c# - 何时创建新任务

Question

我正在学习 C#.NET 4.5 中的任务并行性，我对 example 有点困惑.这是我不明白的代码:

public static Task<string> DownloadStringAsync(string address)
{
  // First try to retrieve the content from cache. 
  string content;
  if (cachedDownloads.TryGetValue(address, out content))
  {
     return Task.FromResult<string>(content);
  }

  // If the result was not in the cache, download the  
  // string and add it to the cache. 
  return Task.Run(async () => // why create a new task here?
  {
     content = await new WebClient().DownloadStringTaskAsync(address);
     cachedDownloads.TryAdd(address, content);
     return content;
  });
}

具体来说，我不明白为什么他们将 DownloadStringTaskAsync() 包装在另一个任务中。 DownloadStringTaskAsync() 不是已经在自己的线程上运行了吗？

这是我的编码方式:

public static async Task<string> DownloadStringAsync(string address)
{
    // First try to retrieve the content from cache.
    string content;
    if (cachedDownloads.TryGetValue(address, out content))
    {
        return content;
    }

    // If the result was not in the cache, download the  
    // string and add it to the cache.
    content = await new WebClient().DownloadStringTaskAsync(address);
    cachedDownloads.TryAdd(address, content);
    return content;
}

两者有什么区别？哪个更好？

Answer 1

好吧，该示例专门展示了如何使用 Task.FromResult，您的第二个代码没有使用它。也就是说，我不同意示例中 Task.Run 的用法。

我自己会这样写:

public static Task<string> DownloadStringAsync(string address)
{
  // First try to retrieve the content from cache.
  string content;
  if (cachedDownloads.TryGetValue(address, out content))
  {
    return Task.FromResult(content);
  }

  // If the result was not in the cache, download the  
  // string and add it to the cache.
  return DownloadAndCacheStringAsync(address);
}

private static async Task<string> DownloadAndCacheStringAsync(string address)
{
  var content = await new WebClient().DownloadStringTaskAsync(address);
  cachedDownloads.TryAdd(address, content);
  return content;
}

另请注意，该示例使用已过时的 WebClient，在新代码中应将其替换为 HttpClient。

总的来说，它看起来像是一个糟糕的 IMO 示例。