public bool SaveContact(Contact currentContact)
 {
     if (this.Store.Any(item => item.Roll == currentContact.Roll))
         return false;
     this.Store.Add(currentContact);
     return true;
 }
 public bool UpdateContacts(Contact contact)
 {
     var currentContact = this.Store.FirstOrDefault(e => e.Roll == contact.Roll);
     if (currentContact != null)
     {
         currentContact.Name = contact.Name;
         currentContact.Age = contact.Age;
         currentContact.Address = contact.Address;
         return true;
     }
     return false;
 }