How to fix a CS0029 error in .NET MAUI app

Question

How to fix a CS0029 error in .NET MAUI app

wire_jp 216

I have a CS0029 error in the WeatherViewModel.cs file a .NET MAUI app, which references the NetworkManager.cs. The NetworkManager.cs file and WeatherViewModel.cs files are shown below: -

The CS0029 error is: -

CS0029 Cannot implicitly convert type 'System.Threading.Tasks.Task<WeatherMap.Models.Weather.CurrentWeather>' to 'WeatherMapModels.Weather.CurrentWeather'

The NetworkManager.cs file and WeatherViewModel.cs files are shown below: -

NetworkManager.cs file

NetworkManager.cs file

public class NetworkManager
    {
        public async Task<CurrentWeather> GetCurrentWeather(int id)
        {
            return await Task<CurrentWeather>.Factory.StartNew(() =>
            {
                string url = $"http://api.openweathermap.org/data/2.5/weather?id={id}&units=metric&appid=921e83b9da8a40a760ad74d5cedd6bbd";
                using var httpClient = new HttpClient();
                var request = new HttpRequestMessage(HttpMethod.Get, "http://api.openweathermap.org/data/2.5/weather?id={id}&units=metric&appid=921e83b9da8a40a760ad74d5cedd6bbd");
                string responseJson = string.Empty;                                                         
                var response = httpClient.Send(request);
                using var reader = new StreamReader(response.Content.ReadAsStream());            
                var responseBody = reader.ReadToEnd();
                using (StreamReader streamReader = new StreamReader(responseBody))
                {
                    responseJson = streamReader.ReadToEnd();
                }
                reader.Close();

                return JsonConvert.DeserializeObject<CurrentWeather>(responseJson);
            });
        }
      }    

    }

WeatherViewModel.cs file
WeatherViewModel.cs
public WeatherViewModel(City city)
        {
            networkManager = new NetworkManager();
            ViewModels = new ObservableCollection<IViewModel>() { new DayViewModel(), new WeekViewModel() };

            try
            {
                CurrentWeather currentWeather = networkManager.GetCurrentWeather(city.Id);  // CS0029 Cannot implicitly convert type 'System.Threading.Tasks.Task<WeatherMap.Models.Weather.CurrentWeather>' to 'WeatherMapModels.Weather.CurrentWeather'
                currentWeather.City = city;
                Forecast = networkManager.GetForecast(city.Id).List; 
                BackgroundSource = currentWeather.Weather[0].Icon;

                MainThread.BeginInvokeOnMainThread(() =>
                {
                    ViewModels[0].Init(currentWeather);
                    ViewModels[1].Init(Forecast);
                });
            }
            catch { }

        }

Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 36,436 Reputation points Microsoft External Staff

2023-06-19T06:34:10.49+00:00
networkManager.GetCurrentWeather method is an async method, and it should be called using the await keyword. For example

public async Task TestAsync() { NetworkManager networkManager = new NetworkManager(); CurrentWeather currentWeather = await networkManager.GetCurrentWeather(2); }

In addition, you call this method in public WeatherViewModel(City city), it looks like the constructor method of WeatherViewModel, right? It's not a good practice to call an async method in constructor method, which will block the UI thread.

I'm not sure what logic you're trying to achieve, can you describe your expected behavior in detail? Would you like to request the API when the page is appeared? Do you set the WeatherViewModel as the Page's BindingContext ? You could try to call the method in OnAppearing of the page.

protected override async void OnAppearing() { base.OnAppearing(); var VM = this.BindingContext as WeatherViewModel; await VM.TestAsync(); }
Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 36,436 Reputation points Microsoft External Staff

2023-06-21T09:00:40.54+00:00

May I know whether your issue has been solved or not? If you're still having problems, please explain in detail what is still going wrong and what you need help with.

1 answer

Your answer

Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 36,436 Reputation points Microsoft External Staff

2023-06-19T06:34:10.49+00:00

networkManager.GetCurrentWeather method is an async method, and it should be called using the await keyword. For example

public async Task TestAsync() { NetworkManager networkManager = new NetworkManager(); CurrentWeather currentWeather = await networkManager.GetCurrentWeather(2); }

In addition, you call this method in public WeatherViewModel(City city), it looks like the constructor method of WeatherViewModel, right? It's not a good practice to call an async method in constructor method, which will block the UI thread.

I'm not sure what logic you're trying to achieve, can you describe your expected behavior in detail? Would you like to request the API when the page is appeared? Do you set the WeatherViewModel as the Page's BindingContext ? You could try to call the method in OnAppearing of the page.

protected override async void OnAppearing() { base.OnAppearing(); var VM = this.BindingContext as WeatherViewModel; await VM.TestAsync(); }
Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 36,436 Reputation points Microsoft External Staff

2023-06-21T09:00:40.54+00:00

May I know whether your issue has been solved or not? If you're still having problems, please explain in detail what is still going wrong and what you need help with.

Answer 1

I tried the following and I am checking with you if this will work: -

NetworkManager.cs file

public class NetworkManager
    {
        public CurrentWeather GetCurrentWeather(int id)
        {
           
               string url = $"http://api.openweathermap.org/data/2.5/weather?id={id}&units=metric&appid=921e83b9da8a40a760ad74d5cedd6bbd";
                using var httpClient = new HttpClient();
                var request = new HttpRequestMessage(HttpMethod.Get, "http://api.openweathermap.org/data/2.5/weather?id={id}&units=metric&appid=921e83b9da8a40a760ad74d5cedd6bbd");
                string responseJson = string.Empty;                                                         
                var response = httpClient.Send(request);
                using var reader = new StreamReader(response.Content.ReadAsStream());            
                var responseBody = reader.ReadToEnd();
                using (StreamReader streamReader = new StreamReader(responseBody))
                {
                    responseJson = streamReader.ReadToEnd();
                }
                reader.Close();

                return JsonConvert.DeserializeObject<CurrentWeather>(responseJson);
            });
        }
      }    

    }

Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 36,436 Reputation points Microsoft External Staff

2023-06-22T05:06:16.98+00:00

Yes, you remove the async method, and synchronize the execution. To wait for a single task to complete, you can also try calling its Task.Wait method. But I have to say, there is high risk blocking the UI thread. The application must wait until the requested task finished. Asynchrony is essential for activities that are potentially blocking.

For more details, you could refer to The Task Asynchronous Programming (TAP) model with async and await" | Microsoft Learn

Task Class (System.Threading.Tasks) | Microsoft Learn

Consume a REST-based web service - .NET MAUI | Microsoft Learn

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How to fix a CS0029 error in .NET MAUI app

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