ArcGIS Pro 2.8 API Reference Guide
ReverseOrientation Method (IGeometryEngine)
Example 

ArcGIS.Core.Geometry Namespace > IGeometryEngine Interface : ReverseOrientation Method
The polygon or polyline whose orientation will be reversed.
Reverse the orientation of the geometry.
Syntax
Multipart ReverseOrientation( 
   Multipart multipart
)
Function ReverseOrientation( _
   ByVal multipart As Multipart _
) As Multipart

Parameters

multipart
The polygon or polyline whose orientation will be reversed.

Return Value

A geometry whose directionality has been reversed.
Exceptions
ExceptionDescription
Multipart is null or empty.
Remarks
Geometry must be either a Polygon or a Polyline. ReverseOrientation changes the direction of the curve without changing the spatial position of the curve. The From Point and To Point of each Segment are interchanged.
Caution should be taken in using ReverseOrientation on Polygons. Since ReverseOrientation changes the direction of each Ring within the Polygon, all Exterior Rings become Interior Rings and vice versa.

GeometryEngine Reverse Orientation

Example
List<Coordinate2D> list2D = new List<Coordinate2D>();
list2D.Add(new Coordinate2D(1.0, 1.0));
list2D.Add(new Coordinate2D(1.0, 2.0));
list2D.Add(new Coordinate2D(2.0, 2.0));
list2D.Add(new Coordinate2D(2.0, 1.0));

Polygon polygon = PolygonBuilder.CreatePolygon(list2D);

Geometry g = GeometryEngine.Instance.ReverseOrientation(polygon);
Polygon gPolygon = g as Polygon;

// gPolygon.Points[0] = 1.0, 1.0
// gPolygon.Points[1] = 2.0, 1.0
// gPolygon.Points[2] = 2.0, 2.0
// gPolygon.Points[3] = 1.0, 2.0
// gPolygon.Points[4] = 1.0, 1.0
// gPolygon.Area = -1 * polygon.Area
Requirements

Target Platforms: Windows 10, Windows 8.1

See Also

Reference

IGeometryEngine Interface
IGeometryEngine Members