支持Cookie并开放了一些特殊设置项的HttpWebClient

 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Net;
 using System.IO;
 using System.Collections.Specialized;
 using System.Web;

 namespace Common.Helpers
 {
     /// <summary>
     /// 网络访问辅助类
     /// </summary>
     public class HttpWebClient : WebClient
     {
         #region 公共属性
         /// <summary>
         /// 浏览器用户标识,默认采用Chrome的标识
         /// </summary>
         public string UserAgent { get; set; }
         /// <summary>
         /// Cookie容器
         /// </summary>
         public CookieContainer CookieContainer { get; set; }
         /// <summary>
         /// 如果 POST 请求需要 100-Continue 响应，则为 true；否则为 false。
         /// </summary>
         public bool Expect100Continue { get; set; }

         private WebResponse m_LastWebResponse = null;
         /// <summary>
         /// 最后一次的响应对象
         /// </summary>
         public WebResponse LastWebResponse { get { return this.m_LastWebResponse; } }

         private int m_Timeout = ;
         /// <summary>
         /// 超时时间，默认120000毫秒(120秒)
         /// </summary>
         public int Timeout
         {
             get { return m_Timeout; }
             set { m_Timeout = value; }
         }

         private HttpWebClientSetting m_HttpWebClientSetting = null;
         /// <summary>
         /// WebClient设置项,该属性始终不会为null
         /// </summary>
         public HttpWebClientSetting HttpWebClientSetting
         {
             get
             {
                 if (m_HttpWebClientSetting == null)
                 {
                     m_HttpWebClientSetting = new HttpWebClientSetting();
                 }
                 return m_HttpWebClientSetting;
             }
             set
             {
                 m_HttpWebClientSetting = value ?? new HttpWebClientSetting();
             }
         }

         /// <summary>
         /// 预处理Web请求对象的委托方法(会在每次获取WebRequest对象后调用)，默认值为null
         /// </summary>
         public Action<HttpWebRequest> PrepareProcessWebRequest { get; set; }
         #endregion

         #region 构造方法
         public HttpWebClient()
             : this(new CookieContainer())
         {
         }

         public HttpWebClient(CookieContainer cookieContainer)
         {
             this.CookieContainer = cookieContainer;
             this.UserAgent = UserAgentValues.FireFox;
             this.Expect100Continue = false;
         }
         #endregion

         #region 重写方法，增加对CookieContainer的支持
         protected override WebRequest GetWebRequest(Uri address)
         {
             if (!string.IsNullOrEmpty(this.UserAgent))
             {
                 this.Headers.Add(HttpRequestHeader.UserAgent, this.UserAgent);
             }

             WebRequest request = base.GetWebRequest(address);
             request.Timeout = this.Timeout;

             if (request is HttpWebRequest)
             {
                 HttpWebRequest httpRequest = request as HttpWebRequest;
                 httpRequest.CookieContainer = this.CookieContainer;
                 httpRequest.ServicePoint.Expect100Continue = this.Expect100Continue; // 取消100-continue

                 //读取自定义设置项
                 if (this.HttpWebClientSetting != null)
                 {
                     httpRequest.AllowAutoRedirect = this.HttpWebClientSetting.AllowAutoRedirect;
                 }

                 //使用外部委托属性处理Request对象
                 if (this.PrepareProcessWebRequest != null)
                 {
                     this.PrepareProcessWebRequest(httpRequest);
                 }
             }

             return request;
         }
         #endregion

         #region 重写方法，增加对响应对象的访问
         protected override WebResponse GetWebResponse(WebRequest request)
         {
             WebResponse response = base.GetWebResponse(request);
             this.m_LastWebResponse = response;
             return response;
         }
         #endregion

         #region (public) 向一个URL用POST提交数据，并返回其响应内容 PostData
         /// <summary>
         /// 向一个URL用POST提交数据，并返回其响应内容
         /// ZhangQingFeng    2014-12-14    Add
         ///    EditLog：
         ///        ZhangQingFeng    2015-05-12    Edit        因WebClient的UpdateValues方法中固定为UTF-8格式进行UrlEncode，因此此处需用UploadString方式来间接实现    --见微软WebClient类源码UploadValuesInternal方法中
         /// </summary>
         /// <param name="url">请求的URL</param>
         /// <param name="data">要提交的数据</param>
         /// <param name="encoding">请求所使用的编码</param>
         /// <param name="responseEncoding">响应内容所使用的编码，为null时使用请求的编码</param>
         /// <returns>响应的内容</returns>
         public string PostData(string url, NameValueCollection data, Encoding encoding, Encoding responseEncoding)
         {
             WebClient client = this;

             /*
             client.Encoding = encoding ?? Encoding.UTF8;

             byte[] response = client.UploadValues(url, "POST", data ?? new NameValueCollection());

             string html = string.Empty;

             if (responseEncoding == null)
             {
                 html = client.Encoding.GetString(response);
             }
             else
             {
                 html = responseEncoding.GetString(response);
             }
              */

             client.Encoding = encoding ?? Encoding.UTF8;
             client.Headers.Add(HttpRequestHeader.ContentType, "application/x-www-form-urlencoded");

             string delimiter = String.Empty;
             StringBuilder values = new StringBuilder();
             foreach (string name in data.AllKeys)
             {
                 values.Append(delimiter);
                 values.Append(HttpUtility.UrlEncode(name, encoding));
                 values.Append("=");
                 values.Append(HttpUtility.UrlEncode(data[name], encoding));
                 delimiter = "&";
             }

             byte[] arrData = client.UploadData(url, "POST", Encoding.ASCII.GetBytes(values.ToString()));
             string html = (responseEncoding ?? client.Encoding).GetString(arrData);

             return html;
         }

         /// <summary>
         /// 向一个URL用POST提交数据，并返回其响应内容
         /// ZhangQingFeng    2014-12-14    Add
         /// </summary>
         /// <param name="url">请求的URL</param>
         /// <param name="data">要提交的数据</param>
         /// <param name="encoding">请求和响应所使用的编码</param>
         /// <returns>响应的内容</returns>
         public string PostData(string url, NameValueCollection data, Encoding encoding)
         {
             return PostData(url, data, encoding, null);
         }

         /// <summary>
         /// 向一个URL用POST提交数据，并返回其响应内容(使用this.Encoding来作请求编码和响应编码)
         /// ZhangQingFeng    2014-12-14    Add
         /// </summary>
         /// <param name="url">请求的URL</param>
         /// <param name="data">要提交的数据</param>
         /// <returns>响应的内容</returns>
         public string PostData(string url, NameValueCollection data)
         {
             return PostData(url, data, this.Encoding);
         }
         #endregion

         #region (public) 向一个URL用POST提交数据，并返回其响应内容 PostData
         /// <summary>
         /// 向一个URL用POST提交数据，并返回其响应内容
         /// ZhangQingFeng    2014-12-14    Add
         /// </summary>
         /// <param name="url">请求的URL</param>
         /// <param name="data">要提交的数据</param>
         /// <param name="encoding">请求和响应内容所使用的编码</param>
         /// <returns>响应的内容</returns>
         public string PostData(string url, Dictionary<string, string> data, Encoding encoding, Encoding responseEncoding)
         {
             NameValueCollection postData = new NameValueCollection();
             if (data != null)
             {
                 foreach (var item in data)
                 {
                     postData.Add(item.Key, item.Value);
                 }
             }
             return PostData(url, postData, encoding, responseEncoding);
         }

         /// <summary>
         /// 向一个URL用POST提交数据，并返回其响应内容
         /// ZhangQingFeng    2014-12-14    Add
         /// </summary>
         /// <param name="url">请求的URL</param>
         /// <param name="data">要提交的数据</param>
         /// <param name="encoding">请求和响应所使用的编码</param>
         /// <returns>响应的内容</returns>
         public string PostData(string url, Dictionary<string, string> data, Encoding encoding)
         {
             return PostData(url, data, encoding, null);
         }

         /// <summary>
         /// 向一个URL用POST提交数据，并返回其响应内容(使用this.Encoding来作请求编码和响应编码)
         /// ZhangQingFeng    2014-12-14    Add
         /// </summary>
         /// <param name="url">请求的URL</param>
         /// <param name="data">要提交的数据</param>
         /// <returns>响应的内容</returns>
         public string PostData(string url, Dictionary<string, string> data)
         {
             return PostData(url, data, this.Encoding);
         }
         #endregion

         #region 辅助类
         /// <summary>
         /// 浏览器用户标识类
         /// </summary>
         public class UserAgentValues
         {
             public static readonly string FireFox = "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:37.0) Gecko/20100101 Firefox/37.0";
             public static readonly string Chrome = "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2272.101 Safari/537.36";
             public static readonly string IE8 = "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; WOW64; Trident/4.0; SLCC2;)";
         }
         #endregion
     }

     /// <summary>
     /// HttpWebClient对象设置类
     /// </summary>
     public class HttpWebClientSetting
     {
         private bool m_AllowAutoRedirect = true;
         /// <summary>
         /// 当响应内容为重定向时客户端是否自动重定向(如果该属性为true，则取到的响应则为重定向后的内容，否则则为响应原文)，默认值为true
         /// </summary>
         public bool AllowAutoRedirect
         {
             get { return m_AllowAutoRedirect; }
             set { m_AllowAutoRedirect = value; }
         }
     }
 }

HttpWebClient

在做页面抓取的过程中，发现自带的WebClient不够灵活，因此做了一个实现。

关于在PostData方法中不使用UploadValues()方法的原因：

1.查看微软的源代码实现时发现，无论设置请求时的Encoding是否为GB2312，在使用WebClient的UploadValues()上传内容时，其内在都是使用UTF-8编码进行UrlEncode，因此传到服务端中的数据中若包含有中文时则一定会乱码，因此重写PostData以规避此问题。

关于HttpWebClientSetting中的AllowAutoRedirect属性：

在WebClient发起请求时，若响应内容为重定向，则WebClient会自动做重定向，因此该类提供此设置项以控制在访问时是否自动做重定向(第二次访问Refer后的网站时会将请求中的Refer头置空，将该AllowAutoRedirect设置为false，然后手动从Response.Header中取出Location对象地址，设置Refer后再访问，则可真实模拟浏览器访问，从而避开一些网站的防抓取设置)

关于HttpWebClient中的LastWebResponse属性：

当存在多次重定向时，系统记录了最后一次返回的内容，从此内容的Header中取出ResponseUri，则可以取到最后返回响应的页面真实地址，从而为下一次的设置请求Refer头作准备。

大约就是如此，后期如有Bug会继续更新。