C#使用semaphore来管理异步下载请求的方法

var semaphor = new Semaphore(50, 50);
// We allow at most 50 threads for crawling
var resultPins = new List<Pin>();
// Results stored here
foreach (var pin in new HashSet<string>(pinIdList))
{
semaphor.WaitOne();
Console.Write(">");
var pinClient = new WebClient();
pinClient.DownloadStringCompleted += (sender, ex) =>
{
var html = ex.Result.Replace("n", "");
pinClient.Dispose();
lock (pinIdList)
{
// do some post-processing and write back the results
}
Console.Write("<");
semaphor.Release();
};
pinClient.DownloadStringAsync(new Uri(string.Format("http://pinterest.com/pin/{0}/", pin)));
}
for (int i = 0; i < 50; i++) semaphor.WaitOne();
// Wait until the last thread ends.
semaphor.Dispose();
Console.WriteLine();