Windows Phone 7中的数据绑定XML源

Question

我在Windows Phone开发中遇到了这个数据绑定问题。在此代码中找不到错误。但我无法创建成功的数据上下文。

public void storeData()
{
    Uri uri = new Uri("http://api.openweathermap.org/data/2.5/weather?q="+         City.city + "&units=metric&mode=xml", UriKind.RelativeOrAbsolute);
    var client = new WebClient();
    client.DownloadStringCompleted += (sender, e) =>
    {
        string xml = e.Result.ToString();
        StringReader stream = new StringReader(xml);
        XmlReader reader = XmlReader.Create(stream);
        while (reader.Read())
        {
            if (reader.NodeType == XmlNodeType.Element && reader.Name == "temperature")
            {
                temp = reader.GetAttribute(0).ToString();
            }
            else if (reader.NodeType == XmlNodeType.Element && reader.Name == "weather")
            {
                condition = reader.GetAttribute(1).ToString();
            }
        }

        weather = new Weather { 
            CityName = City.city, 
            Temperature = temp, 
            Condition = condition     
        };
        DataContext = weather;
    };
    client.DownloadStringAsync(uri);
}