解析Google Directions API时出错（指定的演员表无效）

Question

我是xamarin的新手。我想用json对Google Directions Api的回复进行解析。我为我的对象创建了一个类：

 public class GeocodedWaypoint
{
    public string geocoder_status { get; set; }
    public string place_id { get; set; }
    public List<string> types { get; set; }
}

public class Northeast
{
    public double lat { get; set; }
    public double lng { get; set; }
}

public class Southwest
{
    public double lat { get; set; }
    public double lng { get; set; }
}

public class Bounds
{
    public Northeast northeast { get; set; }
    public Southwest southwest { get; set; }
}

public class Distance
{
    public string text { get; set; }
    public int value { get; set; }
}

public class Duration
{
    public string text { get; set; }
    public int value { get; set; }
}

public class EndLocation
{
    public double lat { get; set; }
    public double lng { get; set; }
}

public class StartLocation
{
    public double lat { get; set; }
    public double lng { get; set; }
}

public class Distance2
{
    public string text { get; set; }
    public int value { get; set; }
}

public class Duration2
{
    public string text { get; set; }
    public int value { get; set; }
}

public class EndLocation2
{
    public double lat { get; set; }
    public double lng { get; set; }
}

public class Polyline
{
    public string points { get; set; }
}

public class StartLocation2
{
    public double lat { get; set; }
    public double lng { get; set; }
}

public class Step
{
    public Distance2 distance { get; set; }
    public Duration2 duration { get; set; }
    public EndLocation2 end_location { get; set; }
    public string html_instructions { get; set; }
    public Polyline polyline { get; set; }
    public StartLocation2 start_location { get; set; }
    public string travel_mode { get; set; }
    public string maneuver { get; set; }
}

public class Leg
{
    public Distance distance { get; set; }
    public Duration duration { get; set; }
    public string end_address { get; set; }
    public EndLocation end_location { get; set; }
    public string start_address { get; set; }
    public StartLocation start_location { get; set; }
    public List<Step> steps { get; set; }
    public List<object> traffic_speed_entry { get; set; }
    public List<object> via_waypoint { get; set; }
}

public class OverviewPolyline
{
    public string points { get; set; }
}

public class Route
{
    public Bounds bounds { get; set; }
    public string copyrights { get; set; }
    public List<Leg> legs { get; set; }
    public OverviewPolyline overview_polyline { get; set; }
    public string summary { get; set; }
    public List<object> warnings { get; set; }
    public List<object> waypoint_order { get; set; }
}

public class DirectionsDto
{
    public List<GeocodedWaypoint> geocoded_waypoints { get; set; }
    public List<Route> routes { get; set; }
    public string status { get; set; }
}

我试图像这样反序列化响应：

JsonValue json = await FetchDataAsync(url);
        DirectionsDto directions = JsonConvert.DeserializeObject<DirectionsDto>(json);

        private async Task<String> FetchDataAsyncXML(string url)
    {
        // Create an HTTP web request using the URL:
        HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(new Uri(url));
        request.ContentType = "application/xml";
        request.Method = "GET";

        // Send the request to the server and wait for the response:
        using (WebResponse response = await request.GetResponseAsync())
        {
            // Get a stream representation of the HTTP web response:
            using (Stream stream = response.GetResponseStream())
            {
                // Use this stream to build a JSON document object:

                return response.GetResponseStream().ToString();

            }
        }
    }

我收到此错误：System.InvalidCastException：指定的强制转换无效。我只需要整个回复中的一件事。我只需要OverviewPolyline。这是在回应的最后。如果使用xml更容易，那么我认为也可以使用xml。

我被困在这里，非常令人沮丧。