Thinking more...

無心工作，剛好又看到有人問怎麼用，所以就牛刀小試一下。基本上用 WebClient 就可以搞定：

//
// Yahoo!搜尋『斷章取義』API http://tw.developer.yahoo.com/cas/
// Yahoo!搜尋『斷章取義』API 技術文件 http://tw.developer.yahoo.com/cas/api.php
//
using System;
using System.IO;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Data;
using System.Text;
using System.Net;

namespace Yahoo
{
    public class CAS
    {
        private string _GetString( byte[] bytes )
        {
            return Encoding.UTF8.GetString(bytes);
        }

        private WebClient GetWebClient()
        {
            WebClient client = new WebClient();
            // client.Proxy = new WebProxy("localhost", 8000);
            client.Encoding = Encoding.UTF8;
            return client;
        }

        private NameValueCollection GetParameters()
        {
            NameValueCollection data = new NameValueCollection();

            // TODO: Place your appid here.
            data.Add("appid", "APgdNPnV34E7WQlhpBYaQvaRWPjwvd8exe094Q_r_7GWEOBFh9UDQY6vqNgZVwhc");
            return data;
        }

        public string WordSegmentation(string content)
        {
            NameValueCollection data = GetParameters();
            data.Add("content", content);

            WebClient client = GetWebClient();
            byte[] responseArray = client.UploadValues("http://asia.search.yahooapis.com/cas/v1/ws", "POST", data);
            return _GetString(responseArray);
        }

        public string Authenticate()
        {
            NameValueCollection data = GetParameters();

            WebClient client = GetWebClient();
            byte[] responseArray = client.UploadValues("http://asia.search.yahooapis.com/cas/v1/AuthBootUp.php", "POST", data);
            return _GetString(responseArray);
        }

        public string KeywordExtraction( string content, int threshold, int maxnum )
        {
            NameValueCollection data = GetParameters();
            data.Add("content", content);
            data.Add("threshold", threshold.ToString());
            data.Add("maxnum", maxnum.ToString());

            WebClient client = GetWebClient();
            byte[] responseArray = client.UploadValues("http://asia.search.yahooapis.com/cas/v1/ke", "POST", data);
            return _GetString(responseArray);
        }

        public static void Main() 
        {
            Yahoo.CAS cas = new Yahoo.CAS();

            // 如果你重新申請 appid 的話，要先作 Authenticate
            // Console.WriteLine( cas.Authenticate() );
            Console.WriteLine( cas.WordSegmentation( "your_text" ));
            Console.WriteLine( cas.KeywordExtraction( "your_text", 30, 10));

            // 得到的字串是 XML，要轉為 DataSet 的話，可以這樣作
            /*
            string text = cas.WordSegmentation( "your_text" );
            TextReader stringReader = new StringReader(text);
            DataSet dataSet1 = new DataSet();
            dataSet1.ReadXml(stringReader);
            // 轉好以後應會有兩個 DataTable，資料都在第二個 Table 裡，也就是 dataSet1.Tables[1]
            */
        }
    }
}