在 WinForm 中打开页面采用POST方式传参http。可以多个参数传递，返回json字符串

//调用方法

Dictionary<string, string> postData = new Dictionary<string, string>();

postData.Add("user", "aaa");

postData.Add("pass", "bbb");

GetPageByPost("http://www.xxx.com/send.aspx", postData, Encoding.UTF8);

/// <summary>

/// 以 Post 方式提交网页数据,获得服务器返回的数据

/// </summary>

/// <param name="url"> Url </param>

/// <param name="postData">Post 数据</param>

/// <param name="encoder">网页编码</param>

/// <returns>服务器返回的数据</returns>

public string GetPageByPost(string url, Dictionary<string, string> postData, Encoding encoder)

{

    string html = "";

    HttpWebRequest webReq = (HttpWebRequest)WebRequest.Create(url);

    webReq.Method = "POST";

    Stream reqStream = null;

    if (postData != null && postData.Count > 0) {

        StringBuilder sb = new StringBuilder();

        foreach (KeyValuePair<string, string> kv in postData) {

            sb.Append(HttpUtility.UrlEncode(kv.Key));

            sb.Append("=");

            sb.Append(HttpUtility.UrlEncode(kv.Value));

            sb.Append("&");

        }

        byte[] data = Encoding.UTF8.GetBytes(sb.ToString().TrimEnd('&'));

        webReq.ContentType = ContentType;

        webReq.ContentLength = data.Length;

        reqStream = webReq.GetRequestStream();

        reqStream.Write(data, 0, data.Length);

    }

    HttpWebResponse webResp = (HttpWebResponse)webReq.GetResponse();

    Stream stream = webResp.GetResponseStream();

    StreamReader sr = new StreamReader(stream, encoder);

    html = sr.ReadToEnd();

    sr.Close();

    stream.Close();

    if (reqStream != null) {

        reqStream.Close();

    }

    return html;

}