private async Task<string> PostResponseContentAsync(string url, string xmlcontent)
        {
            try
            {
                var httpClient = new HttpClient();
                httpClient.DefaultRequestHeaders.ExpectContinue = false;
                httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("text/xml"));
                httpClient.DefaultRequestHeaders.Add("UserAgent", "curl/7.12.1");
                httpClient.DefaultRequestHeaders.Add("Host", "ping.baidu.com");

                StringContent postContent = new StringContent(xmlcontent);
                postContent.Headers.ContentType = new MediaTypeHeaderValue("text/xml");
                
                

                var response = await httpClient.PostAsync(url, postContent);//这行异常 An error occurred while sending the request.
                response.EnsureSuccessStatusCode();
                if (response.StatusCode == System.Net.HttpStatusCode.OK)
                {
                    return await response.Content.ReadAsStringAsync();
                }
                else
                {
                    return null;
                }
            }
            catch (Exception ex)
            {
                return null;
            }

        }

await PostResponseContentAsync("http://ping.baidu.com/sitemap?site=www.39xy.com&resource_name=sitemap&access_token=TTZhhO0M", "<?xml version='1.0' encoding='UTF-8'?><urlset><url><loc><![CDATA[http://www.39xy.com/thread-15-1-1.html]]></loc><lastmod>2013-09-24</lastmod><changefreq>daily</changefreq><priority>0.8</priority></url></urlset>")