SelectedItem/SelectedIndex in DropDown ComboBox

[O.B.]

I have a ComboBox as a DropDown list. The following code is executed
in the form's constructor.

dropDownComboBox.Items.Add("ALL");

The following does nothing.

dropDownComboBox.SelectedItem = 0;

The following also does nothing.

dropDownComboBox.SelectedIndex = 0;

Help?

 

[Misbah Arefin]

try
dropDownComboBox.ClearSelection();
dropDownComboBox.Items[0].Selected = true;