Custom ComboBox for Pocket PC

Question

Hi, I am using CE on a pocket PC. I want to create an intelligent named dialler using a ComboBox. The idea is when the user enters a C say, then all contacts located in the database beginning with C are shown in a drop down Combo box. When the user then enters CH then all contacts with CH are shown in the drop down Combo Box etc. Unfortunately in the Pocket PC environment you are not alowed to freely enter text in a combo box only to select from existing list entries so I want to modify a Conbo Box under CE on Pocket PC to allow a user to enter their own text independently. I know I must Custonise the ComboBox control but am unsure how to start. Can anyome point me in the right direction, e.g. Extend ComboBox, Extend Control etc.