public virtual ReadOnlyPartCollection Parts {get;}

{
  int numParts = polyline.PartCount;
  // get the parts as a readonly collection
  ReadOnlyPartCollection parts = polyline.Parts;
}

{
  ReadOnlyPartCollection polylineParts = polyline.Parts;

  // enumerate the segments to get the length
  double len = 0;
  IEnumerator<ReadOnlySegmentCollection> segmentsPolyline = polylineParts.GetEnumerator();
  while (segmentsPolyline.MoveNext())
  {
    ReadOnlySegmentCollection seg = segmentsPolyline.Current;
    foreach (Segment s in seg)
    {
      len += s.Length;

      // perhaps do something specific per segment type
      switch (s.SegmentType)
      {
        case SegmentType.Line:
          break;
        case SegmentType.Bezier:
          break;
        case SegmentType.EllipticArc:
          break;
      }
    }
  }

  // or use foreach pattern
  foreach (var part in polyline.Parts)
  {
    foreach (var segment in part)
    {
      len += segment.Length;

      // perhaps do something specific per segment type
      switch (segment.SegmentType)
      {
        case SegmentType.Line:
          break;
        case SegmentType.Bezier:
          break;
        case SegmentType.EllipticArc:
          break;
      }
    }
  }
}

{
  // Method 1: Get the start point of the polyline by converting the polyline 
  //    into a collection of points and getting the first point

  // sketchGeometry is a Polyline
  // get the sketch as a point collection
  Geometry sketchGeometry = polyline; // for example
  var pointCol = ((Multipart)sketchGeometry).Points;
  // Get the start point of the line
  var firstPoint = pointCol[0];

  // Method 2: Convert polyline into a collection of line segments 
  //   and get the "StartPoint" of the first line segment.
  var polylineGeom = sketchGeometry as ArcGIS.Core.Geometry.Polyline;
  var polyLineParts = polylineGeom.Parts;

  ReadOnlySegmentCollection polylineSegments = polyLineParts.First();

  //get the first segment as a LineSegment
  var firsLineSegment = polylineSegments.First() as LineSegment;

  //Now get the start Point
  var startPointOfSegment = firsLineSegment.StartPoint;
}

{
  // get the parts as a readonly collection
  ReadOnlyPartCollection parts = polygon.Parts;
}

{
  int numSegments = 0;
  IEnumerator<ReadOnlySegmentCollection> segments = polygon.Parts.GetEnumerator();
  while (segments.MoveNext())
  {
    ReadOnlySegmentCollection seg = segments.Current;
    numSegments += seg.Count;
    foreach (Segment s in seg)
    {
      // do something with the segment
    }
  }
}

IEnumerable<Geometry> MultipartToSinglePart(Geometry inputGeometry)
{
  // list holding the part(s) of the input geometry
  List<Geometry> singleParts = new List<Geometry>();

  // check if the input is a null pointer or if the geometry is empty
  if (inputGeometry == null || inputGeometry.IsEmpty)
    return singleParts;

  // based on the type of geometry, take the parts/points and add them individually into a list
  switch (inputGeometry.GeometryType)
  {
    case GeometryType.Envelope:
      singleParts.Add(inputGeometry.Clone() as Envelope);
      break;
    case GeometryType.Multipatch:
      singleParts.Add(inputGeometry.Clone() as Multipatch);
      break;
    case GeometryType.Multipoint:
      var multiPoint = inputGeometry as Multipoint;

      foreach (var point in multiPoint.Points)
      {
        // add each point of collection as a standalone point into the list
        singleParts.Add(point);
      }
      break;
    case GeometryType.Point:
      singleParts.Add(inputGeometry.Clone() as MapPoint);
      break;
    case GeometryType.Polygon:
      var polygonNew = inputGeometry as Polygon;

      foreach (var polygonPart in polygonNew.Parts)
      {
        // use the PolygonBuilderEx turning the segments into a standalone 
        // polygon instance
        singleParts.Add(PolygonBuilderEx.CreatePolygon(polygonPart));
      }
      break;
    case GeometryType.Polyline:
      var polylineNew = inputGeometry as Polyline;

      foreach (var polylinePart in polyline.Parts)
      {
        // use the PolylineBuilderEx turning the segments into a standalone
        // polyline instance
        singleParts.Add(PolylineBuilderEx.CreatePolyline(polylinePart));
      }
      break;
    case GeometryType.Unknown:
      break;
    default:
      break;
  }
  return singleParts;
}