Public Sub AddBindingSourceParsed(ByVal TextBox As TextBox, ByVal BindingSource _
  As BindingSource, _
                                        ByVal DataField As String)
 
        Dim ab As New Binding("Text", BindingSource, DataField)
        AddHandler ab.Parse, AddressOf OnChangeParse
        TextBox.DataBindings.Add(ab)
 
    End Sub
 
Public Sub OnChangeParse(ByVal sender As System.Object, ByVal e As _
  ConvertEventArgs)
 
        If (CStr(e.Value) = "") Then
            e.Value = System.DBNull.Value
        End If
 
    End Sub

AddBindingSourceParsed(dieTextBox, dieBindingSource, dasDatenfeld)