<Autodesk.Revit.Attributes.Journaling(Autodesk.Revit.Attributes.JournalingMode.NoCommandData)> _
<Autodesk.Revit.Attributes.Transaction(Autodesk.Revit.Attributes.TransactionMode.Automatic)> _
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.Id.IntegerValue = CInt(BuiltInCategory.OST_Windows))
                    Dim isHostedByRoof As Boolean = (instance.Host.Category.Id.IntegerValue = CInt(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 = revit.Application.Application.Create.NewPlane(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