public sealed class ReadOnlySegmentCollection : System.Collections.Generic.IEnumerable<Segment>, System.Collections.Generic.IEnumerable<T>, System.Collections.Generic.IReadOnlyCollection<Segment>, System.Collections.IEnumerable  

{
  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;
}

{
  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
    }
  }
}