我正在编写一个C＃UWP程序来跟踪天气数据。当试图访问WeatherPage类中的行Debug.WriteLine("DEBUG IN WEATHERPAGE: " + myForecast.mysortedDays[1][1].desc);时，我得到“对象引用未设置为对象的实例”。 sortedDays是一个嵌套列表，我需要能够完全访问。我知道在我的GetWeather方法中sortedDays正在填充，但在从另一个类引用时为null。我觉得这是一个调用setter方法的情况，但不是我无法用链表成功完成这个
namespace WeatherForecast
{
/// <summary>
/// An empty page that can be used on its own or navigated to within a Frame.
/// </summary>
public sealed partial class WeatherPage : Page
{
Forecast myForecast;
public WeatherPage()
{
this.InitializeComponent();
myForecast = new Forecast();
// Task task =myForecast.GetWeather("id=2964179");
myForecast.GetWeather("id=2964179");
Debug.WriteLine("DEBUG IN WEATHERPAGE: " + myForecast.sortedDays[1][1].desc);
}
}
}
namespace WeatherForecast
{
class Forecast
{
public List<List<WeatherController>> sortedDays { get; set; }
public RootObject result { get; set; }
public Forecast()
{
}
public async void GetWeather(string cCode)
{
// DEBUG
Debug.WriteLine("DEBUG: Started getWeather");
string cityCode = cCode;
string apiKey = "myapikey";
// string cityCode = "id=2964179";
string url = "myurl" + cityCode + apiKey;
var uri = new Uri(url);
using (HttpClient client = new HttpClient())
{
using (HttpResponseMessage response = await client.GetAsync(uri))
{
using (IHttpContent content = response.Content)
{
var json = await content.ReadAsStringAsync();
result = JsonConvert.DeserializeObject<RootObject>(json);
// SortWeather();
// create a list of weatherController lists to hold each day
// made public for global access
List<List<WeatherController>> sortedDays = new List<List<WeatherController>>();
// sortedDays = new List<List<WeatherController>>();
//create a list of weatherController objects to hold each hourly interval
List<WeatherController> sortedHours = new List<WeatherController>();
// a base time
DateTime prevDate = Convert.ToDateTime("2000-01-01");
int counter = 0;
// iterate through result list
for (int i = 0; i < result.list.Count(); i++)
{
// if the date is greater than the previous date add the sortedHours to sortedDays
if (Convert.ToDateTime(result.list[counter].dt_txt).Date > prevDate.Date && counter != 0)
{
sortedDays.Add(sortedHours);
sortedHours = new List<WeatherController>();
}
WeatherController wController = new WeatherController
{
dtime = result.list[counter].dt_txt,
dayOfWeek = (Convert.ToDateTime(result.list[counter].dt_txt).DayOfWeek).ToString(),
temp = result.list[counter].main.temp,
humidity = result.list[counter].main.humidity,
desc = result.list[counter].weather[0].description,
windSpeed = result.list[counter].wind.speed
};
sortedHours.Add(wController);
prevDate = Convert.ToDateTime(result.list[counter].dt_txt);
counter++;
}
// add any left over sortedHours to sortedDays
if (sortedHours != null)
{
sortedDays.Add(sortedHours);
}
Debug.WriteLine("DEBUG: Finished getWeather");
}
}
}
}
}
}
答案 0 :(得分：1)
您正在分配具有相同名称的本地变量。 见A点 所以基本上你从来没有为预测的assigne sortedDays属性。您必须执行 Point B 中的代码 或添加以下行：
this.sortedDays = sortedDays;
namespace WeatherForecast
{
/// <summary>
/// An empty page that can be used on its own or navigated to within a Frame.
/// </summary>
public sealed partial class WeatherPage : Page
{
Forecast myForecast;
public WeatherPage()
{
this.InitializeComponent();
myForecast = new Forecast();
// Task task =myForecast.GetWeather("id=2964179");
// POINT C
myForecast.GetWeather("id=2964179").GetAwaiter().GetResult();
//POINT D
Debug.WriteLine("DEBUG IN WEATHERPAGE: " + myForecast.sortedDays[1][1].desc);
}
}
}
namespace WeatherForecast
{
class Forecast
{
public List<List<WeatherController>> sortedDays { get; set; }
public RootObject result { get; set; }
public Forecast()
{
}
public async Task GetWeather(string cCode)
{
// DEBUG
Debug.WriteLine("DEBUG: Started getWeather");
string cityCode = cCode;
string apiKey = "myapikey";
// string cityCode = "id=2964179";
string url = "myurl" + cityCode + apiKey;
var uri = new Uri(url);
using (HttpClient client = new HttpClient())
{
using (HttpResponseMessage response = await client.GetAsync(uri))
{
using (IHttpContent content = response.Content)
{
var json = await content.ReadAsStringAsync();
result = JsonConvert.DeserializeObject<RootObject>(json);
// SortWeather();
// create a list of weatherController lists to hold each day
// made public for global access
// POINT A
List<List<WeatherController>> sortedDays = new List<List<WeatherController>>();
// POINT B
// sortedDays = new List<List<WeatherController>>();
//create a list of weatherController objects to hold each hourly interval
List<WeatherController> sortedHours = new List<WeatherController>();
// a base time
DateTime prevDate = Convert.ToDateTime("2000-01-01");
int counter = 0;
// iterate through result list
for (int i = 0; i < result.list.Count(); i++)
{
// if the date is greater than the previous date add the sortedHours to sortedDays
if (Convert.ToDateTime(result.list[counter].dt_txt).Date > prevDate.Date && counter != 0)
{
sortedDays.Add(sortedHours);
sortedHours = new List<WeatherController>();
}
WeatherController wController = new WeatherController
{
dtime = result.list[counter].dt_txt,
dayOfWeek = (Convert.ToDateTime(result.list[counter].dt_txt).DayOfWeek).ToString(),
temp = result.list[counter].main.temp,
humidity = result.list[counter].main.humidity,
desc = result.list[counter].weather[0].description,
windSpeed = result.list[counter].wind.speed
};
sortedHours.Add(wController);
prevDate = Convert.ToDateTime(result.list[counter].dt_txt);
counter++;
}
// add any left over sortedHours to sortedDays
if (sortedHours != null)
{
sortedDays.Add(sortedHours);
}
Debug.WriteLine("DEBUG: Finished getWeather");
}
}
}
}
}
}
<强> 编辑：
此外，您正在使用async void方法，因此您无需等待任务，因此您将在GetWeather方法结束之前到达 POINT D 中的行。
您可以更改GetWeather方法以返回任务，然后使用 POINT C 同步调用它。虽然我会在Loading或Loaded等事件中执行此操作并使用该语句
await myForecast.GetWeather("id=2964179");
谢谢，