using System;
using System.Net;

  public class CSiteMonitor
  {
      public struct UrlHeaderInfo
      {
          public DateTime _lastModified;
          public string _statusCode;
          public string _errorMessage;
      }

      /// <summary>
      /// آیا آدرس اینترنتی وارد شده معتبر است؟
      /// </summary>
      /// <param name="url">آدرس مورد نظر جهت بررسی</param>
      /// <returns></returns>
      public static bool IsValidURL(string url)
      {
          try
          {
              Uri uri = new Uri(url);
              return (uri.Scheme == Uri.UriSchemeHttp) || (uri.Scheme == Uri.UriSchemeHttps);
          }
          catch { return false; }
      }
      /// <summary>
      /// آدرس اینترنتی جهت بررسی
      /// </summary>
      /// <param name="url"></param>
      /// <returns></returns>
      /// <exception cref="ArgumentException">آدرس اینترنتی وارد شده معتبر نیست</exception>
      public static UrlHeaderInfo GetSiteHeaderInfo(string url)
      {
          if (!IsValidURL(url))
              throw new ArgumentException("آدرس اینترنتی وارد شده معتبر نیست", "url");

          UrlHeaderInfo hhi = new UrlHeaderInfo { _lastModified = DateTime.Now, _statusCode = "NOK!", _errorMessage = string.Empty };

          HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
          //request.Proxy
          request.Method = "HEAD";
          request.AllowAutoRedirect = true;
          request.UserAgent = "Mozilla/5.0 (Windows; U; Windows NT 5.0; ; rv:1.8.0.7) Gecko/20060917 Firefox/1.9.0.1";
          request.Timeout = 1000 * 300;
          request.AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate;
        
          try
          {
              using (HttpWebResponse response = (HttpWebResponse) request.GetResponse())
              {
                  hhi._statusCode = response.StatusCode.ToString();
                  hhi._lastModified = response.LastModified;
              }
          }
          catch (Exception ex)
          {
              hhi._errorMessage = ex.Message;
          }

          return hhi;
      }
  }

CSiteMonitor.UrlHeaderInfo info = CSiteMonitor.GetSiteHeaderInfo("http://www.google.com");
MessageBox.Show(info._statusCode);