How to customize a comobox

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VB

Dear friends

I am a student in programming languages (currently vb.net) and i would like to create an application.

My appplication will use a very special combo box.
The user must be able to add new items on the control.
Set the control should work i 2 modes
1) to show the items sorted
2)or show the most used selected items first.

Also must have the ability to hide some items.
Items will be retrieved and saved into a db.

Could you please give me some ideas?
If you have some source code to study i will appreciate it also.

Regards