如何調用異步方法？

Question

我想做一個簡單的應用程序，獲取當前位置在Windows 8中，但我無法找到await關鍵字的正確語法。

錯誤提示： 錯誤1'await'操作符只能在異步方法中使用。考慮使用「異步」修飾符標記此方法，並將其返回類型更改爲「任務」。

的代碼如下：

public MainPage() 
     { 
      this.InitializeComponent(); 

      TextBlock txt = new TextBlock();   
      var location = await InitializeLocationServices(); 
      txt.Text = location; 

      Grid.SetRow(txt, 0); 
      Grid.SetColumn(txt, 1); 
      //InitializeLocationServices(); 
     } 

     /// <summary> 
     /// Invoked when this page is about to be displayed in a Frame. 
     /// </summary> 
     /// <param name="e">Event data that describes how this page was reached. The Parameter 
     /// property is typically used to configure the page.</param> 
     protected override void OnNavigatedTo(NavigationEventArgs e) 
     { 
     } 

     private async Task<string> InitializeLocationServices() 
     { 
      //Initialize geolocator object 
      Geolocator geoLocator = new Geolocator(); 
      if (null != geoLocator) 
       try 
       { 
        //Try resolve the current location 
        var position = await geoLocator.GetGeopositionAsync(); 
        if (null != position) 
        { 
         string city = position.CivicAddress.City; 
         string country = position.CivicAddress.Country; 
         string state = position.CivicAddress.State; 
         string zip = position.CivicAddress.PostalCode; 
         string msg = "I am located in " + country; 
         if (city.Length > 0) 
          msg += ", city of " + city; 
         if (state.Length > 0) 
          msg += ", " + state; 
         if (zip.Length > 0) 
          msg += " near zip code " + zip; 
         return msg; 
        } 
        return string.Empty; 
       } 
       catch (Exception) 
       { 
        //Nothing to do - no GPS signal or some timeout occured.n . 
        return string.Empty; 
       } 
      return string.Empty; 
     } 

Answer 1

所以它不會起作用，因爲你是在構造函數中調用它。

我不熟悉Win8，但是從OnNavigatedTo的描述來看「///屬性通常用於配置頁面。」

它可能是初始化的好候選者。讓我們試着移動它：

protected async override void OnNavigatedTo(NavigationEventArgs e) 
{ 

    TextBlock txt = new TextBlock();   
    var location = await InitializeLocationServices(); 
    txt.Text = location; 

    Grid.SetRow(txt, 0); 
    Grid.SetColumn(txt, 1); 
} 

Answer 2

記住，你的函數返回Task<string>，所以怎麼弄你回來string兩次？

return string.Empty; 

在旁註。我無法理解這樣的檢查

Geolocator geoLocator = new Geolocator(); 
     if (null != geoLocator)