ArcGIS Pro 2.6 API Reference Guide
Project Method (IGeometryEngine)
Example 

ArcGIS.Core.Geometry Namespace > IGeometryEngine Interface : Project Method
The geometry to be projected.
The spatial reference to which the geometry will be projected.
Projects the given geometry to a new spatial reference. Same as GeometryEngine.ProjectEx(geometry, ProjectionTransformation.Create(geometry.SpatialReference, outputSpatialReference)); or, if both spatial references have vertical coordinate systems same as GeometryEngine.ProjectEx(geometry, ProjectionTransformation.CreateWithVertical(geometry.SpatialReference, outputSpatialReference));
Syntax

Parameters

geometry
The geometry to be projected.
outputSpatialReference
The spatial reference to which the geometry will be projected.

Return Value

The projected geometry.
Exceptions
ExceptionDescription
Geometry is null or empty.
The method is not implemented for GeometryBag.
geometry.SpatialReference or output spatial reference is null.
geometry.SpatialReference or output spatial reference is unknown coordinate system.
Example
MapPoint pt = MapPointBuilder.CreateMapPoint(1.0, 3.0, SpatialReferences.WGS84);
Geometry result = GeometryEngine.Instance.Project(pt, SpatialReferences.WebMercator);
MapPoint projectedPt = result as MapPoint;
// create the polygon
List<MapPoint> pts = new List<MapPoint>();
pts.Add(MapPointBuilder.CreateMapPoint(1.0, 1.0, SpatialReferences.WGS84));
pts.Add(MapPointBuilder.CreateMapPoint(1.0, 2.0, SpatialReferences.WGS84));
pts.Add(MapPointBuilder.CreateMapPoint(2.0, 2.0, SpatialReferences.WGS84));
pts.Add(MapPointBuilder.CreateMapPoint(2.0, 1.0, SpatialReferences.WGS84));

Polygon polygon = PolygonBuilder.CreatePolygon(pts);
// ensure it is simple
bool isSimple = GeometryEngine.Instance.IsSimpleAsFeature(polygon);

// create the spatial reference to project to
SpatialReference northPole = SpatialReferenceBuilder.CreateSpatialReference(102018);   // north pole stereographic 

// project
Geometry geometry = GeometryEngine.Instance.Project(polygon, northPole);
Requirements

Target Platforms: Windows 10, Windows 8.1, Windows 7

See Also

Reference

IGeometryEngine Interface
IGeometryEngine Members