var postUrl = "**/LoginInfo.ashx";  
var postString = "method=CheckPW&id=4454556289&pwd=&checkword=8888&sign=";  
HttpWebRequest httpRequset = null;  
HttpWebResponse httpResponse = null;  
try  
{  
//POST登录信息获取返回结果，结果为0是为登录成功  
httpRequset = (HttpWebRequest)HttpWebRequest.Create(postUrl);//创建http 请求  
httpRequset.Method = "POST";//POST 提交  
httpRequset.ContentType = "application/x-www-form-urlencoded";//以上信息在监听请求的时候都有的直接复制过来  
httpRequset.Headers.Add("X-Requested-With", "XMLHttpRequest");  
httpRequset.Accept = "*/*";  
httpRequset.Referer = "***";  
httpRequset.Headers.Add("Accept-Language", "zh-Hans-CN,zh-Hans;q=0.7,ja;q=0.3");  
httpRequset.Headers.Add("Accept-Encoding", "gzip, deflate");  
httpRequset.UserAgent = "Mozilla/5.0 (Windows NT 10.0; WOW64; Trident/7.0; rv:11.0) like Gecko";  
httpRequset.KeepAlive = true;  
httpRequset.Host = "xxx.com";  
//httpRequset.Headers.Add("Access-Control-Allow-Origin", "*");//可跨域  
httpRequset.Headers.Add("Pragma", "no-cache");  
httpRequset.Headers.Add("DNT", "1");  
httpRequset.ServicePoint.Expect100Continue = false;  
byte[] bytes = System.Text.Encoding.UTF8.GetBytes(postString);  
httpRequset.ContentLength = bytes.Length;  
Stream stream = httpRequset.GetRequestStream();  
stream.Write(bytes, 0, bytes.Length);  
stream.Close();//以上是POST数据的写入  

httpResponse = (HttpWebResponse)httpRequset.GetResponse();//获得 服务端响应  
var s = string.Empty;  
using (Stream responsestream = httpResponse.GetResponseStream())  
{  
using (StreamReader sr = new StreamReader(responsestream, System.Text.Encoding.UTF8))  
{  
content = sr.ReadToEnd();   //返回信息  

}  

//WebClient 写法
WebClient webClient = new WebClient();
NameValueCollection formData = new NameValueCollection();
formData["username"] = "testuser";
formData["password"] = "mypassword";

byte[] responseBytes = webClient.UploadValues(postUrl , "POST", formData);
string responsefromserver = Encoding.UTF8.GetString(responseBytes);
Console.WriteLine(responsefromserver);
webClient.Dispose();