Private Sub TextBox1_Validating(ByVal sender As Object, _
  ByVal e As System.ComponentModel.CancelEventArgs) Handles TextBox1.Validating
 
  ' Eingabe prüfen
  Try
    Dim value As Double = CDbl(TextBox1.Text)
    TextBox1.Text = value.ToString("f")
  Catch ex As Exception
    e.Cancel = True
  End Try
End Sub

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