Its not that easy as we think to manipulate a collection that is bound to a combobox/listbox. This works out of the box if you are using a DataView but if you are using something like an ArrayList to bind you comboBox control, then the changes in the binded collection will not directly update the control collection.
However you can force this by calling SuspendBinding() and ResumeBinding() on the BindingContext. You might need to do something like this everytime you manipulate the collection..
BindingManagerBase bindingManagerBase = BindingContext[_documentTypeDTOList];
_documentTypeDTOList.Add(documentType); // manipulating and adding a new type at run
_documentTypeComboBox.DataSource = __documentTypeDTOList;
_documentTypeComboBox.DisplayMember = "Name";