如何运行MSDN代码示例"How to: Create Pre-Computed Tasks"？

Question

尝试在。net 4.0异步CTP中运行c#代码示例"How to: Create precomputed Tasks" (MSDN)，已做了更改:

• Task.FromResult—> TaskEx.FromResult
• Task.FromResult—> TaskEx.FromResult
• Task.Run(async () => -> TaskEx.RunEx(async () =>
  (也试过TaskEx.Run(async () =>)

获取编译错误:

因为' System.Func<System.Threading.Tasks.Task> '是异步的并且返回任务中，return关键字不能后跟对象表达式

如何更改代码以运行此示例?

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:

这个代码示例没有我的更新异步CTP运行在。net 4.5?

Answer 1

c#编译器在VS2012中得到了增强，在async lambdas(即Func<Task>及其家族)存在的情况下，可以更智能地处理泛型重载解析。因为你是在VS2010上，所以你必须在这里和那里帮助它。

线索在您的错误消息中显示的async lambda的类型:System.Func<System.Threading.Tasks.Task> .

应该是System.Func<System.Threading.Tasks.Task<System.String>>。

那么我就试试这个:

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