Simple Event Sourcing using NMF

Linqpad script.cs

void Main()
{
	var events = new ObservableCollection<Event>();

	var state = new NotifyCollection<object>();

	events.OnAdd(next =>
	{
		if (next is UserWasAdded)
		{
			var userWasAdded = (UserWasAdded)next;
			state.Add(new User() { UserName = userWasAdded.UserName });
		}

		if (next is UserWasDeleted)
		{
			var userWasDeleted = (UserWasDeleted)next;
			var user = state.OfType<User>().Single(u => u.UserName == userWasDeleted.UserName);
			user.Deleted = true;
		}
	});



	var activeUsers = state.WithUpdates().OfType<User>().Where(u => u.Deleted == false);

	("Active users: " + activeUsers.Count()).Dump();

	events.Add(new UserWasAdded() { UserName = "Harry" });

	("Active users: " + activeUsers.Count()).Dump();

	events.Add(new UserWasDeleted() { UserName = "Harry" });

	("Active users: " + activeUsers.Count()).Dump();
}

class Event { }
class UserWasAdded : Event { public string UserName { get; set; } }
class UserWasDeleted : Event { public string UserName { get; set; } }
 
class User : INotifyPropertyChanged { 
	public string UserName { get; set; }
	bool _deleted;
	public bool Deleted
	{
		get { return _deleted; } set{ _deleted = value; OnPropertyChanged();} 
	}
	
	public event PropertyChangedEventHandler PropertyChanged;
	
	protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
	{
		PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
	}
}

@bwanner Am I using this incorrectly? I get InvalidOperationException: Sequence contains no elements on line 6

edit: ah you need to provide a seed to Aggregate if the collection is empty.