Drawn objects disappearing after set time. VB - vb.net

Question

I have code that makes a list of 11 circle objects and draws them to a picture box in a VB windows form. They are supposed to move across it and when they reach the end, restart at the beginning.
The circles get populated correctly, and move as they are supposed to, but after two cycles once they reach the end they disappear and don't reset to the beginning. Here is the code i am currently using.
Public Class frmContent
Private chocolatemark As New List(Of circlemark)
Public Sub frmContent_Load(sender As Object, e As EventArgs) Handles MyBase.Load
For i As Integer = 0 To 10
chocolatemark.Add(New circlemark(Rnd() * 630 + 1, Rnd() * 220 + 140))
Next
End Sub
Private Sub rndGen_Tick(sender As Object, e As EventArgs) Handles rndGen.Tick
picturebox.Invalidate()
End Sub
Private Sub picturebox_Paint(sender As Object, e As PaintEventArgs) Handles picturebox.Paint
For i As Integer = 0 To 10
chocolatemark(i).update()
chocolatemark(i).draw(e)
Next
End Sub
End Class
Public Class circlemark
Private pos As Point = New Point(0, 0)
Sub New(ByVal x As Double, ByVal y As Double)
pos.X = x
pos.Y = y
End Sub
Public Sub draw(ByRef e As PaintEventArgs)
e.Graphics.DrawEllipse(Pens.Red, pos.X, pos.Y, 5, 5)
End Sub
Public Sub update()
If pos.X < 640 Then
pos.X += globalvalue.speed
End If
If pos.X > 640 Then
pos.X = 0
End If
End Sub
End Class
Public Class globalvalue
Public Shared speed As Integer = 5
End Class
Does anyone know why this might be happening?

Answer 1

If pos.X < 640 Then
pos.X += globalvalue.speed
End If
If pos.X > 640 Then
pos.X = 0
End If
You created a black hole for the objects, one they can never escape from. Once the value reaches 640, they'll get stuck forever. Can't get higher, can't get back to 0. It should of course be >= 640.
This problem became hard to diagnose (although the debugger can easily show you) because you used a "magic number". 640 isn't actually the size of the picture box. So you couldn't see them being stuck. Never use magic numbers. PictureBox.ClientSize.Width is the correct value to use, minus the object size. Don't hard-code that either, turns in a fleck of dust on a "retina" display.