DropDownItems.Add
does not call the DropDownItems
setter. You're not setting the value of the property, just calling a method on its current value.
You need to say something like
uc1.DropDownItems = new List<DropdownItemObject>() {
new DropdownItemObject("ITEM 998") };
in order to call the setter code.
You could also expose your own Add
method on your UserControl class, which would in turn call comboPrimary.Items.Add
.
public void Add(DropdownItemObject item) {
comboPrimary.Items.Add(item);
}