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> segments = polylineParts.GetEnumerator();
while (segments.MoveNext())
{
  ReadOnlySegmentCollection seg = segments.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
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 startPoint = 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
  }
}