如何運行MSDN代碼示例「如何創建預計算任務」？

Question

試圖在.NET運行C＃代碼示例"How to: Create Pre-Computed Tasks" (MSDN) 4.0異步CTP，在作出改變：

• Task.FromResult --->TaskEx.FromResult
• Task.FromResult --->TaskEx.FromResult
• Task.Run(async() => --->TaskEx.RunEx(async() =>
  （也試過TaskEx.Run(async() =>）

對接總體性他編譯錯誤：

Since ' System.Func<System.Threading.Tasks.Task> ' is async and returns Task, a return keyword must not be followed by an object expression

如何更改代碼以運行此示例？

using System; 
using System.Collections.Concurrent; 
using System.Diagnostics; 
using System.Linq; 
using System.Net; 
using System.Threading.Tasks; 

// Demonstrates how to use Task<TResult>.FromResult to create a task 
// that holds a pre-computed result. 
class CachedDownloads 
{ 
    // Holds the results of download operations. 
    static ConcurrentDictionary<string, string> cachedDownloads = 
    new ConcurrentDictionary<string, string>(); 

    // Asynchronously downloads the requested resource as a string. 
    public static Task<string> DownloadStringAsync(string address) 
    { 
    // First try to retrieve the content from cache. 
    string content; 
    if (cachedDownloads.TryGetValue(address, out content)) 
    { 

     return TaskEx //****** Task.FromResult ---> TaskEx.FromResult 
      .FromResult<string>(content); 
    } 

    // If the result was not in the cache, download the 
    // string and add it to the cache. 
    return TaskEx.RunEx //****** Task.Run --> TaskEx.RunEx (TaskEx.Run) 
     (async() =>  
    { 
     content = await new WebClient().DownloadStringTaskAsync(address); 
     cachedDownloads.TryAdd(address, content); 
     return content;//*****Error 
    }); 
    } 

    static void Main(string[] args) 
    { 
    // The URLs to download. 
    string[] urls = new string[] 
     { 
     "http://msdn.microsoft.com", 
     "http://www.contoso.com", 
     "http://www.microsoft.com" 
     }; 

    // Used to time download operations. 
    Stopwatch stopwatch = new Stopwatch(); 

    // Compute the time required to download the URLs. 
    stopwatch.Start(); 
    var downloads = from url in urls 
        select DownloadStringAsync(url); 
    TaskEx.WhenAll(downloads)//*** Task.WhenAll --> TaskEx.WhenAll 
      .ContinueWith(results => 
    { 
     stopwatch.Stop(); 

     // Print the number of characters download and the elapsed time. 
     Console.WriteLine 
      ("Retrieved {0} characters. Elapsed time was {1} ms." 
       , results.Result.Sum(result => result.Length) 
       , stopwatch.ElapsedMilliseconds); 
    }) 
    .Wait(); 

    // Perform the same operation a second time. The time required 
    // should be shorter because the results are held in the cache. 
    stopwatch.Restart(); 
    downloads = from url in urls 
       select DownloadStringAsync(url); 

    TaskEx.WhenAll(downloads)//*** Task.WhenAll --> TaskEx.WhenAll 
      .ContinueWith(results => 
    { 
     stopwatch.Stop(); 

     // Print the number of characters download and the elapsed time. 
     Console.WriteLine("Retrieved {0} characters. Elapsed time was {1} ms.", 
     results.Result.Sum(result => result.Length), 
     stopwatch.ElapsedMilliseconds); 
    }) 
    .Wait(); 
    } 
} 

更新：

難道沒有我的異步CTP更新此代碼示例運行在.NET 4.5？

Answer 1

C＃編譯器在VS2012中進行了增強，以便在存在async lambda（即Func<Task>和系列）的情況下更加智能化。既然你是在VS2010上，那麼你必須在這裏和那裏幫助你。

線索在您的錯誤消息System.Func<System.Threading.Tasks.Task>中顯示的async lambda的類型中。

它是什麼應該是System.Func<System.Threading.Tasks.Task<System.String>>。你爲什麼現在的RTM版本可使用CTP

return TaskEx.RunEx<string> //****** Task.Run --> TaskEx.RunEx (TaskEx.Run) 
    (async() =>  
{ 
    content = await new WebClient().DownloadStringTaskAsync(address); 
    cachedDownloads.TryAdd(address, content); 
    return content;//*****Error 
});