如何在AsAsyncOperation中進行投射？

Question

我在WinRT IAsyncOperation，async等待崩潰課程。我試圖用IAsyncOperation包裝任務，但沒有想出一個簡單的方法來執行投射。

public interface IInfo{} 
public class InfoList : IInfo{} 

public class Parser { 
    private async Task<InfoList> ParseList() { 
     var returnList = new InfoList(); 

     HttpClient client = new HttpClient(); 
     Task<string> htmlTask = client.GetStringAsync("http://stackoverflow.com"); 

     string html = await htmlTask; 
     // parsing logic... 
     return returnList; 
    } 

    public IAsyncOperation<IInfo> Parse() 
    { 
     return AsyncInfo.Run(_ => 
      Task.Run<IInfo>(async() => 
      { 
       var returnList = await ParseList(); 
       return returnList; 
      }) 
     ); 
     // I have tried following but causes InvalidCastException 
     //return (IAsyncOperation<IInfo>)ParseList().AsAsyncOperation<InfoList>(); 
    } 
} 

Answer 1

public IAsyncOperation<IInfo> Parse() 
{ 
    return AsyncInfo.Run(async _ => (IInfo) await ParseList()); 
} 

或

public IAsyncOperation<IInfo> Parse() 
{ 
    return AsyncInfo.Run(_ => ParseList().ContinueWith(t => (IInfo)t.Result)); 
} 

Answer 2

的IAsyncOperation<>類型不是協變的，這意味着你可以不投一個IAsyncOperation<InfoList>到IAsyncOperation<IInfo>。

嘗試讓ParseList方法返回Task<IInfo>。