Visual Basic

Dim number as Int32
Try
   number = Int32.Parse(textBox.Text)
Catch
  // Show Messagebox here

Private Sub ListBox1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
    Dim notSelectableIndices As Integer() = New Integer() {2, 5, 8}

    Dim index As Integer
    For Each index In notSelectableIndices
        If (ListBox1.GetSelected(index)) Then
            ListBox1.SetSelected(index, False)
        End If
    Next
End Sub

Dim index As Integer
Dim idexArray(2) As Integer
Dim strEval As String
Dim i As Integer = 0
Dim j As Integer = 0

'Evaluate which items in the list box should not be
'allowed to be selected
'If the list was larger this would be set globally once and used
For i = 0 To lbEducationalMaterials.Items.Count - 1
    strEval = Microsoft.VisualBasic.Left(lbEducationalMaterials.Items.Item(i), 2)
    If strEval = "--" Then
        idexArray(j) = lbEducationalMaterials.Items.IndexOf(lbEducationalMaterials.Items.Item(i))
        j += 1
    End If
Next

'set the total bound of the array
i = idexArray.GetUpperBound(0)

'identify and unselect items
For Each index In idexArray
    If (lbEducationalMaterials.GetSelected(index)) Then
        lbEducationalMaterials.SetSelected(index, False)
    End If
Next