Create a Filled Region to use as a Mask

I am off in the mountains again today, after a long break due to bad weather and too many other activities.

Meanwhile, here is a pretty old item that has been hanging around in my to-do list for much too long, so let's get it out there to you:

Question: How can I programmatically create a masked region?

Answer: I am sorry to say you currently cannot.

Hoewever, one of the API features added in Revit 2013 was the abiility to create a filled region.

The FilledRegion class was extended to offer the ability to create a new filled region, get its boundaries, and apply a linestyle to all its boundary segments.

So, while there is no straightforward method to create a mask region, the closest we can get is by creating a filled region using the FilledRegion.Create method. However, a filled region is not the same object as a mask region. A filled region has a fill pattern, and its type can be changed. The mask region type cannot be changed. If you set a filled region's fill pattern to a solid blank pattern, it will look like a mask region.

Here is a sample code fragment showing how to create a FilledRegion object:

  FilteredElementCollector fillRegionTypes
    = new FilteredElementCollector( doc )
      .OfClass( typeof( FilledRegionType ) );
  IEnumerable<FilledRegionType> myPatterns =
      from pattern in fillRegionTypes.Cast<FilledRegionType>()
      where pattern.Name.Equals( "Diagonal Crosshatch" )
      select pattern;
  foreach( FilledRegionType frt in fillRegionTypes )
    List<CurveLoop> profileloops
      = new List<CurveLoop>();
    XYZ[] points = new XYZ[5];
    points[0] = new XYZ( 0.0, 0.0, 0.0 );
    points[1] = new XYZ( 10.0, 0.0, 0.0 );
    points[2] = new XYZ( 10.0, 10.0, 0.0 );
    points[3] = new XYZ( 0.0, 10.0, 0.0 );
    points[4] = new XYZ( 0.0, 0.0, 0.0 );
    CurveLoop profileloop = new CurveLoop();
    for( int i = 0; i < 4; i++ )
      Line line = Line.CreateBound( points[i],
        points[i + 1] );
      profileloop.Append( line );
    profileloops.Add( profileloop );
    ElementId activeViewId = doc.ActiveView.Id;
    FilledRegion filledRegion = FilledRegion.Create(
      doc, frt.Id, activeViewId, profileloops );

You could simulate the creation of a mask region by modifying this to find a filled region type whose fill pattern is blank.