/// <summary>
        /// Implement this method as an external command for Revit.
        /// </summary>
        /// <param name="commandData">An object that is passed to the external application
        /// which contains data related to the command,
        /// such as the application object and active view.</param>
        /// <param name="message">A message that can be set by the external application
        /// which will be displayed if a failure or cancellation is returned by
        /// the external command.</param>
        /// <param name="elements">A set of elements to which the external application
        /// can add elements that are to be highlighted in case of failure or cancellation.</param>
        /// <returns>Return the status of the external command.
        /// A result of Succeeded means that the API external method functioned as expected.
        /// Cancelled can be used to signify that the user cancelled the external operation
        /// at some point. Failure should be returned if the application is unable to proceed with
        /// the operation.</returns>
        public virtual Result Execute(ExternalCommandData commandData
                                      , ref string message, ElementSet elements)
        {
            try
            {
                Document document           = commandData.Application.ActiveUIDocument.Document;
                Autodesk.Revit.DB.View view = commandData.View;

                ElementId alignmentId = AlignmentSelectionFilter.SelectAlignment(document);
                Alignment alignment   = Alignment.Get(document.GetElement(alignmentId));

                TaskDialog td = new TaskDialog("Alignment and Station Label Properties");
                td.MainContent =
                    GetAlignmentProperties(alignment) +
                    GetAlignmentHorizontalCurveTypes(alignment) +
                    GetAlignmentStationLabelsProperties(alignment);
                td.Show();

                return(Result.Succeeded);
            }
            catch (Exception ex)
            {
                message = ex.Message;
                return(Result.Failed);
            }
        }
        /// <summary>
        /// Initiate alignment selection.
        /// </summary>
        /// <param name="document"></param>
        /// <returns></returns>
        public static ElementId SelectAlignment(Autodesk.Revit.DB.Document document)
        {
            UIDocument       uidoc        = new UIDocument(document);
            ISelectionFilter selFilter    = new AlignmentSelectionFilter();
            Reference        alignmentRef = uidoc.Selection.PickObject
                                                (ObjectType.Element, selFilter, "Select an alignment");

            return(alignmentRef == null ? ElementId.InvalidElementId : alignmentRef.ElementId);
        }
        const double labelTextOffset      = 0.005;   // 5mm, defined in paper space
        #endregion

        #region InterfaceImplementation
        /// <summary>
        /// Implement this method as an external command for Revit.
        /// </summary>
        /// <param name="commandData">An object that is passed to the external application
        /// which contains data related to the command,
        /// such as the application object and active view.</param>
        /// <param name="message">A message that can be set by the external application
        /// which will be displayed if a failure or cancellation is returned by
        /// the external command.</param>
        /// <param name="elements">A set of elements to which the external application
        /// can add elements that are to be highlighted in case of failure or cancellation.</param>
        /// <returns>Return the status of the external command.
        /// A result of Succeeded means that the API external method functioned as expected.
        /// Cancelled can be used to signify that the user cancelled the external operation
        /// at some point. Failure should be returned if the application is unable to proceed with
        /// the operation.</returns>
        public virtual Result Execute(ExternalCommandData commandData
                                      , ref string message, ElementSet elements)
        {
            try
            {
                Document document           = commandData.Application.ActiveUIDocument.Document;
                Autodesk.Revit.DB.View view = commandData.View;

                // find major and minor station arrow styles
                FilteredElementCollector fec            = new FilteredElementCollector(document);
                Element majorStationLeaderArrowheadType = fec.OfClass(typeof(ElementType))
                                                          .Cast <ElementType>()
                                                          .FirstOrDefault(x => x.Name.Contains(majorStationLeaderArrowheadName));
                Element minorStationLeaderArrowheadType = fec.OfClass(typeof(ElementType))
                                                          .Cast <ElementType>()
                                                          .FirstOrDefault(x => x.Name.Contains(minorStationLeaderArrowheadName));
                if (majorStationLeaderArrowheadType == null || minorStationLeaderArrowheadType == null)
                {
                    TaskDialog td = new TaskDialog("Missing arrowheads");
                    td.MainContent = "In Manage>Additional Settings>Arrowheads, create two styles of arrowheads, named\r\n" +
                                     "Major Station Arrowhead and Minor Station Arrowhead";
                    td.Show();

                    return(Result.Failed);
                }

                // find major and minor station label types; if not there, create them
                SpotDimensionType majorLabelType = fec.OfClass(typeof(SpotDimensionType))
                                                   .Cast <SpotDimensionType>()
                                                   .FirstOrDefault(sdt => sdt.StyleType == DimensionStyleType.AlignmentStationLabel && sdt.Name.Contains(majorStationSetLabelTypeName));

                SpotDimensionType minorLabelType = fec.OfClass(typeof(SpotDimensionType))
                                                   .Cast <SpotDimensionType>()
                                                   .FirstOrDefault(sdt => sdt.StyleType == DimensionStyleType.AlignmentStationLabel && sdt.Name.Contains(minorStationSetLabelTypeName));

                SpotDimensionType curvatureLabelType = fec.OfClass(typeof(SpotDimensionType))
                                                       .Cast <SpotDimensionType>()
                                                       .FirstOrDefault(sdt => sdt.StyleType == DimensionStyleType.AlignmentStationLabel && sdt.Name.Contains(horizontalCurvatureChangeLabelTypeName));

                using (Transaction t = new Transaction(document, "Create major station labels"))
                {
                    t.Start();
                    if (majorLabelType == null)
                    {
                        // create major station label type with the given arrowhead style
                        majorLabelType = document.GetElement(AlignmentStationLabel.CreateRecommendedTypeForSet(document)) as SpotDimensionType;
                        majorLabelType.get_Parameter(BuiltInParameter.SPOT_ELEV_LEADER_ARROWHEAD).Set(majorStationLeaderArrowheadType.Id);
                        majorLabelType.Name = majorStationSetLabelTypeName;
                    }

                    if (minorLabelType == null)
                    {
                        // create minor station label type with the given arrowhead style
                        // exclude the station text, which leave only the arrowheads
                        // make the minor station's color grey
                        // make the text 60% of the original value, in case text is later turned on
                        minorLabelType = document.GetElement(AlignmentStationLabel.CreateRecommendedTypeForSet(document)) as SpotDimensionType;
                        minorLabelType.get_Parameter(BuiltInParameter.SPOT_ELEV_LEADER_ARROWHEAD).Set(minorStationLeaderArrowheadType.Id);
                        minorLabelType.get_Parameter(BuiltInParameter.ALIGNMENT_STATION_LABEL_INCLUDE_STATION).Set(0);
                        minorLabelType.get_Parameter(BuiltInParameter.LINE_COLOR).Set(8421504 /* 127*2^0 + 127*2^8 + 127*2^16: grey */);
                        Parameter textSizeParam = minorLabelType.get_Parameter(BuiltInParameter.TEXT_SIZE);
                        textSizeParam.Set(textSizeParam.AsDouble() * 0.6);
                        minorLabelType.Name = minorStationSetLabelTypeName;
                    }

                    if (curvatureLabelType == null)
                    {
                        // create a new label type, based on the default alignment station label type,
                        // but with some adjustments to the label contents, as described below
                        ElementType defaultAlignmentLabelType = document.GetElement(
                            document.GetDefaultElementTypeId(ElementTypeGroup.AlignmentStationLabelType)) as ElementType;

                        curvatureLabelType = defaultAlignmentLabelType.Duplicate(horizontalCurvatureChangeLabelTypeName) as SpotDimensionType;

                        curvatureLabelType.get_Parameter(BuiltInParameter.SPOT_COORDINATE_BASE).Set(1);              // "Shared" coordinate base

                        // Label position and content
                        curvatureLabelType.get_Parameter(BuiltInParameter.SPOT_ELEV_ROTATE_WITH_COMPONENT).Set(0);         // do not rotate with component
                        curvatureLabelType.get_Parameter(BuiltInParameter.SPOT_ELEV_TEXT_ORIENTATION).Set(0);              // horizontal text
                        curvatureLabelType.get_Parameter(BuiltInParameter.SPOT_ELEV_TEXT_LOCATION).Set(0);                 // text location above leader
                        curvatureLabelType.get_Parameter(BuiltInParameter.ALIGNMENT_STATION_LABEL_INCLUDE_STATION).Set(1); // include station
                        curvatureLabelType.get_Parameter(BuiltInParameter.SPOT_COORDINATE_INCLUDE_ELEVATION).Set(0);       // do not include elevation
                        curvatureLabelType.get_Parameter(BuiltInParameter.SPOT_ELEV_BOT_VALUE).Set(0);                     // do not include bottom value
                        curvatureLabelType.get_Parameter(BuiltInParameter.SPOT_ELEV_TOP_VALUE).Set(0);                     // do not include top value
                        curvatureLabelType.get_Parameter(BuiltInParameter.ALIGNMENT_STATION_LABEL_IND_STATION).Set("");    // empty station indicator

                        // Text
                        curvatureLabelType.get_Parameter(BuiltInParameter.DIM_TEXT_BACKGROUND).Set(0);                              // nontransparent text
                        curvatureLabelType.get_Parameter(BuiltInParameter.LINE_COLOR).Set(255 /* 255*2^0 + 0*2^8 + 0*2^16: red */); // text in red color
                        Parameter textSizeParam = curvatureLabelType.get_Parameter(BuiltInParameter.TEXT_SIZE);
                        textSizeParam.Set(textSizeParam.AsDouble() * 0.6);                                                          // text size 60% of default

                        // Leader
                        curvatureLabelType.get_Parameter(BuiltInParameter.SPOT_ELEV_LEADER_ARROWHEAD).Set(ElementId.InvalidElementId); // no leader arrowhead
                    }
                    t.Commit();
                }

                // create major and minor station label sets
                ElementId alignmentId = AlignmentSelectionFilter.SelectAlignment(document);
                Alignment alignment   = Alignment.Get(document.GetElement(alignmentId));

                // start placement from a multiple of the major station interval
                // make sure to compute the multiple in the proper unit system and then convert it back to internal units for further use
                double labelSetsPlacementStartStation = UnitUtils.ConvertFromInternalUnits(alignment.DisplayedStartStation, UnitTypeId.StationingMeters);
                labelSetsPlacementStartStation =
                    Math.Ceiling(labelSetsPlacementStartStation / majorStationInterval) * majorStationInterval;
                labelSetsPlacementStartStation =
                    UnitUtils.ConvertToInternalUnits(labelSetsPlacementStartStation, UnitTypeId.StationingMeters);

                var majorStations = new List <double>();
                using (Transaction t = new Transaction(document, "Create major station labels"))
                {
                    t.Start();
                    AlignmentStationLabelSetOptions options = new AlignmentStationLabelSetOptions();
                    options.Interval     = UnitUtils.ConvertToInternalUnits(majorStationInterval, UnitTypeId.StationingMeters);
                    options.Offset       = UnitUtils.ConvertToInternalUnits(labelTextOffset, UnitTypeId.StationingMeters);
                    options.StartStation = labelSetsPlacementStartStation;
                    options.EndStation   = alignment.DisplayedEndStation;
                    options.TypeId       = majorLabelType.Id;

                    var labels = AlignmentStationLabel.CreateSet(alignment, view, options);
                    foreach (var label in labels)
                    {
                        majorStations.Add(label.Station);
                    }
                    t.Commit();
                }

                using (Transaction t = new Transaction(document, "Create minor station labels"))
                {
                    t.Start();
                    AlignmentStationLabelSetOptions options = new AlignmentStationLabelSetOptions();
                    options.Interval = UnitUtils.ConvertToInternalUnits(minorStationInterval, UnitTypeId.StationingMeters);
                    // setting text offset specification can be skipped,
                    // as in this example the minor station labels do not include any label text, only leader arrowheads
                    options.StartStation = labelSetsPlacementStartStation;
                    options.EndStation   = labelSetsPlacementStartStation + UnitUtils.ConvertToInternalUnits(majorStationInterval, UnitTypeId.StationingMeters);
                    options.TypeId       = minorLabelType.Id;

                    // delete the minor station labels which overlap with the major ones
                    var labels = AlignmentStationLabel.CreateSet(alignment, view, options);
                    foreach (var label in labels)
                    {
                        foreach (var majorStation in majorStations)
                        {
                            if (MathComparisonUtils.IsAlmostEqual(label.Station, majorStation))
                            {
                                label.Element.Pinned = false;
                                document.Delete(label.Element.Id);
                                break;
                            }
                        }
                    }

                    t.Commit();
                }

                if (view.ViewType == ViewType.FloorPlan || view.ViewType == ViewType.CeilingPlan || view.ViewType == ViewType.EngineeringPlan)
                {
                    IList <HorizontalCurveEndpoint> curveEndpoints = alignment.GetDisplayedHorizontalCurveEndpoints();
                    using (TransactionGroup tg = new TransactionGroup(document, "Create horizontal curvature changes labels"))
                    {
                        tg.Start();

                        double previousStation = alignment.DisplayedStartStation;
                        foreach (var curveEndpoint in curveEndpoints)
                        {
                            using (Transaction t = new Transaction(document, "Create one horizontal curvature change label"))
                            {
                                double thisStation = curveEndpoint.Station;
                                // skip placing curvature labels at the start and end points of the alignment
                                if (MathComparisonUtils.IsAlmostEqual((alignment.DisplayedStartStation), thisStation) ||
                                    MathComparisonUtils.IsAlmostEqual((alignment.DisplayedEndStation), thisStation))
                                {
                                    continue;
                                }

                                t.Start();

                                AlignmentStationLabelOptions options = new AlignmentStationLabelOptions(thisStation);
                                options.HasLeader = false;
                                options.TypeId    = curvatureLabelType.Id;

                                AlignmentStationLabel label = AlignmentStationLabel.Create(alignment, view, options);

                                // regeneration is necessary before the label's positional properties (such as Origin)L can be properly evaluated
                                document.Regenerate();

                                // set the shoulder and end to coincide, creating a leader pointing along the view's up direction
                                SpotDimension dim = label.Element as SpotDimension;

                                XYZ leaderDirection = view.UpDirection;
                                // compute the distance to the previous label
                                // if the previous label is too close, flip the placement direction
                                {
                                    var    dimBBox   = dim.get_BoundingBox(view);
                                    double dimOffset = Math.Abs(dimBBox.Max.X - dimBBox.Min.X);
                                    if (MathComparisonUtils.IsGreaterThanOrAlmostEqual(dimOffset, thisStation - previousStation))
                                    {
                                        leaderDirection = leaderDirection.Negate();
                                    }
                                }

                                dim.HasLeader              = true;
                                dim.LeaderHasShoulder      = true;
                                dim.LeaderShoulderPosition = dim.Origin +
                                                             leaderDirection * UnitUtils.ConvertToInternalUnits(labelTextOffset, UnitTypeId.StationingMeters) * view.Scale;
                                dim.LeaderEndPosition = dim.LeaderShoulderPosition;
                                dim.TextPosition      = dim.LeaderShoulderPosition;

                                previousStation = thisStation;
                                t.Commit();
                            }
                        }
                        tg.Assimilate();
                    }
                }

                return(Result.Succeeded);
            }
            catch (Exception ex)
            {
                message = ex.Message;
                return(Result.Failed);
            }
        }