异步/等待HttpClient请求

Question

我在一个类中有下一个代码，该类以XML格式从服务器获取数据：

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Net.Http;
using System.Text;
using System.Threading.Tasks;
using Windows.Security.Cryptography.Certificates;
using Windows.Storage;
using Windows.Storage.Streams;

namespace TestProj.WebServiceModels
{
public class MyClient
{
     async public static Task<string> fetchContentAsync()
    {
        string url = "https://blabla.com/test/data?format=xml";
        HttpClientHandler aHandler=new HttpClientHandler();
        aHandler.ClientCertificateOptions=ClientCertificateOption.Automatic;
        HttpClient aClient=new HttpClient(aHandler);
        Uri requestUri=new Uri(url);
        HttpRequestMessage request=new HttpRequestMessage(HttpMethod.Get, requestUri);
        var result=await aClient.GetAsync(requestUri, HttpCompletionOption.ResponseContentRead);
        var responseHeader=result.Headers;
        var responseBody=await result.Content.ReadAsStringAsync();
        return responseBody;        
    }
}
}

在我的主页上，我有一个函数触发get方法并失败！！：

 public MainPage()
    {
        this.InitializeComponent();
        fillChartsObject();
    }

private async void fillChartsObject()
    {
        try
        {
            var response = await MyClient.fetchContentAsync();
            responseString = response.ToString();
        }
        catch
        {
            var dlg = new MessageDialog("You are totaly screwed!");
            dlg.ShowAsync();
        }
    }

但是，当我在Button上单击它时，它将运行！ 怎么可能？！ 如何解决？

当它失败时-它不会问我要使用哪个证书，否则-不会。

错误：请求已中止：无法创建SSL / TLS安全通道。

System.Net.Http.HttpRequestException не обработано пользовательским кодом
HResult=-2146233088
Message=An error occurred while sending the request.
Source=mscorlib
StackTrace:
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at     System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at SaC.WebServiceModels.MyClient.<fetchContentAsync>d__0.MoveNext() in c:\Users\C5195923\Documents\Visual Studio 2012\Projects\SaC\SaC\WebServiceModels\MyClient.cs:line 31
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at SaC.MainPage.<fillChartsObject>d__96.MoveNext() in c:\Users\foma\Documents\Visual Studio 2012\Projects\SaC\SaC\MainPage.xaml.cs:line 1546
  InnerException: System.Net.WebException
   HResult=-2146233079
   Message=The request was aborted: Could not create SSL/TLS secure channel.
   Source=System
   StackTrace:
        at System.Net.HttpWebRequest.EndGetResponse(IAsyncResult asyncResult)
        at System.Net.Http.HttpClientHandler.GetResponseCallback(IAsyncResult ar)
   InnerException:

代码2：

protected override void OnNavigatedTo(NavigationEventArgs e)
    {
        fillChartsObject();
    }



 private async void fillChartsObject()
    {

        String responseString = "";
            var response = await MyClient.fetchContentAsync();
            responseString = response.ToString();

    }

错误：

   System.Net.Http.HttpRequestException не обработано пользовательским кодом
   HResult=-2146233088
   Message=An error occurred while sending the request.
   Source=mscorlib
   StackTrace:
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at SAPSalesCentral.WebServiceModels.MyClient.<fetchContentAsync>d__0.MoveNext() in c:\Users\C5195923\Desktop\NewNewRevision\SAPSalesCentral\SAPSalesCentral\WebServiceModels\MyClient.cs:line 31
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at SAPSalesCentral.MainPage.<fillChartsObject>d__94.MoveNext() in c:\Users\foma\Desktop\NewNewRevision\SAPSalesCentral\SAPSalesCentral\MainPage.xaml.cs:line 1539
  InnerException: System.Net.WebException
   HResult=-2146233079
   Message=The request was aborted: Could not create SSL/TLS secure channel.
   Source=System
   StackTrace:
        at System.Net.HttpWebRequest.EndGetResponse(IAsyncResult asyncResult)
        at System.Net.Http.HttpClientHandler.GetResponseCallback(IAsyncResult ar)
   InnerException:

附加功能：

ShowLoadingScreen(); // displays semi-transparent background over the whole form and progressRing (native Windows 8 spinner) on it.
GetSettings(); //gets data from local settings storage;
CheckFileExists(); // ASYNC! Checks if data is already stored on a device, if not, gets from web
fillListBox(); // Just populates listbox with hardcoded data;
PopulateHelpMenu(); // Just populates listbox with hardcoded data;
(App.Current as App).Main = this;

编辑：

public MainPage()
    {
        this.InitializeComponent();

        //#region Initialization
        ShowLoadingScreen();
        //fillChartsObject();
        GetSettings();
        CheckFileExists();
        fillListBox();
        PopulateHelpMenu();
        (App.Current as App).Main = this;
        //#endregion
    }


 protected async override void OnNavigatedTo(NavigationEventArgs e)
    {
        await Task.Delay(5000);
        await fillChartsObject(); 
        //await fillChartsObject();
        //fillChartsObject();
        ////dtm = DataTransferManager.GetForCurrentView();
        ////dtm.DataRequested += OnDataRequested;
        ////SettingsPane.GetForCurrentView().CommandsRequested += BlankPage_CommandsRequested;
    }

         private async void CheckFileExists()
    {
        //vpnAvailability = await VPN.Check(); //When i uncoment this - it fails again. This just sends request to a link, if it fails = no VPN if not = is VPN.
        ro = new List<RootObject>();
        StorageFile File;
        try
        {
            #region If file exists
            File = await Windows.Storage.ApplicationData.Current.LocalFolder.GetFileAsync("UserData.txt");
            if (File != null)
            {
                string JSON = await FileIO.ReadTextAsync(File);
                if (JSON != "")
                {
                    //If file exists get data
                    try
                    {
                        ro = JsonConvert.DeserializeObject<List<RootObject>>(JSON);
                        fillGamificationLine(ro);
                    }
                    catch
                    {
                        getGamificationDataFromServer();
                    }
                }
                else
                {
                    throw new FileNotFoundException();
                }
            }
            #endregion
        }
        catch (FileNotFoundException)
        {
            //If file doesn't exist - get data from server
            getGamificationDataFromServer();
        }
    }

 //The idea was that when all UserData is being populated from server or locally, the request of table data (only from service) is being proceeded parallel.

多亏了Kris Vandermotten，我遇到了这个魔术：

失败：

protected async override void OnNavigatedTo(NavigationEventArgs e)
    {
        await fillChartsObject(); 
    }

作品：

protected async override void OnNavigatedTo(NavigationEventArgs e)
    {
        await Task.Delay(1);
        await fillChartsObject(); 
    }

Answer 1

这可能是一个时序问题，您的代码运行得太早。 您应该先让UI初始化代码完成。

尝试这个：

public MainPage()
{
    this.InitializeComponent();
}

protected override async void OnNavigatedTo(NavigationEventArgs e)
{
    // allow other queued messages to be processed first
    // this may work as well: await Task.Yield();
    await Task.Delay(1);
    // now continue
    await fillChartsObject();
}

private async void fillChartsObject()
{
    try
    {
        var response = await MyClient.fetchContentAsync();
        responseString = response.ToString();
    }
    catch
    {
        var dlg = new MessageDialog("You are totally screwed!");
        dlg.ShowAsync();
    }
}