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> 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;

// 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
  }
}

/// <summary>
/// This method takes an input multi part geometry and breaks the parts into individual standalone geometries.
/// </summary>
/// <param name="inputGeometry">The geometry to be exploded into the individual parts.</param>
/// <returns>An enumeration of individual parts as standalone geometries. The type of geometry is maintained, i.e.
/// if the input geometry is of type Polyline then each geometry in the return is of type Polyline as well.
/// If the input geometry is of type Unknown then an empty list is returned.</returns>
public 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 polygon = inputGeometry as Polygon;

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