multithreading - ManualResetEvent.WaitOne 阻塞所有线程

Question

我有以下代码

ThreadPool.QueueUserWorkItem(new WaitCallback(DownloadAsync), apiMethod);
downloadHandle.WaitOne();

DownloadAsync 在哪里

private void DownloadAsync(object _uri)
        {
            var url = _uri as string;
            WebClient client = new WebClient();
            client.DownloadStringCompleted += new DownloadStringCompletedEventHandler(client_DownloadStringCompleted);
            client.DownloadStringAsync(new Uri(GLOBALS.MAIN_API_URL + url, UriKind.Absolute));
        }

void client_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
        {
            result = e.Result;
            downloadHandle.Set();
        }

所以我的问题是 downloadHandle.Set() 永远不会调用。但我不明白为什么？我为 DownloadAsync 创建了一个新线程，并且 downloadHandle.WaitOne() 不应该阻止他。

我需要的是创建一个同步方法而不是异步。

谢谢!

UPD:使用异步调用更新源代码。

Answer 1

client.DownloadString 是同步方法，所以你完成的处理程序永远不会被调用。需要调用异步版本:client.DownloadStringAsync()

您可以阅读有关 DownloadStringAsync 的更多信息在 msdn 上。如果您依赖于应该调用某些代码的事实，那么将代码放入 try-catch block 并处理异常也是明智的。

您的代码可能如下所示:

private void DownloadAsync(object _uri)
{
    try
    {
        var url = _uri as string;
        WebClient client = new WebClient();
        client.DownloadStringCompleted += new DownloadStringCompletedEventHandler(client_DownloadStringCompleted);
        client.DownloadStringAsync(new Uri(GLOBALS.MAIN_API_URL + url, UriKind.Absolute));
    }
    catch //appropriate exception
    {
       //Handle exception (maybe set downloadHandle or report an error)
    }
}

void client_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
{
    result = e.Result;
    downloadHandle.Set();
}