//多线程并发时，默认最多只能建立两个持久连接，后续请求也会阻塞，
//增大默认连接数限制可解决此问题

//C#默认只允许两个持久连接，http协议的RFC文档也有此建议
System.Net.ServicePointManager.DefaultConnectionLimit = 256;

WebRequest wr = WebRequest.Create(URL);
try
{
  WebResponse res = wr.GetResponse();
  /*
   * do something
  */
  res.Close();
  wr.Abort();
}
catch(WebException ex)
{
  res.Close();
  wr.Abort();
  throw ex;
}