我正在学习C#.NET 4.5中的任务并行,我对
example有点困惑.这是我不理解的代码：
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在另一个任务中包装DownloadStringTaskAsync(). DownloadStringTaskAsync()是否已经在自己的线程上运行？
这是我编写它的方式：
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？哪一个更好？
解决方法
好吧,这个例子具体说明了如何使用你的第二个代码不使用的Task.FromResult.也就是说,我不同意示例中Task.Run的用法.
我这样写,我自己：
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,该示例使用过时的WebClient,在新代码中应使用HttpClient替换.
总的来说,它看起来只是一个不好的例子IMO.
