Xamarin Forms，HttpClient 类 - GetStringAsync 和 GetAsync 返回空值并快速退出函数

Question

当我运行我的代码时，在调用 GetStringAsync 或 GetAsync 的函数中，这 2 个调用返回 null 值并立即退出我的函数（之后的所有代码都未到达）

我制作了一个可从网络浏览器 (url reached on browser) 访问的网络 api，也在模拟器浏览器 (Browser on the Android Emulator) 中，然后我尝试制作一个设法与之通信的 xamarin 表单

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));
    }
}

或

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));
    }
}

我希望var result = await httpClient.GetStringAsync(uri);

或

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

返回一些东西，但实际输出为空，之后的代码未到达，因为这 2 个调用立即从函数中退出

Answer 1

避免混合 async/await 和阻塞调用，如 .Wait() 或 .Result

重构您的代码，使其始终保持异步。

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;
    }

}