Hi,

Please refer below code.

C#

protected void Save(object sender, EventArgs e)
{
    List<string> dropDownSelected = new List<string>();
    foreach (ListItem item in lstFruits.Items)
    {
        if (item.Selected)
        {
            dropDownSelected.Add(item.Value);
        }
    }
    string selected = string.Join(",", dropDownSelected);
    // Write code to save into Database.
}

I hope this will help you out.