how to serialize a directories from a path ?

Question

Hello, I develop my first application for a client has the objective of managing quotes and invoices, at first I have to choose the directory that contains the quotes. But just once and after it uses the application normally,The selected files are added to a list <directory> with the code below

 

if (listBox2.SelectedIndex >= 0)
{
    foreach (DirectoryInfo dr in listBox2.SelectedItems)
    {

        Dossiers.ls_dossier_devis.Add(dr);

    }
}
what i have to do ?