public bool Within( 
   Geometry geometry1,
   Geometry geometry2
)

{
  List<Polygon> internalRings = new List<Polygon>();

  // explode the parts of the polygon into a list of individual geometries
  // see the "Get the individual parts of a multipart feature"
  // snippet for MultipartToSinglePart
  var parts = MultipartToSinglePart(polygon); //any polygon

  // get an enumeration of clockwise geometries (area > 0) ordered by the area
  var clockwiseParts = parts.Where(geom => ((Polygon)geom).Area > 0)
                        .OrderByDescending(geom => ((Polygon)geom).Area);

  // for each of the exterior rings
  foreach (var part in clockwiseParts)
  {
    // add the first (the largest) ring into the internal collection
    if (internalRings.Count == 0)
      internalRings.Add(part as Polygon);

    // use flag to indicate if current part is within the already selection polygons
    bool isWithin = false;

    foreach (var item in internalRings)
    {
      if (GeometryEngine.Instance.Within(part, item))
        isWithin = true;
    }

    // if the current polygon is not within any polygon of the internal collection
    // then it is disjoint and needs to be added to 
    if (isWithin == false)
      internalRings.Add(part as Polygon);
  }

  PolygonBuilderEx outerRings = new PolygonBuilderEx();
  // now assemble a new polygon geometry based on the internal polygon collection
  foreach (var ring in internalRings)
  {
    outerRings.AddParts(ring.Parts);
  }

  // the final geometry of the outer rings
  var outerRingsResult = outerRings.ToGeometry();
}

{
    // build a polygon      
    List<MapPoint> pts =
    [
      MapPointBuilderEx.CreateMapPoint(1.0, 1.0),
  MapPointBuilderEx.CreateMapPoint(1.0, 2.0),
  MapPointBuilderEx.CreateMapPoint(2.0, 2.0),
  MapPointBuilderEx.CreateMapPoint(2.0, 1.0),
];

    Polygon poly = PolygonBuilderEx.CreatePolygon(pts);

    // an inner point
    MapPoint innerPt = MapPointBuilderEx.CreateMapPoint(1.5, 1.5);
    bool within = GeometryEngine.Instance.Within(innerPt, poly);   // within = true

    // point on a boundary
    within = GeometryEngine.Instance.Within(pts[0], poly);     // within = false

    // an interior line
    MapPoint innerPt2 = MapPointBuilderEx.CreateMapPoint(1.25, 1.75);
    List<MapPoint> innerLinePts = [innerPt, innerPt2];

    polyline = PolylineBuilderEx.CreatePolyline(innerLinePts);
    within = GeometryEngine.Instance.Within(polyline, poly);   // within = true

    // a line that crosses the boundary
    MapPoint outerPt = MapPointBuilderEx.CreateMapPoint(3, 1.5);
    List<MapPoint> crossingLinePts = [innerPt, outerPt];

    polyline = PolylineBuilderEx.CreatePolyline(crossingLinePts);
    within = GeometryEngine.Instance.Within(polyline, poly);     // within = false

    // polygon in polygon
    Envelope env = EnvelopeBuilderEx.CreateEnvelope(innerPt, innerPt2);
    within = GeometryEngine.Instance.Within(env, poly);      // within = true
  }