Private Sub TextBox1_KeyPress(ByVal sender As Object, ByVal e As _
  System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
        Select Case Asc(e.KeyChar)
            Case 48 To 57 '0-9
            Case 8 'Backspace
            Case Else
                MsgBox("Bitte nur numerische Werte eingeben")
        End Select
End Sub

Private Sub TextBox1_Validated(ByVal sender As Object, ByVal e As _
  System.EventArgs) Handles TextBox1.Validated
        Dim _i As Single = 0
 
        For _i = 0 To Me.TextBox1.Text.Length - 1
            Select Case Asc(Me.TextBox1.Text.Chars(_i))
                Case 48 To 57 '0-9
                Case 8 'Backspace
                Case Else
                    MsgBox("Der Wert " & Me.TextBox1.Text.Chars(_i).ToString & _
                      " ist leider nicht numerisch")
            End Select
 
        Next
    End Sub