Xamarin Forms, HttpClient class - GetStringAsync and GetAsync return null value and exit from function quickly

Question

when I run my code, in my function that call GetStringAsync or GetAsync, this 2 calls returns null value and immediately exit from my function (all code after is unreached )

I make a web api that is is reachable from web browser (url reached on browser) , also in the emulator browser ( Browser on the Android Emulator ) then i try to make a xamarin forms that manages to communicate with it

class MainPageViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;



    public MainPageViewModel()
    {
        GetEmployees();
    }



    private async void GetEmployees()
    {
        using (var httpClient = new HttpClient())
        {
            var uri = "http://192.168.1.135:8092/api/Masters/GetEmployees/";

            var result = await httpClient.GetStringAsync(uri);


            var EmployeeList = JsonConvert.DeserializeObject<List<Employee>>(result);

            Employees = new ObservableCollection<Employee>(EmployeeList);
        }

    }

    ObservableCollection<Employee> _employees;

    public ObservableCollection<Employee> Employees
    {

        get
        {
            return _employees;
        }

        set
        {
            _employees = value;
            OnPropertyChanged(nameof(Employee));

        }

    }

    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

or

class MainPageViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    private const string ApiBaseAddress = "http://192.168.1.135:8092/api/Masters/GetEmployees/";

    public MainPageViewModel()
    {
        GetEmployees();
    }

    private HttpClient CreateClient()
    {
        var httpClient = new HttpClient
        {
            BaseAddress = new Uri(ApiBaseAddress)
        };

        httpClient.DefaultRequestHeaders.Accept.Clear();
        httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

        return httpClient;
    }

    private async void GetEmployees()
    {

        using (var httpClient = CreateClient())
        {
            var response = await httpClient.GetAsync(ApiBaseAddress).ConfigureAwait(false);

            var test = response;

            if (response.IsSuccessStatusCode)
            {
                var json = await response.Content.ReadAsStringAsync().ConfigureAwait(false);

                if (!string.IsNullOrWhiteSpace(json))
                {
                    var EmployeeList = JsonConvert.DeserializeObject<List<Employee>>(json);

                    Employees = new ObservableCollection<Employee>(EmployeeList);

                }

            }

            response.Dispose();
        }
    }

    ObservableCollection<Employee> _employees;

    public ObservableCollection<Employee> Employees
    {

        get
        {
            return _employees;
        }

        set
        {
            _employees = value;
            OnPropertyChanged(nameof(Employee));

        }

    }

    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

I expect that var result = await httpClient.GetStringAsync(uri);

or

 var response = await httpClient.GetAsync(ApiBaseAddress).ConfigureAwait(false);

returns something but the actual output is null and the code after is unreached because this 2 calls exit immediatly from the function

yes, I already said that with the browser it works, only with calls, unfortunately, does not work — AccountForWorks
– AccountForWorks, Commented Aug 12, 2019 at 7:37
Maybe your running emulator (Android or iOS) don't have access to that url, can you start your emulator, use the installed browser and try to enter into that url? — ganchito55
– ganchito55, Commented Aug 12, 2019 at 7:53
using (var httpClient = CreateClient()) is wrong, HttpClient is intended to be instantiated once per application, rather than per-use. learn.microsoft.com/en-us/dotnet/api/… — Ive
– Ive, Commented Aug 12, 2019 at 8:04

Lucas Zhang · Accepted Answer · 2019-08-12 08:30:25Z

1

Avoid mixing async/await and blocking calls like .Wait() or .Result

Refactor your code to be async all the way through.

public MainPageViewModel()
{
   var EmployeeList =  GetEmployees();
   Employees = new ObservableCollection<Employee>(EmployeeList.Result);
}



private async Task<List<Employee>> GetEmployees()
{
   using (var httpClient = CreateClient())
   {
     var response = await httpClient.GetAsync(ApiBaseAddress).ConfigureAwait(false);

     var test = response;

     if (response.IsSuccessStatusCode)
     {
       var json = await response.Content.ReadAsStringAsync().ConfigureAwait(false);

       if (!string.IsNullOrWhiteSpace(json))
       {
          var EmployeeList = JsonConvert.DeserializeObject<List<Employee>>(json);

          return EmployeeList;

       }                    

     }

     return null;
    }

}

answered Aug 12, 2019 at 8:30

Lucas Zhang

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5 Comments

AccountForWorks Over a year ago

Throws Exception: Cleartext HTTP traffic to 192.168.1.135 not permitted

Lucas Zhang Over a year ago

Check stackoverflow.com/questions/45940861/…

AccountForWorks Over a year ago

I've already read it but for me is not working. I belive my problem is the ssl certificate because if i try to connecting to web api with https, I can't

Lucas Zhang Over a year ago

Did you try to change another url?

AccountForWorks Over a year ago

Ok, problem solved adding these lines of code into AndroidManifest.xml <base-config cleartextTrafficPermitted="true"> <trust-anchors> <certificates src="system" /> </trust-anchors> </base-config>

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