the above CODE IS ONLY FOR TEST (PUT IT IN A BUTTON CLICK EVENT AND TRY IT)
AGAIN IT IS ONLY FOR TEST

let's say if you want the textbox autocomplete list will be the combobox values
then you can put this in the form load event:-

Code:
      TextBox1.AutoCompleteSource = AutoCompleteSource.CustomSource
        TextBox1.AutoCompleteMode = AutoCompleteMode.SuggestAppend
and in the event that you use to add values to the combo box you can use:-
Code:
TextBox1.AutoCompleteCustomSource.Add(ComboBox1.Items(x))