Many flat combobox controls out there are not based on the standard ComboBox control that is supplied by .NET, instead they have their own interfaces and requires a customized type of ComboBoxItem when inserting into the Items container. If you already have code written based on the standard .NET ComboBox, changing to the flat look may also require you to modify your codes. I have personally experienced that as a problem, and someone else has brought that up as a problem as well in my recent article DateTimePicker appears Flat.