.cs

public class VerbalizedEvent
{
    private readonly EventData _eventData;

    public VerbalizedEvent(MqttDelivery delivery)
    {
        Delivery = delivery;

        if ( delivery.TryGetPayload<PayloadWithEventData>(out var payload))
        {
            _eventData = payload.EventData;

            Title = _eventData?.Title ?? new();
            Description = _eventData?.Description ?? new();
            Items =
                _eventData?.Items
                    .ExceptNull()
                    .ToList()
                ?? new List<DataItem>();
        }

        Categories = new( Items
            .Where(item => item.IsOfType(DataItemType.Category))
            .Select(item => item.ID.ToEnumOrDefault<EventCategoryType>()));
    }

    public MqttDelivery Delivery { get; }

    public DateTimeOffset Timestamp =>
        Delivery.Header.Timestamp.GetValueOrDefault();

    public string Schema =>
        Delivery.Header.Schema;

    public string Name =>
        Delivery.Header.Name;

    public ApiQualifiedName QualifiedName =>
        (Schema, Name);

    public LocalizedString Title { get; } =
        new();

    public LocalizedString Description { get; } =
        new();

    public HashSet<EventCategoryType> Categories { get; } =
        new();

    public IReadOnlyList<DataItem> Items { get; } =
        new List<DataItem>();

    public List<DataItem> Sources =>
        Items.Where( item => item.Traits.HasFlag(DataItemTraitType.Source)).ToList();

    public List<DataItem> Targets =>
        Items.Where( item => item.Traits.HasFlag(DataItemTraitType.Target)).ToList();

    public List<DataItem> CategoryItems =>
        Items.Where( item => item.IsOfType(DataItemType.Category)).ToList();

    public List<DataItem> Reasons =>
        Items.Where( item => item.IsOfType(DataItemType.Reason)).ToList();

    public List<DataItem> Operators =>
        Items.Where( item => item.IsOfType(DataItemType.Operator)).ToList();

    public List<DataItem> OperatorGroups =>
        Items.Where( item => item.IsOfType(DataItemType.OperatorGroup)).ToList();

    public List<DataItem> IdentifierCodes =>
        Items.Where( item => item.IsOfType(DataItemType.IdentifierCode)).ToList();

    public List<DataItem> Identifiers =>
        Items.Where( item => item.IsOfType(DataItemType.Identifier)).ToList();

    public List<DataItem> Subjects =>
        Items.Where( item => item.IsOfType(DataItemType.Subject)).ToList();

    public List<DataItem> Departments =>
        Items.Where( item => item.IsOfType(DataItemType.Department)).ToList();

    public List<DataItem> Territories =>
        Items.Where( item => item.IsOfType(DataItemType.Territory)).ToList();

    public List<DataItem> TerritoryGroups =>
        Items.Where( item => item.IsOfType( DataItemType.TerritoryGroup ) ).ToList();

    public List<DataItem> AccessPoints =>
        Items.Where( item => item.IsOfType( DataItemType.AccessPoint ) ).ToList();

    public List<DataItem> Nodes =>
        Items.Where( item => item.IsOfType( DataItemType.Node ) ).ToList();

    public List<DataItem> Devices =>
        Items.Where( item => item.IsOfType( DataItemType.Device ) ).ToList();

    public List<DataItem> Objects =>
        Items.Where( item => item.IsOfType(DataItemType.Object ) ).ToList();

    public List<DataItem> Images =>
        Items.Where( item => item.IsOfType(DataItemType.Image)).ToList();
}