Projection prototype

gistfile1.cs

    //There is also a set of (extension) methods
    //such as Add, Update, etc.
    //all these methods use AddOrUpdate inside.
    public interface IDocumentWriter<in TKey, TDocument>
    {
            //The idea of AddOrUpdate is borrowed from ConcurrentDictionary:
            //AddOrUpdate(TKey key, Func<TKey, TView> createNewToAdd, Func<TView, TView> updateExisting);
	    TDocument AddOrUpdate(TKey key, Func<TDocument> addFactory, Func<TDocument, TDocument> updateFactory);
	    bool TryDelete(TKey key);
        bool Exists(TKey key);
    }

RegistrationStatusProjection.cs

    //The idea of a projection is to accumulate and maintain some persisted state
    //based on events
    public sealed class RegistrationStatusProjection
    {
        //just a writer who knows how to persist a projection
        //The first type parameter is a key, the second is the PVM type.
        //Look at IDocumentWriter interface in this gist
        private IDocumentWriter<Guid, RegistrationStatusView> _writer;

        public RegistrationStatusProjection(IDocumentStore factory)
        {
            //create a writer.
            //in my case it is a file system writer that will create a file like:
            //{Guid}-RegistrationStatusView.pvm
            //where the content is just a RegistrationStatusView serialized into JSON.
            //Obviously it can be anything: MongoDB persister, SQL 1NF, RavenDB, SOLR, OLAP, etc.
            _writer = factory.GetWriter<Guid, RegistrationStatusView>();
        }

        //In my case I use a simple "When(IEvent)" convention which allows me to keep
        //my projections clean and simple.
        //Another way could be to implement a bunch of IHandle<TEvent> interfaces,
        //but I really prefer conventions when possible.
        public void When(RegistrationStarted evt)
        {
            //my writer interface has a set of (extension) methods
            //such as Add, Update, etc.
            _writer.Add(evt.RegistrationId, new RegistrationStatusView
            {
                Status = RegistrationProcessStatus.InProgress,
                Login = evt.Security.Login,
                DisplayName = evt.Profile.DisplayName,
                Email = evt.Security.Email,
                Errors = new List<string>()
            });
        }

        public void When(RegistrationFailed evt)
        {
            //here I am updating the existing registration projection
            //AddOrUpdate will load one for me and put it as a "view" parameter
            //into my anonymous function.
            //There I just change some properties and the view will be saved back.
            _writer.AddOrUpdate(evt.RegistrationId, view =>
            {
                view.Status = RegistrationProcessStatus.Failed;
                view.Errors = evt.Errors.ToList();
            });
        }

        public void When(RegistrationCompleted evt)
        {
            _writer.AddOrUpdate(evt.RegistrationId, view =>
            {
                view.Errors = new List<string>();
                view.Status = RegistrationProcessStatus.Successful;
            });
        }

RegistrationStatusView.cs

    //Damn simple PVM
    [Serializable]
    public sealed class RegistrationStatusView
    {
        public string Login { get; set; }
        public string Email { get; set; }
        public string DisplayName { get; set; }
        public List<string> Errors { get; set; }

        public RegistrationProcessStatus Status { get; set; }
    }

    public enum RegistrationProcessStatus
    {
        InProgress = 0,
        Successful = 1,
        Failed = 2
    }