https://backendless.com/documentation/users/dotnet/users_relations_with_geo_points.htm
In the example mentioned there:
List<GeoPoint> locations = new List<GeoPoint>(); locations.Add( geoPoint1 ); locations.Add( geoPoint2 );
//This is not working user.AddProperty( “locations”, locations );
//This is working
user.AddProperty( “locations”, locations.ToArray() );