UWP-Weather-App.exe中发生类型为'System.NullReferenceException'的异常，但未在用户代码中处理

Question

I've been following a tutorial of a UWP Weather application that uses an API key.

I get a System.NullReferenceException .

This is the method it happens in, at the line that begins with RootObject :

namespace UWP_Weather_App
{
    public sealed partial class MainPage : Page
    {
        public MainPage()
        {
            this.InitializeComponent();
        }

        private async void Button_Click(object sender, RoutedEventArgs e)
        {
            RootObject myWeather = await OpenWeatherMapProxy.GetWeather(20.0, 30.0);
            ResultTextBlock.Text = myWeather.name + " - " + myWeather.main.temp + " - " + myWeather.weather[0].description;
        }
    }
}

Answer 1

According to your description, I'd suppose you are following this UWP Weather tutorial . I used your code with this tutorial's source code and reproduced your issue.

The System.NullReferenceException error occurred at the line ResultTextBlock.Text = myWeather.name + " - " + myWeather.main.temp + " - " + myWeather.weather[0].description;

This is because the URL you've used in GetWeather method is not right. The result returned by this URL doesn't contain weather information and when it is deserialized to RootObject , most properties will be null.

To retrieve current weather data by geographic coordinates, we can use following API:

api.openweathermap.org/data/2.5/weather?lat={lat}&lon={lon}

And starting from 9 October 2015 this API requires a valid APPID for access. For more info, please see Call current weather data for one location and How to use API key in API call .

So we can change GetWeather method like following to fix this error.

public async static Task<RootObject> GetWeather(double lat, double lon)
{
    var http = new HttpClient();
    var response = await http.GetAsync($"http://api.openweathermap.org/data/2.5/weather?lat={lat}&lon={lon}&appid={APIKEY}&units=metric");
    var result = await response.Content.ReadAsStringAsync();
    var data = JsonConvert.DeserializeObject<RootObject>(result);
    return data;
}

Please note that I tried with your API key, but I got 401 Invalid API key error, so please make sure you are using the right key. And also, we need to change Main.pressure and Wind.deg 's type to double as their value are not int . If we don't change the type, we may get Newtonsoft.Json.JsonReaderException while deserialization.