BuildingCoder.CmdNewDuctSystem.Execute C# (CSharp) Method

        public Result Execute(
            ExternalCommandData commandData,
            ref string message,
            ElementSet elements)
        {
            UIApplication app = commandData.Application;
              UIDocument uidoc = app.ActiveUIDocument;
              Document doc = uidoc.Document;

              Transaction tx = new Transaction( doc,
            "New Duct System" );

              tx.Start();

              ConnectorSet connectorSet = new ConnectorSet();

              Connector baseConnector = null;

              ConnectorSetIterator csi;

              // select a Parallel Fan Powered VAV
              // and some Supply Diffusers prior to running
              // this command

              //ElementSet selection = uidoc.Selection.Elements; // 2014

              foreach( ElementId id in uidoc.Selection.GetElementIds() ) // 2015
              {
            Element e = doc.GetElement( id );

            if( e is FamilyInstance )
            {
              FamilyInstance fi = e as FamilyInstance;

              Family family = fi.Symbol.Family;

              // assume the selected Mechanical Equipment
              // is the base equipment for new system:

              if( family.FamilyCategory.Name
            == "Mechanical Equipment" )
              {
            // find the "Out" and "SupplyAir" connectors
            // on the base equipment

            if( null != fi.MEPModel )
            {
              csi = fi.MEPModel.ConnectorManager
                .Connectors.ForwardIterator();

              while( csi.MoveNext() )
              {
                Connector conn = csi.Current as Connector;

                if( conn.Direction == FlowDirectionType.Out
                  && conn.DuctSystemType == DuctSystemType.SupplyAir )
                {
                  baseConnector = conn;
                  break;
                }
              }
            }
              }
              else if( family.FamilyCategory.Name == "Air Terminals" )
              {
            // add selected Air Terminals to
            // connector set for new mechanical system

            csi = fi.MEPModel.ConnectorManager
              .Connectors.ForwardIterator();

            csi.MoveNext();

            connectorSet.Insert( csi.Current as Connector );
              }
            }
              }

              // create a new SupplyAir mechanical system

              MechanicalSystem ductSystem = doc.Create.NewMechanicalSystem(
            baseConnector, connectorSet, DuctSystemType.SupplyAir );

              tx.Commit();
              return Result.Succeeded;
        }