Cách 1:

public string Post(string url, string data)
{


    string vystup = null;
    try
    {
        //Our postvars
        byte[] buffer = Encoding.ASCII.GetBytes(data);
        //Initialisation, we use localhost, change if appliable
        HttpWebRequest WebReq = (HttpWebRequest)WebRequest.Create(url);
        //Our method is post, otherwise the buffer (postvars) would be useless
        WebReq.Method = "POST";
        //We use form contentType, for the postvars.
        WebReq.ContentType = "application/x-www-form-urlencoded";
        //The length of the buffer (postvars) is used as contentlength.
        WebReq.ContentLength = buffer.Length;
        //We open a stream for writing the postvars
        Stream PostData = WebReq.GetRequestStream();
        //Now we write, and afterwards, we close. Closing is always important!
        PostData.Write(buffer, 0, buffer.Length);
        PostData.Close();
        //Get the response handle, we have no true response yet!
        HttpWebResponse WebResp = (HttpWebResponse)WebReq.GetResponse();
        //Let's show some information about the response
        Console.WriteLine(WebResp.StatusCode);
        Console.WriteLine(WebResp.Server);

        //Now, we read the response (the string), and output it.
        Stream Answer = WebResp.GetResponseStream();
        StreamReader _Answer = new StreamReader(Answer);
        vystup = _Answer.ReadToEnd();

        //Congratulations, you just requested your first POST page, you
        //can now start logging into most login forms, with your application
        //Or other examples.
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
    return vystup.Trim() + "\n";

}

trong đó data có dạng

string data = "param1=value1&parm2=value2";

Cách 2:

public static string HttpPost(string URI, string Parameters)
{
    // Create a 'HttpWebRequest' object.
    HttpWebRequest req = (HttpWebRequest)WebRequest.Create(URI);

    CookieContainer cookieJar = new CookieContainer();
    req.CookieContainer = cookieJar;
    req.Referer = "http://captcha.chat.yahoo.com/go/captchat/";
    req.Accept = "text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5";
    req.UserAgent = "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.6) Gecko/20070725 Firefox/2.0.0.6";
    req.KeepAlive = true;
    req.Referer = URI;
    req.ContentType = "application/x-www-form-urlencoded";
    req.Method = "POST";


    byte[] bytes = System.Text.Encoding.UTF8.GetBytes(Parameters);
    req.ContentLength = bytes.Length;

    Stream os = req.GetRequestStream();
    os.Write(bytes, 0, bytes.Length);
    os.Close();

    WebResponse resp = req.GetResponse();
    if (resp == null) return null;

    StreamReader sr = new StreamReader(resp.GetResponseStream());
    return sr.ReadToEnd().Trim();
}

Note: Parameters in my method is the same as

str = "question=" & CaptChaQuestion & "&.intl=us&answer=" & CaptChaAnswer in VB

Cách 3:

using System;
using System.Net;
using System.Web;
using System.Collections;
using System.IO;

namespace WebRequestExample
{

    class WebPostRequest
    {
        WebRequest theRequest;
        HttpWebResponse theResponse;
        ArrayList  theQueryData;

        public WebPostRequest(string url)
        {
            theRequest = WebRequest.Create(url);
            theRequest.Method = "POST";
            theQueryData = new ArrayList();
        }

        public void Add(string key, string value)
        {
            theQueryData.Add(String.Format("{0}={1}",key,HttpUtility.UrlEncode(value)));
        }

        public string GetResponse()
        {
            // Set the encoding type
            theRequest.ContentType="application/x-www-form-urlencoded";

            // Build a string containing all the parameters
            string Parameters = String.Join("&",(String[]) theQueryData.ToArray(typeof(string)));
            theRequest.ContentLength = Parameters.Length;

            // We write the parameters into the request
            StreamWriter sw = new StreamWriter(theRequest.GetRequestStream());
            sw.Write(Parameters);
            sw.Close();

            // Execute the query
            theResponse =  (HttpWebResponse)theRequest.GetResponse();
            StreamReader sr = new StreamReader(theResponse.GetResponseStream());
            return sr.ReadToEnd();
        }

    }

    class MainClass
    {
        public static void Main(string[] args)
        {
            WebPostRequest myPost = new WebPostRequest("http://www.dijksterhuis.org/test/post.php");
            myPost.Add("keyword","void");
            myPost.Add("data","hello&+-[]");
            Console.WriteLine(myPost.GetResponse());
        }
    }
}

Cách 4:

public class RemotePost
{
    private System.Collections.Specialized.NameValueCollection Inputs = new System.Collections.Specialized.NameValueCollection();
    private string Url = string.Empty;

    /// <summary>
    /// number of seconds for timeout
    /// </summary>
    public int Timeout = 0;

    /// <summary>
    /// Url to post to
    /// </summary>
    public RemotePost(string url)
    {
        Url = url;
    }

    /// <summary>
    /// Parameters to add to the post
    /// </summary>
    /// <param name="name">parameter name</param>
    /// <param name="value">parameter value</param>
    public void Add(string name, string value)
    {
        Inputs.Add(name, value);
    }

    /// <summary>
    /// Posts data to server using POST method
    /// </summary>
    /// <returns></returns>
    public string Post()
    {
        string postData = String.Empty;

        int keyscount = Inputs.Keys.Count;

        for (int i = 0; i < keyscount; i++)
            postData += "&" + Inputs.Keys[i] + "=" + HttpUtility.UrlEncode(Inputs[Inputs.Keys[i]]);

        if (postData.Length > 0)
            postData = postData.Substring(1);

        HttpWebRequest wr = WebRequest.Create(Url) as HttpWebRequest;
        wr.Method = "POST";
        wr.ContentType = "application/x-www-form-urlencoded";

        using (StreamWriter requestWriter = new StreamWriter(wr.GetRequestStream()))
        {
            requestWriter.Write(postData);
        }

        return GetResponse(wr);
    }

    /// <summary>
    /// Posts data to server using GET method
    /// </summary>
    /// <returns></returns>
    public string Get()
    {
        string postData = String.Empty;

        int keyscount = Inputs.Keys.Count;

        for (int i = 0; i < keyscount; i++)
            postData += "&" + Inputs.Keys[i] + "=" + HttpUtility.UrlEncode(Inputs[Inputs.Keys[i]]);

        if (postData.Length > 0)
            postData = postData.Substring(1);

        string postUrl = Url.Contains("?") ? (Url + "&" + postData) : (Url + "?" + postData);

        HttpWebRequest wr = WebRequest.Create(postUrl) as HttpWebRequest;
        wr.Method = "GET";

        return GetResponse(wr);
    }

    private string GetResponse(HttpWebRequest wr)
    {
        wr.Timeout = Timeout * 1000; //millisecond timeout

        HttpWebResponse response = (HttpWebResponse)wr.GetResponse();

        if (response.StatusCode != HttpStatusCode.OK)
            return String.Empty;

        using (Stream responseStream = response.GetResponseStream())
        {
            using (StreamReader readerStream = new StreamReader(responseStream, System.Text.Encoding.UTF8))
            {
                return readerStream.ReadToEnd();
            }
        }
    }
}

RemotePost req = new RemotePost(postUrl);
req.Timeout = 3;

req.Add("FirstName", "First");
req.Add("LastName", "Last");

string response = req.Post();
string response = req.Get();

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