发送多个WebRequest并行

我想发送多个WebRequest。我使用Parallel.For循环来做到这一点，但循环运行一次，第二次它在获得响应时给出错误。

错误:

操作已超时

代码:

Parallel.For(0, 10, delegate(int i) {
    HttpWebRequest request = (HttpWebRequest)WebRequest.Create(
        new Uri("http://www.mysite.com/service"));
    string dataToSend = "Data";
    byte[] buffer = System.Text.Encoding.GetEncoding(1252).
        GetBytes(dataToSend);
    request.Method = "POST";
    request.ContentType = "application/x-www-form-urlencoded";
    request.ContentLength = buffer.Length;
    request.Host = "www.mysite.com";
    Stream requestStream = request.GetRequestStream();
    requestStream.Write(buffer, 0, buffer.Length);
    requestStream.Close();
    HttpWebResponse response = (HttpWebResponse)request.GetResponse();
});