如果这确实是CodeProject中的文件，则很可能您在尝试下载文件之前忘记了身份验证(登录).我建议您改用System.Net.HttpWebRequest类:
http://msdn.microsoft.com/en-us/library/system.net. httpwebrequest.aspx [ ^ ].

以下是几个问题，并附有解释方法的答案:
http://***.com/questions/9841344/c-sharp-https-登录并下载文件 [ ^ ]，
http://***.com/questions/4699938/如何使用httpwebrequest-and-httpwebresponse-classcookies下载文件 [

If this is really a file from CodeProject, most likely you forgot to authenticate (login) before trying to download a file. I would advise that you use the class System.Net.HttpWebRequest instead:
http://msdn.microsoft.com/en-us/library/system.net.httpwebrequest.aspx[^].

Here are a couple of question with answers explaining how to do it:
http://***.com/questions/9841344/c-sharp-https-login-and-download-file[^],
http://***.com/questions/4699938/how-to-download-the-file-using-httpwebrequest-and-httpwebresponse-classcookies[^].

—SA

private void button1_Click(object sender, EventArgs e)
       {
  string url = downloadURL.Text;
           WebBrowser bweb = new WebBrowser();
           bweb.DocumentCompleted += new WebBrowserDocumentCompletedEventHandler(bweb_DocumentCompleted);
           bweb.Navigate(new Uri(url));

       }

       void bweb_DocumentCompleted(object sender, webBrowserDocumentCompletedEventArgs e)
       {
           string url = downloadURL.Text;
           WebClient client = new WebClient();
           client.Credentials = new NetworkCredential("ravi", "ravi");
           client.BaseAddress = "http://www.codeproject.com";
           string FileName = url.Substring(url.LastIndexOf("/") + 1,
                           (url.Length - url.LastIndexOf("/") - 1));
           client.DownloadFile(url, savePath.Text + FileName);
       }