use the below code in the librabry and pass your combobox as reference parameter
public void LoadCombobox(ref ComboBox mycombobox)
{
mycombobox.Items.Clear();
mycombobox.Items.Add("Rajesh");
mycombobox.Items.Add("Anuhya");
}
Call the method
LoadCombobox(ref ComboBox1);
Thanks
--RA