VB - Textbox-Extension nur numerische Werte zulassen mit Ein-/Aus

        ''' <summary>
        ''' Nur Eingabe von Zahlen zulassen {0-9/,/-}
        ''' </summary>
        ''' <param name="T">Typ TextBox</param>
        ''' <param name="allow">Ein-/Ausschalten numerische Eingabe</param>
        ''' <remarks></remarks>
        <Extension()> _
       Public Sub AllowNumeric(ByVal T As TextBox, ByVal allow As Boolean)
            If T IsNot Nothing Then
                If allow Then
                    AddHandler T.KeyPress, AddressOf DoubleKeyPresser
                Else
                    RemoveHandler T.KeyPress, AddressOf DoubleKeyPresser
                End If
            End If
        End Sub


        Private Sub DoubleKeyPresser(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs)
            Select Case e.KeyChar
                Case ControlChars.Tab, ControlChars.Back : Exit Sub
                Case ControlChars.Cr : e.Handled = True : SendKeys.Send("{TAB}") : Exit Sub
                Case "0"c To "9"c : Exit Sub
                Case "-"c
                    If _CountStrings(CType(sender, TextBox).Text, CChar("-")) = 0 Then Exit Sub
                Case ","c       ' nur maximal 1 Komma zulassen
                    If _CountStrings(CType(sender, TextBox).Text, CChar(",")) = 0 Then Exit Sub
                Case "."c
                    If _CountStrings(CType(sender, TextBox).Text, CChar(",")) = 0 Then e.KeyChar = ","c : Exit Sub
            End Select

            e.Handled = True
        End Sub

        Private Function _CountStrings(ByVal str As String, ByVal regexStr As Char) As Integer
            Dim regex As New RegularExpressions.Regex(regexStr)
            Return regex.Matches(str).Count
        End Function