ArcGIS Pro 2.8 API Reference Guide
Area Method (GeometryEngine)
Example 

ArcGIS.Core.Geometry Namespace > GeometryEngine Class : Area Method
The geometry of which the area will be calculated.
Gets the area of the geometry. This is a planar measurement using 2D Cartesian mathematics to compute the area.
Syntax
public double Area( 
   Geometry geometry
)
Public Function Area( _
   ByVal geometry As Geometry _
) As Double

Parameters

geometry
The geometry of which the area will be calculated.

Return Value

The computed area in the same units as the geometry's spatial reference unit. If the geometry is empty, then zero is returned.
Exceptions
ExceptionDescription
Geometry is null.
The method is not implemented for GeometryBag.
Example
var g1 = PolygonBuilder.FromJson("{\"rings\": [ [ [0, 0], [10, 0], [10, 10], [0, 10] ] ] }");
double d = GeometryEngine.Instance.Area(g1);
// d = -100.0         //negative due to wrong ring orientation
d = GeometryEngine.Instance.Area(GeometryEngine.Instance.SimplifyAsFeature(g1));
// d = 100.0        // feature has been simplifed; ring orientation is correct
List<Coordinate2D> outerCoordinates = new List<Coordinate2D>();
outerCoordinates.Add(new Coordinate2D(10.0, 10.0));
outerCoordinates.Add(new Coordinate2D(10.0, 20.0));
outerCoordinates.Add(new Coordinate2D(20.0, 20.0));
outerCoordinates.Add(new Coordinate2D(20.0, 10.0));

// define the inner polygon as anti-clockwise
List<Coordinate2D> innerCoordinates = new List<Coordinate2D>();
innerCoordinates.Add(new Coordinate2D(13.0, 13.0));
innerCoordinates.Add(new Coordinate2D(17.0, 13.0));
innerCoordinates.Add(new Coordinate2D(17.0, 17.0));
innerCoordinates.Add(new Coordinate2D(13.0, 17.0));

// Builder constructors need to run on the MCT.
ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
{
  // use the PolygonBuilder as we wish to manipulate the parts
  using (PolygonBuilder pb = new PolygonBuilder(outerCoordinates))
  {
    Polygon donut = pb.ToGeometry();
    double area = donut.Area;       // area = 100

    pb.AddPart(innerCoordinates);
    donut = pb.ToGeometry();

    area = donut.Area;    // area = 84.0

    area = GeometryEngine.Instance.Area(donut);    // area = 84.0
  }
});


// builderEx constructors don't need to run on the MCt
PolygonBuilderEx pbEx = new PolygonBuilderEx(outerCoordinates);
Polygon donutEx = pbEx.ToGeometry() as Polygon;
double areaEx = donutEx.Area;       // area = 100

pbEx.AddPart(innerCoordinates);
donutEx = pbEx.ToGeometry() as Polygon;

areaEx = donutEx.Area;    // area = 84.0

areaEx = GeometryEngine.Instance.Area(donutEx);    // area = 84.0

Requirements

Target Platforms: Windows 10, Windows 8.1

See Also

Reference

GeometryEngine Class
GeometryEngine Members