public bool Within( 
   Geometry geometry1,
   Geometry geometry2
)

/// <summary>
/// The methods retrieves the outer ring(s) of the input polygon.
/// </summary>
/// <param name="inputPolygon">Input Polygon.</param>
/// <returns>The outer most (exterior, clockwise) ring(s) of the polygon. If the input is null or empty, a null pointer is returned.</returns>
public Polygon GetOutermostRings(Polygon inputPolygon)
{
  if (inputPolygon == null || inputPolygon.IsEmpty)
    return null;

  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(inputPolygon);

  // 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);
  }

  // return the final geometry of the outer rings
  return outerRings.ToGeometry();
}

// build a polygon      
List<MapPoint> pts = new List<MapPoint>();
pts.Add(MapPointBuilderEx.CreateMapPoint(1.0, 1.0));
pts.Add(MapPointBuilderEx.CreateMapPoint(1.0, 2.0));
pts.Add(MapPointBuilderEx.CreateMapPoint(2.0, 2.0));
pts.Add(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 = new List<MapPoint>();
innerLinePts.Add(innerPt);
innerLinePts.Add(innerPt2);

Polyline 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 = new List<MapPoint>();
crossingLinePts.Add(innerPt);
crossingLinePts.Add(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