Json-file (Api-Response) deserialized to C# Class works with Winforms/Console but I do get an Error with ASP.Net Blazor! Why?

I started looking into Openweathermap API (weather Data). The Api response is a Json file. I managed to get everything working with MS WinForms in (VisualStudio).

Now I wanted to do the same on my BlazorWebpage. But I cant get it to work. I am stuck maybe you could point me in the right direction.

In the Winforms project I succsessfull call the Api with this code:

    private async Task CallWeatherApiAsync()     {         oneDayWeatherInfo = new OneDayWeatherDataModel();         using (HttpResponseMessage response = await client.GetAsync(client.BaseAddress))         {             if (response.IsSuccessStatusCode)             {                 oneDayWeatherInfo= await response.Content.ReadAsAsync<OneDayWeatherDataModel>();              }         }      } 

Everything from the api response data "goes" in the right places. The WeatherDataModel is the C# Class structur "from" the jsonfile. The Classes looking like this (on first read skip this):

namespace Blazor_WeatherApi.Models { public class OneDayWeatherDataModel {     //Name of the City     public string Name { get; set; }      public Main Main { get; set; }      public Coord Coord { get; set; }     public List<Weather> Weather { get; set; }     public Wind Wind { get; set; }      public Clouds Clouds { get; set; }     public Sys Sys { get; set; }      public int Visibility { get; set; }     public int Dt { get; set; }     public int Timezone { get; set; }     public int Id { get; set; }     public int Cod { get; set; } }  public class Main {     public double Temp { get; set; }     public double Feels_like { get; set; }     public double Temp_min { get; set; }     public double Temp_max { get; set; }     public int Pressure { get; set; }     public int Humidity { get; set; } }  public class Coord {     public double Lon { get; set; }     public double Lat { get; set; } }  public class Weather {     public int Id { get; set; }     public string Main { get; set; }     public string Description { get; set; }     public string Icon { get; set; } }  public class Wind {     public double Speed { get; set; }     public int Deg { get; set; }     public double Gust { get; set; } }  public class Clouds {     public int All { get; set; } }  public class Sys {     public int Type { get; set; }     public int Id { get; set; }     public double Message { get; set; }     public string Country { get; set; }     public int Sunrise { get; set; }     public int Sunset { get; set; } } } 

Now to my problem I created a blazor project and I can call the data from the root ModelClass like Name (first Propertie in the class Model) But I cant use anything else that is one class deeper besides root class. E.g. If I want to get the temperatur value root->Main->Temp then I get an error. I used the same ClassModel in the winforms project there it was successful.

Here the Blazor code that works only for the Properties from the "root class":

@page "/weather" @inject HttpClient http;  <h3>ApiCall</h3>   <button class="btn btn-primary" @onclick="@GetWeatherData">Get Weather</button>   <table>      <tr>        <td>@weatherModel.Name</td>     </tr>     <tr>        <td>@weatherModel.Dt</td>     </tr>    @*<tr>         <td>@weatherModel.Main.Temp</td>       // IF I DECOMMENT THIS 3 LINES I GET the ERROR     </tr>*@   </table>  @code {    OneDayWeatherDataModel weatherModel = new OneDayWeatherDataModel();      private async Task GetWeatherData()     {       weatherModel = await http.GetJsonAsync<OneDayWeatherDataModel>         ("https://api.openweathermap.org/data/2.5/weather?id=...");     }  } 

If I comment the 3 lines out I have a working blazer api call and I get the name of the city from the web-api-call from openweathermap. BUT…

The Error occures when I decomment @weatherModel.Main.Temp the project freezes an Visual Studio tells me it is in HoldModus (System.NullReferenceException: "Object reference not set to an instance of an object.")

This happens at the moment I browse to the page NOT after I click the button ??? Any idea what I did wrong? And sorry that the post is so long and intertwined 🙂

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1 Answer(s)

The problem is that you are manually instantiating the class in the following line and not setting the ‘Main’ property to anything, and therfore it is null

OneDayWeatherDataModel weatherModel = new OneDayWeatherDataModel();

You could ensure that you populate the property or use access it with the following to avoid the null reference error:


However you might also want to consider something like the below so that that table doesnt show unless it’s got the data from the API:

@if (weatherModel != null) { <table>      <tr>        <td>@weatherModel.Name</td>     </tr>     <tr>        <td>@weatherModel.Dt</td>     </tr>    <tr>         <td>@weatherModel.Main.Temp</td>            </tr>   </table> }  @code {    OneDayWeatherDataModel weatherModel = null;      private async Task GetWeatherData()     {       weatherModel = await http.GetJsonAsync<OneDayWeatherDataModel>         ("https://api.openweathermap.org/data/2.5/weather?id=...");     }  } 
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