public static Nullable <Tbase> Sum <Tobj, Tbase>(this IEnumerable <Tobj> lista, Func <Tobj, Nullable <Tbase> > prop) where Tobj : class where Tbase : struct { Nullable <Tbase> soma = null; foreach (var obj in lista) { soma = soma.Add(prop.Invoke(obj)); } return(soma); }