Get User Location

Hi, I have an object with a field distance like this:

public double distance {
            get {
                var pos1= //USER LOCATION
                var pos2=new Xamarin.Forms.Labs.Services.Geolocation.Position()
                {
                    Latitude = x, //attribute of the object
                    Longitude = y
                };
                return PositionExtensions.DistanceFrom (pos1,pos2);
            }
        }

How can I retrieve the user location?