C#のWebRequestとWebClientでCookie認証をする方法(と、mixiボイスへの投稿)

// WebRequestによるCookie認証
// POSTしてCookieContainerに書き込む
var data = Encoding.ASCII.GetBytes(string.Join("&",
    new[] { "next_url=/home.pl", "email=めるあど", "password=ぱすわど" }));
var cookieContainer = new CookieContainer();
var req = (HttpWebRequest)WebRequest.Create("https://mixi.jp/login.pl");
req.CookieContainer = cookieContainer;
req.Method = "POST";
req.ContentType = "application/x-www-form-urlencoded";
req.ContentLength = data.Length;
using (var stream = req.GetRequestStream())
{
    stream.Write(data, 0, data.Length);
}
var res = req.GetResponse(); // ここでCookieContainerに書き込まれる
 
// 以下、そのCookieを使えばアクセスし放題
var reqLog = (HttpWebRequest)WebRequest.Create("https://mixi.jp/show_log.pl");
reqLog.CookieContainer = cookieContainer; // CookieContainerセット
var resLog = reqLog.GetResponse();
using (var stream = resLog.GetResponseStream())
using (var sr = new StreamReader(stream, Encoding.GetEncoding("euc-jp")))
{
    Console.WriteLine(sr.ReadToEnd()); // アクセスできてるのを確認
}

// WebClientならポストは超簡単！
var wc = new WebClient { Encoding = Encoding.GetEncoding("euc-jp") };
wc.UploadValues("https://mixi.jp/login.pl", new NameValueCollection
{
    {"next_url", "/home.pl"},
    {"email", "めるあど"},
    {"password", "ぱすわど"}
});
// じゃあCookieはどうするの？というと、ResponseHeaderから自前で抽出します
var setCookie = wc.ResponseHeaders[HttpResponseHeader.SetCookie];
var cookies = Regex.Split(setCookie, "(?<!expires=.{3}),")
    .Select(s => s.Split(';').First().Split('='))
    .Select(xs => new { Name = xs.First(), Value = string.Join("=", xs.Skip(1).ToArray()) })
    .Select(a => a.Name + "=" + a.Value)
    .ToArray();
var cookie = string.Join(";", cookies);
// 以降は取得したCookieをHeaderに設定しておけばOk
wc.Headers[HttpRequestHeader.Cookie] = cookie;
var result = wc.DownloadString("https://mixi.jp/show_log.pl");
Console.WriteLine(result); // アクセスできてるのを確認

class CustomWebClient:WebClient
{
    private CookieContainer cookieContainer = new CookieContainer();
 
    // WebClientはWebRequestのラッパーにすぎないので、
    // GetWebRequestのところの動作をちょっと横取りして書き換える
    protected override WebRequest GetWebRequest(Uri address)
    {
        var request = base.GetWebRequest(address);
        if (request is HttpWebRequest)
        {
            (request as HttpWebRequest).CookieContainer = cookieContainer;
        }
        return request;
    }
}
 
// WebClientを継承してちょっと書き換えてやれば一番簡単
var cwc = new CustomWebClient { Encoding = Encoding.GetEncoding("euc-jp") };
cwc.UploadValues("https://mixi.jp/login.pl", new NameValueCollection
{
    {"next_url", "/home.pl"},
    {"email", "める"},
    {"password", "ぱす"}
});
var result = cwc.DownloadString("https://mixi.jp/show_log.pl");
Console.WriteLine(result); // アクセスできてるのを確認

static void Main(string[] args)
{
    var encoding = Encoding.GetEncoding("euc-jp");
    // ログイン
    var cwc = new CustomWebClient { Encoding = encoding };
    cwc.UploadValues("https://mixi.jp/login.pl", new NameValueCollection
    {
        {"next_url", "/home.pl"},
        {"email", "めーる"},
        {"password", "ぱすわど"}
    });
 
    // 投稿に必要なpost_keyをhtmlから取り出す
    var echo = cwc.DownloadString("https://mixi.jp/recent_echo.pl");
    var postKey = Regex.Match(echo, "id=\"post_key\" value=\"(.+?)\"").Groups[1].Value;
 
    // 例なので簡易化するため認証無しのTwitterステータスを取得します
    // HttpUtilityの利用にはSystem.Webの参照設定が別途必要
    var id = "自分の(じゃなくてもいいけど)TwitterID";
    var texts = XDocument.Load("http://twitter.com/statuses/user_timeline/" + id + ".xml")
        .Descendants("status")
        .Select(x => HttpUtility.HtmlDecode(x.Element("text").Value))
        .Reverse();
    foreach (var text in texts)
    {
        // mixiボイスに投稿(UTF-8以外の日本語の投稿はUploadValuesが使えない(泣)
        cwc.Headers[HttpRequestHeader.ContentType] = "application/x-www-form-urlencoded";
        cwc.UploadString("http://mixi.jp/add_echo.pl", string.Join("&", new[]
        {
            "body=" + HttpUtility.UrlEncode(text,　encoding),
            "post_key=" + postKey,
            "redirect=recent_echo"
        }));
    }
}