public void Unfollow(string userName, string followerUserName)
{
if (userName.IsNullOrEmpty())
{
throw new ArgumentNullException(nameof(userName));
}
if (userName.IsNullOrEmpty())
{
throw new ArgumentNullException(nameof(userName));
}
var followers = GetFollowers(userName);
if (!followers.Select(p => p.UserName).Contains(followerUserName))
{
throw new BusinessLogicException("User is already unfollow");
}
_profileProvider.Unfollow(userName, followerUserName);
}