Private Sub Form1_MouseDown(ByVal sender As Object, ByVal e As _
  System.Windows.Forms.MouseEventArgs) Handles MyBase.MouseDown
Static a As Integer
Dim i As Integer
a += 1
 
For i = 0 To MenuItem1.MenuItems.Count - 1
MenuItem4.MenuItems(i).Visible = Int((a + i) / 2) = (i + a) / 2
Next
ContextMenu1.Show(Button1, New System.Drawing.Point(e.X, e.Y))
End Sub

Private Sub Form1_MouseDown(ByVal sender As Object, ByVal e As _
  System.Windows.Forms.MouseEventArgs) Handles MyBase.MouseDown
MenuItem1.MenuItems.Add(InputBox("Titel"))
ContextMenu1.Show(Button1, New System.Drawing.Point(e.X, e.Y))
End Sub