DuctTypes.cs

    [NodeName("DuctTypes")]
    [NodeCategory("MEP.Ducts")]
    [NodeDescription("Get All Duct Type From Model Dropdown")]
    [IsDesignScriptCompatible]
    public class DuctTypeUi : DSDropDownBase
    {
        const string NoDuctTypes = "No types were found.";

        /// <summary>
        /// </summary>
        public DuctTypeUi() : base("DuctType")
        {
        }

        /// <summary>
        /// </summary>
        /// <param name="currentSelection"></param>
        /// <returns></returns>
        protected override SelectionState PopulateItemsCore(string currentSelection)
        {
            Items.Clear();

            Document doc = DocumentManager.Instance.CurrentDBDocument;
            List<Element> vtList = new FilteredElementCollector(doc).OfClass(typeof(DuctType)).ToList();

            if (vtList.Count == 0)
            {
                Items.Add(new DynamoDropDownItem(NoDuctTypes, null));
                SelectedIndex = 0;
                return SelectionState.Done;
            }

            foreach (DuctType v in vtList) Items.Add(new DynamoDropDownItem(v.Name, v));
            Items = Items.OrderBy(x => x.Name).ToObservableCollection();
            return SelectionState.Restore;
        }

        /// <summary>
        /// </summary>
        /// <param name="inputAstNodes"></param>
        /// <returns></returns>
        public override IEnumerable<AssociativeNode> BuildOutputAst(List<AssociativeNode> inputAstNodes)
        {
            AssociativeNode node;

            if (SelectedIndex == -1)
            {
                node = AstFactory.BuildNullNode();
            }
            else
            {
                DuctType ductType = Items[SelectedIndex].Item as DuctType;


                StringNode idNode = AstFactory.BuildStringNode(ductType.UniqueId);
                BooleanNode falseNode = AstFactory.BuildBooleanNode(true);

                node = AstFactory.BuildFunctionCall(
                    new Func<string, bool, object>(ElementSelector.ByUniqueId),
                    new List<AssociativeNode> {idNode, falseNode});
            }

            return new[] {AstFactory.BuildAssignment(GetAstIdentifierForOutputIndex(0), node)};
        }
    }
}