public ReadOnlyPointCollection Points {get;}

// get the points as a readonly Collection
ReadOnlyPointCollection pts = polyline.Points;
int numPts = polyline.PointCount;

// OR   get an enumeration of the points
IEnumerator<MapPoint> enumPts = polyline.Points.GetEnumerator();

// OR   get the point coordinates as a readonly list of Coordinate2D
IReadOnlyList<Coordinate2D> coordinates = polyline.Copy2DCoordinatesToList();

// OR   get the point coordinates as a readonly list of Coordinate3D
IReadOnlyList<Coordinate3D> coordinates3D = polyline.Copy3DCoordinatesToList();

// OR   get a subset of the collection as Coordinate2D using preallocated memory

IList<Coordinate2D> coordinate2Ds = new List<Coordinate2D>(10);   // allocate some space
ICollection<Coordinate2D> subsetCoordinates2D = coordinate2Ds;    // assign
pts.Copy2DCoordinatesToList(1, 2, ref subsetCoordinates2D);       // copy 2 elements from index 1 into the allocated list
                                                                  // coordinate2Ds.Count = 2
                                                                  // do something with the coordinate2Ds

// without allocating more space, obtain a different set of coordinates
pts.Copy2DCoordinatesToList(5, 9, ref subsetCoordinates2D);       // copy 9 elements from index 5 into the allocated list
                                                                  // coordinate2Ds.Count = 9


// OR   get a subset of the collection as Coordinate3D using preallocated memory

IList<Coordinate3D> coordinate3Ds = new List<Coordinate3D>(15);   // allocate some space
ICollection<Coordinate3D> subsetCoordinates3D = coordinate3Ds;    // assign
pts.Copy3DCoordinatesToList(3, 5, ref subsetCoordinates3D);       // copy 5 elements from index 3 into the allocated list
                                                                  // coordinate3Ds.Count = 5


// OR   get a subset of the collection as MapPoint using preallocated memory

IList<MapPoint> mapPoints = new List<MapPoint>(7);   // allocate some space
ICollection<MapPoint> subsetMapPoint = mapPoints;    // assign
pts.CopyPointsToList(1, 4, ref subsetMapPoint);      // copy 4 elements from index 1 into the allocated list
                                                     // mapPoints.Count = 4

// 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 points as a readonly Collection
ReadOnlyPointCollection pts = polygon.Points;

// get an enumeration of the points
IEnumerator<MapPoint> enumPts = polygon.Points.GetEnumerator();

// get the point coordinates as a readonly list of Coordinate2D
IReadOnlyList<Coordinate2D> coordinates = polygon.Copy2DCoordinatesToList();

// get the point coordinates as a readonly list of Coordinate3D
IReadOnlyList<Coordinate3D> coordinates3D = polygon.Copy3DCoordinatesToList();