Geometry ImportFromJson( 
   JsonImportFlags importFlags,
   string jsonString
)

// MapPoint
string inputString = 
  "{\"x\":1,\"y\":2,\"spatialReference\":{\"wkid\":4326,\"latestWkid\":4326}}";
Geometry geometry = GeometryEngine.Instance.ImportFromJson(
  JsonImportFlags.JsonImportDefaults, inputString);

MapPoint importPoint = geometry as MapPoint;
// importPoint = 1, 2
// importPoint.SpatialReference.WKid = 4326

// use the MapPointBuilderEx convenience method
MapPoint importPoint2 = MapPointBuilderEx.FromJson(inputString);
// importPoint2 = 1, 2
// impointPoint2.SpatialReference.Wkid = 4326

string outputString = GeometryEngine.Instance.ExportToJson(
  JsonExportFlags.JsonExportDefaults, importPoint);
// outputString =
//   "{\"x\":1,\"y\":2,\"spatialReference\":{\"wkid\":4326,\"latestWkid\":4326}}"

string outputString2 = importPoint.ToJson();

inputString = 
  "{\"spatialReference\":{\"wkid\":4326},\"z\":3,\"m\":4,\"x\":1,\"y\":2}";
importPoint = GeometryEngine.Instance.ImportFromJson(
  JsonImportFlags.JsonImportDefaults, inputString) as MapPoint;
// importPoint.HasM = true
// importPoint.HasZ = true
// importPoint.X = 1
// importPoint.Y = 2
// importPoint.M = 4
// importPoint.Z = 3

importPoint2 = MapPointBuilderEx.FromJson(inputString);

// export to json - skip spatial reference
outputString = GeometryEngine.Instance.ExportToJson(
              JsonExportFlags.JsonExportSkipCRS, importPoint);
// outputString = "{\"x\":1,\"y\":2,\"z\":3,\"m\":4}"

// export from mappoint, skipping the sr - same as GeometryEngine.Instance.ExportToJson w JsonExportFlags.JsonExportSkipCRS
outputString2 = importPoint.ToJson(true);

//
// Multipoint
//
List<Coordinate2D> coords = new List<Coordinate2D>()
{
  new Coordinate2D(100, 200),
  new Coordinate2D(201, 300),
  new Coordinate2D(301, 400),
  new Coordinate2D(401, 500)
};

Multipoint multipoint = MultipointBuilderEx.CreateMultipoint(
  coords, SpatialReferences.WebMercator);

inputString = 
  "{\"points\":[[100,200],[201,300],[301,400],[401,500]],\"spatialReference\":{\"wkid\":3857}}";
Multipoint importMultipoint = 
  GeometryEngine.Instance.ImportFromJson(
         JsonImportFlags.JsonImportDefaults, inputString) as Multipoint;
// importMultipoint.IsEqual(multipoint) = true

ReadOnlyPointCollection points = importMultipoint.Points;
// points.Count = 4
// points[0] = 100, 200
// points[1] = 201, 300
// points[2] = 301, 400
// points[3] = 401, 500

// use the MultipointbuilderEx convenience method
Multipoint importMultipoint2 = MultipointBuilderEx.FromJson(inputString);
// importMultipoint2.IsEqual(multipoint) = true

// export to json
outputString = GeometryEngine.Instance.ExportToJson(
                JsonExportFlags.JsonExportDefaults, multipoint);
// outputString = inputString

// or use the multipoint itself
outputString2 = multipoint.ToJson();

//
// Polyline
//
Polyline polyline = PolylineBuilderEx.CreatePolyline(
                         coords, SpatialReferences.WebMercator);

// export without the spatial reference
outputString = GeometryEngine.Instance.ExportToJson(
                        JsonExportFlags.JsonExportSkipCRS, polyline);
// import
geometry = GeometryEngine.Instance.ImportFromJson(
             JsonImportFlags.JsonImportDefaults, outputString);
Polyline importPolyline = geometry as Polyline;
// importPolyline.SpatialReference = null


points = importPolyline.Points;
// points.Count = 4
// points[0] = 100, 200
// points[1] = 201, 300
// points[2] = 301, 400
// points[3] = 401, 500

// use the polylineBuilderEx convenience method 
Polyline importPolyline2 = PolylineBuilderEx.FromJson(outputString);
// importPolyline2 = importPolyline

outputString2 = importPolyline2.ToJson();
// outputString2 = outputString

//
// Polygon
//
Polygon polygon = PolygonBuilderEx.CreatePolygon(
                              coords, SpatialReferences.WebMercator);

// export without the spatial reference
outputString = GeometryEngine.Instance.ExportToJson(
                          JsonExportFlags.JsonExportSkipCRS, polygon);
// import
geometry = GeometryEngine.Instance.ImportFromJson(
                     JsonImportFlags.JsonImportDefaults, outputString);

Polygon importPolygon = geometry as Polygon;
// importPolygon.SpatialReference = null
points = importPolygon.Points;
// points.Count = 5

// polygonBuilderEx convenience method
Polygon importPolyon2 = PolygonBuilderEx.FromJson(outputString);
// importPolygon2 = importPolygon

// export from the polygon
outputString2 = importPolyon2.ToJson(true);

// Empty polygon
polygon = PolygonBuilderEx.CreatePolygon(SpatialReferences.WebMercator);
outputString = GeometryEngine.Instance.ExportToJson(
                               JsonExportFlags.JsonExportDefaults, polygon);
importPolygon = GeometryEngine.Instance.ImportFromJson(
                    JsonImportFlags.JsonImportDefaults, outputString) as Polygon;

// importPolygon.IsEmpty = true
// importPolygon.SpatialReference.Wkid = 3857