C# 下载文件

public static string _defaultUserAgent = "Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/50.0.2661.102 UBrowser/5.7.15319.202 Safari/537.36";   //定义默认浏览器类型

	public static int _timeout = 35000;    //定义请求超时时间

	public void DoGetImage(string url, string path)
        {
            HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);
            req.ServicePoint.Expect100Continue = false;
            req.Method = "GET";
            req.KeepAlive = true;
            req.UserAgent = _defaultUserAgent;
            req.Timeout = _timeout;//3秒
            Uri aurl = new Uri(url);
            
            //通过url 获取域名
           // string host = "img.alicdn.com";
            req.Host = aurl.Host;
            req.Headers.Add("Accept-Encoding", "gzip,deflate");
            //req.Headers.Add("Accept", "image/webp,image/*,*/*;q=0.8");
            req.Headers.Add("Accept-Language", "zh-CN,zh;q=0.8");

            string imgType = "jpeg";//通过url 获取图片的扩展名

            imgType = url.Substring(url.LastIndexOf('.')+1);
            if (imgType == "jpg") imgType = "jpeg";
            req.ContentType = "image/" + imgType;
            Random rad = new Random();
            int rnum = rad.Next(1000, 9999);
            path += "/" + DateTime.Now.ToString("HHmmssfff") + rnum + "." + imgType;

            using (HttpWebResponse rsp = (HttpWebResponse)req.GetResponse())
            {

                System.IO.Stream stream = null;
                try
                {
                    // 以字符流的方式读取HTTP响应
                    using (stream = rsp.GetResponseStream())
                    {
                        using (Image am = Image.FromStream(stream))
                        {
                            am.Save(path);
                            am.Dispose();
                        }
                        stream.Close();
                        stream.Dispose();
                        stream = null;
                    }
                }
                finally
                {
                    // 释放资源
                    if (stream != null) stream.Close();
                    if (rsp != null) rsp.Close();  
                }
            }
            req = null;
        }

调用：
DoGetImage(“http://img.alicdn.com/imgextra/i1/1736826123/1736826123.jpg”, “D://imgs//”);