how to have multiselect option in combobox in c# windows app

Question

HI,
        i have many options in my combobox and with the input  form the combobox i am passing it to the database to sort the data in the datagridview. i want to add a new field name "select all" if i select this option,  i need to select all the value in the combobox. any one help me plz.