public async Task ChangeNameAsync(
[NotNull] Author author,
[NotNull] string newName)
{
Check.NotNull(author, nameof(author));
Check.NotNullOrWhiteSpace(newName, nameof(newName));
var existingAuthor = await _authorRepository.FindByNameAsync(newName);
if (existingAuthor != null && existingAuthor.Id != author.Id)
{
throw new AuthorAlreadyExistsException(newName);
}
author.ChangeName(newName);
}