I think there might be a bug with the GeoJson.Polygon() method. It doesn’t set the BoundingBox property. It leaves it null.
[Fact]
public void MongodbGeoPolygonTest()
{
var firstPosition = GeoJson.Geographic(-74.28919785518953, 40.88312519489477);
var coordinates = new GeoJson2DGeographicCoordinates[]
{
firstPosition,
GeoJson.Geographic(-74.19603266794998, 40.882917293906864),
GeoJson.Geographic(-74.19543240486455, 41.01368469640098),
GeoJson.Geographic(-74.28878156709355, 41.01389355183413),
firstPosition,
};
var polygon = GeoJson.Polygon(coordinates);
Assert.True(polygon.BoundingBox != null);
}