How to calculate Sum datatime? under format HH:MM:SS using Linq Lamba

i have column Duree_Comm in my class 

 public partial class Staxe
    {
        public System.DateTime Date_Comm { get; set; }
        public System.DateTime Heure_Comm { get; set; }
        public string N_Poste { get; set; }
        public Nullable<System.DateTime> Duree_Comm { get; set; }
    }
}
 example values Duree_Comm  in my  class , i want calculate sum Duree_Comm  using Linq lambda  and the result be under format hh:mm:ss thanks