WorkItem.cs

public class WorkItem : Aggregate
{
    public WorkItem(EntityId id, string description, TimeSpan retryInterval)
    {
        Announce(new WorkItemCreatedEvent(id, description, retryInterval));
    }

    public string Description { get; private set; }
    public Progress Progress { get; private set; }
    public TimeSpan RetryInterval { get; private set; }
    public Uri ResultLocation { get; private set; }

    private void Handle(ReportProgressCommand command)
    {
        Announce(command.Progress == Progress.Complete
            ? new ProgressReportedEvent(Id, Progress, command.ResultLocation)
            : new ProgressReportedEvent(Id, Progress, command.Progress));
    }

    private void Observe(WorkItemCreatedEvent e)
    {
        OnCreated(e.WorkItemId, e.When);

        Description = e.Description;
        RetryInterval = e.RetryInterval;

        Progress = Progress.Accepted;
    }

    private void Observe(ProgressReportedEvent e)
    {
        OnModified(e.When);

        Progress = e.NewProgress;
        ResultLocation = e.ResultLocation;
    }
}