Auto-complete IsReadOnly And Combobox IsReadOnly?

I have an app that uses a textbox with an autocomplete source and have come to realize that there may be times (to be decided at run time) where the list should be read-only. So ideally at run time if the list was considered R/O and the user's last typed character resulted in a no match, I would throw out the last key and go back to where we last had a match. When not in R/O mode, I would simply create a new entry on the list. Turns out the AutoCompleteStringCollection has a IsReadOnly property which is ReadOnly. So, when would it ever be true? Where does it dervive the value from?

So, the next step was that I thought that maybe a textbox is not the right control to use in this situation; maybe a combobox is. I notice that a combobox has an IsEditable and IsReadOnly properties that seem promising. I swithch the textbox control to a combobox and can't find the properties. I realize they are supported in Framework 3.0 and 3.5 and I am targeting 2.0. So I decide to bite the bullet and raise the target, but still can't access the properties for the combobox; what else do I need to do.And finally, when I am fooling around with the combobox set to 'simple' mode and select an item from the autocomplete list and hit enter, the combobox ends up blank; which seems to be a bug that was reported several years ago and still has not been fixed!