concrete-subject.cs

public class Sukhpinder : ICelebrityInstagram
{
    private readonly List<IFollower> _posts = new List<IFollower>();
    private string _post;
    public string FullName => "Sukhpinder Singh";

    public string Post {
        get { return _post; }
        set
        {
            Notify(value);
        }
    }

    public void AddFollower(IFollower follower)
    {
        _posts.Add(follower);
    }

    public void Notify(string post)
    {
        _post = post;
        foreach (var item in _posts)
        {
            item.Update(this);
        }
    }

    public void RemoveFollower(IFollower follower)
    {
        _posts.Remove(follower);
    }
}