private void CreateOverlay()
{
// Create polygon builder and add polygon corners into it
PolygonBuilder builder = new PolygonBuilder(SpatialReferences.WebMercator);
builder.AddPoint(new MapPoint(-20e5, 20e5));
builder.AddPoint(new MapPoint(20e5, 20e5));
builder.AddPoint(new MapPoint(20e5, -20e5));
builder.AddPoint(new MapPoint(-20e5, -20e5));
// Get geometry from the builder
Polygon polygonGeometry = builder.ToGeometry();
// Create symbol for the polygon
SimpleFillSymbol polygonSymbol = new SimpleFillSymbol(
SimpleFillSymbolStyle.Solid,
System.Drawing.Color.Yellow,
null);
// Create new graphic
Graphic polygonGraphic = new Graphic(polygonGeometry, polygonSymbol);
// Create overlay to where graphics are shown
_polygonOverlay = new GraphicsOverlay();
_polygonOverlay.Graphics.Add(polygonGraphic);
// Add created overlay to the MapView
_myMapView.GraphicsOverlays.Add(_polygonOverlay);
}