c# 如何用WebClient实现网页登陆?

c# 如何用WebClient实现网页登陆? 并保持登陆状态,下载所需要的文件.
我虽然可以用webbrowser可以实现,不过会跳出 WIN系统的  下载框体 出来.
           
//wbtest 的当前页面是dkp.178.com也就是登陆页面
                HtmlElement ClickBtn = null;
                HtmlDocument doc = wbtest.Document;
                //搜寻表单中的元素.
                for (int i = 0; i < doc.All.Count; i++)
                {
                    if (doc.All[i].TagName.ToUpper().Equals("INPUT"))
                    {
                        switch (doc.All[i].Name)
                        {
                            case "email":    //用户名文本框 的 name
                                doc.All[i].InnerText = "username";
                                break;
                            case "password": //密码文本框 的 name
                                doc.All[i].InnerText = "password";
                                break;
                        }
                    }
                                             //按钮框 的 元素名,提交按钮是用BUTTON实现的.中间内容是登 陆
                    if (doc.All[i].TagName.ToUpper().Equals("BUTTON"))
                    {
                        if (doc.All[i].InnerText == "登 录")
                        {
                            ClickBtn = doc.All[i];
                        }
                    }
                }
                ClickBtn.InvokeMember("Click");   //执行按扭操作
                int interval = 2000; //2 秒
                 //执行登陆后的下载操作,以178的DKP登陆后下载界面为例
                wbtest.Navigate("http://dkp.178.com/action=download&option=log&guild_id=50519&setting=midkp");
有什么办法实现不跳框直接保存在想要的目录下.或者用WebClient登陆?