public async Task <Guid> AddContactAsync(Contact contact)
{
// First we will fetch address book along with the contact we are trying to add.
// This is just in case the contact already exists.
var addressBook = await _addressBookRepository.GetContactAsync(contact.Name, contact.Address);
addressBook.RegisterNewContact(contact);
await _addressBookRepository.SaveAsync(addressBook);
return(contact.Id);
}