>Actully I wanat to allow my user to type code directlly in combo because some times the # of elements in combo may be up to 300-400 and its difficult to search from combo.

With that number of items you might want to think of moving away from a combo and instead have textbox with a button that says something like "find account". You could then display a listbox of accounts they could choose. You are starting to get near the limit of what a combo can display and also what users can stand to see in a combo. Just a suggestion.

>I want the Combo not dropdown list & want to manage the things here, because ny user normally type the code instead of selection !

It looks like Roi got the answer you were looking for.