ReferenceIntersector

<Autodesk.Revit.Attributes.Journaling(Autodesk.Revit.Attributes.JournalingMode.NoCommandData)> _
<Autodesk.Revit.Attributes.Transaction(Autodesk.Revit.Attributes.TransactionMode.Manual)> _
Public Class RayProjection
   Implements IExternalCommand
   Public Function Execute(revit As ExternalCommandData, ByRef message As String, elements As ElementSet) As Autodesk.Revit.UI.Result Implements IExternalCommand.Execute
      Dim doc As Document = revit.Application.ActiveUIDocument.Document

      Dim selectedIds As ICollection(Of ElementId) = revit.Application.ActiveUIDocument.Selection.GetElementIds()

      ' If skylight is selected, process it.
      Dim skylight As FamilyInstance = Nothing
      If selectedIds.Count = 1 Then
         For Each id As ElementId In selectedIds
            Dim e As Element = doc.GetElement(id)
            If TypeOf e Is FamilyInstance Then
               Dim instance As FamilyInstance = TryCast(e, FamilyInstance)
               Dim isWindow As Boolean = instance.Category.BuiltInCategory = BuiltInCategory.OST_Windows
               Dim isHostedByRoof As Boolean = instance.Host.Category.BuiltInCategory = BuiltInCategory.OST_Roofs

               If isWindow AndAlso isHostedByRoof Then
                  skylight = instance
               End If
            End If
         Next
      End If

      If skylight Is Nothing Then
         message = "Please select one skylight."
         Return Result.Cancelled
      End If

      ' Calculate the height
      Dim line As Line = CalculateLineAboveFloor(doc, skylight)

      ' Create a model curve to show the distance
      Dim plane As Plane = plane.CreateByNormalAndOrigin(New XYZ(1, 0, 0), line.GetEndPoint(0))
      Dim sketchPlane__1 As SketchPlane = SketchPlane.Create(doc, plane)

      Dim curve As ModelCurve = doc.Create.NewModelCurve(line, sketchPlane__1)

      ' Show a message with the length value
      TaskDialog.Show("Distance", "Distance to floor: " & [String].Format("{0:f2}", line.Length))

      Return Result.Succeeded
   End Function

   ' <summary>
   ' Determines the line segment that connects the skylight to the nearest floor.
   ' </summary>
   ' <returns>The line segment.</returns>
   Private Function CalculateLineAboveFloor(doc As Document, skylight As FamilyInstance) As Line
      ' Find a 3D view to use for the ReferenceIntersector constructor
      Dim collector As New FilteredElementCollector(doc)
      Dim isNotTemplate As Func(Of View3D, Boolean) = Function(v3) Not (v3.IsTemplate)
      Dim view3D As View3D = collector.OfClass(GetType(View3D)).Cast(Of View3D)().First(isNotTemplate)

      ' Use the center of the skylight bounding box as the start point.
      Dim box As BoundingBoxXYZ = skylight.BoundingBox(view3D)
      Dim center As XYZ = box.Min.Add(box.Max).Multiply(0.5)

      ' Project in the negative Z direction down to the floor.
      Dim rayDirection As New XYZ(0, 0, -1)

      Dim filter As New ElementClassFilter(GetType(Floor))

      Dim refIntersector As New ReferenceIntersector(filter, FindReferenceTarget.Face, view3D)
      Dim referenceWithContext As ReferenceWithContext = refIntersector.FindNearest(center, rayDirection)

      Dim reference As Reference = referenceWithContext.GetReference()
      Dim intersection As XYZ = reference.GlobalPoint

      ' Create line segment from the start point and intersection point.
      Dim result As Line = Line.CreateBound(center, intersection)
      Return result
   End Function
End Class