string json = "{\"hasM\":true,\"paths\":[[[-3000,-2000,10],[-2000,-2000,15],[-1000,-2000,20],[0,-2000,0],[1000,-2000,20],[2000,-2000,30],[3000,-2000,10],[4000,-2000,5]]],\"spatialReference\":{\"wkid\":3857}}";
Polyline polyline = PolylineBuilderEx.FromJson(json);
Multipoint multipoint = GeometryEngine.Instance.GetPointsAtM(polyline, 10, 500);
// multiPoint.PointCount = 4
// multipoint.Points[0] X= -3000, Y= -2500 M= 10
// multipoint.Points[1] X= -500, Y= -2500 M= 10
// multipoint.Points[2] X= 500, Y= -2500 M= 10
// multipoint.Points[3] X= 3000, Y= -2500 M= 10