Any idea for create a combobox-like control

I am combining a combobox-like control (named mycombo) with a textbox and a
form (named comboform) in which there is a listbox with same size of form.

The control mycombo has a method named dropdown in which mycombo show
comboform and set its location beside the bottom of the textbox.

I dont know what I do is a good idea or not.

I mean the mycombo look like a real combobox. However, the MyCombo.form
doesn't really belong to MyCombo.textbox or MyCombo, which is an indepedant
control. When user click on another application to make the mainform which
mycombo belong to deactive, the mycombo.textbox disappear with the mainform,
but the mycombo.form just hang up there. Another example is, if the user
drag the mainform to move from location A to location B, mycombo.textbox
move together, but mycombo.form doesn't.

I don't know how to make mycombo.form has a same behave with
mycombo.textbox. Form control is the topest level control which cann't
assign parent property.

Any idea or suggestion?

---Frank