name WebRequestMethods 在当前上下文中不存在，并且找不到 FTPWebResopnce

我正在为文件下载进行编码，但它是WebRequestMethods不存在显示。

        FtpWebRequest request = (FtpWebRequest)WebRequest.Create("ftp://something.com/temp.exe");
        request.Method = WebRequestMethods.Ftp.DownloadFile;
        request.Credentials = new NetworkCredential("admin", "1234");
        FtpWebResponse response = (FtpWebResponse)request.GetResponse();
        Stream responseStream = response.GetResponseStream();
        FileStream file = File.Create(@"c:'temp'temp.exe");
        byte[] buffer = new byte[32 * 1024];
        int read;
        //reader.Read(
        while ((read = responseStream.Read(buffer, 0, buffer.Length)) > 0)
        {
            file.Write(buffer, 0, read);
        }
        file.Close();
        responseStream.Close();
        response.Close();

以上是我的代码请指导我。！