing)
            _PRNG.NextBytes(buffer)
            If Block.IsHeld Then
                Block.Exit()
            End If
        End Sub

        Public Shared Function NextD() As Double
            Block.Enter(Nothing)
            NextD = _PRNG.NextDouble
            If Block.IsHeld Then
                Block.Exit()
            End If
        End Function

        Public Shared Function NextI() As Integer
            Block.Enter(Nothing)
            NextI = _PRNG.Next
            If Block.IsHeld Then
                Block.Exit()
            End If
        End Function

        Public Shared Function NextI(maxValXclu As Integer) As Integer
            Block.Enter(Nothing)
            NextI = _PRNG.Next(maxValXclu)
            If Block.IsHeld Then
                Block.Exit()
            End If
        End Function

        Public Shared Function NextI(minValIlu As Integer, maxValXclu As Integer) As Integer
            Block.Enter(Nothing)
            NextI = _PRNG.Next(minValIlu, maxValXclu)
            If Block.IsHeld Then
                Block.Exit()
            End If
        End Function
    End Class