[TransactionAttribute(Autodesk.Revit.Attributes.TransactionMode.Manual)]
public class cmdPickPointIn3d : IExternalCommand
{
    public Result Execute(ExternalCommandData commandData, ref string messages, ElementSet elements)
    {
        UIApplication app = commandData.Application;
        Document doc = app.ActiveUIDocument.Document;

        XYZ point_in_3d;

        if (PickFaceSetWorkPlaneAndPickPoint( app.ActiveUIDocument, out point_in_3d))
        {
            TaskDialog.Show("3D Point Selected",
                "3D point picked on the plane"
                + " defined by the selected face: "
                + PointString(point_in_3d));

            return Result.Succeeded;
        }
        else
        {
            messages = "3D point selection failed";
            return Result.Failed;
        }

        return Result.Succeeded;
    }
    string PointString(XYZ p)
    {
        return string.Format("({0},{1},{2})",
            RealString(p.X),
            RealString(p.Y),
            RealString(p.Z));
    }
    string PointString(UV p)
    {
        return string.Format("({0},{1})",
            RealString(p.U),
            RealString(p.V));
    }
    string RealString(double a)
    {
        return a.ToString("0.##");
    }
    bool PickFaceSetWorkPlaneAndPickPoint(UIDocument uidoc, out XYZ point_in_3d)
    {
        point_in_3d = null;

        Document doc = uidoc.Document;

        Reference r = uidoc.Selection.PickObject(
            ObjectType.Face,
            "Please select a planar face to define work plane");

        Element e = doc.get_Element(r.ElementId);

        if (null != e)
        {
            PlanarFace face
                = e.GetGeometryObjectFromReference(r)
                as PlanarFace;

            if (face != null)
            {
                Plane plane = new Plane(
                    face.Normal, face.Origin);

                Transaction t = new Transaction(doc);

                t.Start("Temporarily set work plane"
                    + " to pick point in 3D");

                SketchPlane sp = doc.Create.NewSketchPlane(
                    plane);

                uidoc.ActiveView.SketchPlane = sp;
                uidoc.ActiveView.ShowActiveWorkPlane();

                try
                {
                    point_in_3d = uidoc.Selection.PickPoint(
                        "Please pick a point on the plane"
                        + " defined by the selected face");
                }
                catch (OperationCanceledException)
                {
                }

                t.RollBack();
            }
        }
        return null != point_in_3d;
    }
}