c#这个爬虫代码会出现内存泄漏吗？

Question

抓取某帖子的图片，内存从一开始不到10MB涨到最后33MB，为什么内存占用会多这么多，已经用了using (){}代码段了，还能怎样优化才能减少内存占用。

代码入口是BtnImageClick()，其中inputTid是用户输入的帖子tid是一串数字

/*
 * Created by SharpDevelop.
 * User: licre
 * Date: 2021/3/18
 * Time: 4:58
 * 下载nga论坛网页版的图片
 * To change this template use Tools | Options | Coding | Edit Standard Headers.
 */
using System;
using System.Collections.Generic;
using System.Drawing;
using System.Windows.Forms;
// 用户
using System.Net;
using System.Text.RegularExpressions;
using System.IO;
using System.Threading;

namespace ngan6
{
    /// <summary>
    /// Description of MainForm.
    /// </summary>
    public partial class MainForm : Form
    {
        static string default_cookie = @"ngaPassportUid=61471889; ngaPassportCid=X947obm5v6mjjorchujde3b6ab2pvbv7vgk8itjo; ";
        static string user_agent = "Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.192";


        public MainForm()
        {
            //
            // The InitializeComponent() call is required for Windows Forms designer support.
            //
            InitializeComponent();
            
            //
            // TODO: Add constructor code after the InitializeComponent() call.
            //
        }
        
        public string download_json(string tid, int page_num){
            string link = "https://bbs.nga.cn/read.php?tid=" + tid + "&lite=js&__output=8&page=" + Convert.ToString(page_num);
            string json_body = "";
            
                
                using (WebClient dl = new WebClient()){
                    dl.Headers.Add("User-agent", MainForm.user_agent);
                    dl.Headers.Add("Cookie", MainForm.default_cookie);
                    json_body = dl.DownloadString(link);
                }
            
            if (String.IsNullOrEmpty(json_body)){
                throw new Exception("下载json失败");
                
            }
            return json_body;
        }


        /// <summary>
        /// 获取帖子基本信息
        /// </summary>
        /// <param name="html_body">json文本</param>
        /// <returns></returns>
        public int summarise(string tid, string html_body=""){
            if (String.IsNullOrEmpty(html_body)){
                html_body = "333replies\":94,\"authorid\"666\"authorid\":594999,\"subject\":\"大哥们护栏多少钱一米啊\",\"type\":33554436,\"fid\"";
                // 供测试用
            }
            Regex reply1 = new Regex("replies\":(\\d+),\"authorid");
            Match m1 = reply1.Match(html_body);
            if (m1.Groups.Count == 0){
                throw new Exception("无法找到帖子回复数");
            }
//            Regex subject1 = new Regex("subject\":\"(.*?)\",\"type");
//            Match m2 = subject1.Match(html_body);
//            if (m2.Groups.Count == 0){
//                throw new Exception("无法找到帖子主题");
//            }
        
            //ti.subject = m2.Groups[1].Value;
//            WarnMsg(ti.subject);
        
            int reply_count = Convert.ToInt16(m1.Groups[1].Value);
            int page_count = 1 + reply_count / 20;
            return page_count;
        }
        /// <summary>
        /// 下载各个页面的json文本
        /// </summary>
        /// <param name="tid"></param>
        /// <param name="last_page_num"></param>
        /// <returns></returns>
        public string[] scan_pages(string tid){
            
            // 先下载第一页
            string page1 = download_json(tid, 1);
            int page_count = summarise(tid, page1);
            string[] pages = new string[page_count];
            pages[0] = page1;
            
            for (int page_num = 2; page_num <= page_count; page_num +=1){
                string raw = download_json(tid, page_num);
                Thread.Sleep(1000);
                int pos1 = raw.IndexOf("\"__R\"");
                int pos2 = raw.IndexOf("\"__T\"");
                string page = raw.Substring(pos1, pos2-pos1); // 提取回复正文
                pages[page_num-1] = page;
            }
            return pages;
        }
        
        
        /// <summary>
        /// 从页面文本提取图片和音频链接
        /// </summary>
        /// <param name="pages"></param>
        /// <returns></returns>
        public Dictionary<string, string> refine_image_links(string[] pages){
            // 图片名字->图片链接
            Dictionary<string, string> images = new Dictionary<string, string>();
            //string bb = "初照人？][img]https://img.nga.178.com/attachments/mon_202006/14/-7da9Q5-6dm6Z27T3cS1z4-12s.jpg[/img][/collapsetent\":\"[img]./mon_202103/19/-7da9Q9506-4la9KhT3cSmo-8r.png[/img]<br/>政府工程";
            Regex img_re = new Regex(@"\[img\](.*?)\[\/img\]", RegexOptions.Multiline);
            
            foreach (string page in pages){
                // 提取回复内容
                
                
                
                MatchCollection m = img_re.Matches(page);
            
                for (int r = 0; r < m.Count; r += 1){
                    string link = m[r].Groups[1].Value;
                    if (link.Contains("[")){
                        // 地址最后的[/img]文本被截断，地址中会含有其他bbcode或者中文字符,因此舍弃
                        continue;
                    }
                    string name = link.Substring(link.LastIndexOf("/")+1);
                    if (images.ContainsKey(name)){ continue; }
                    if (link.StartsWith("./")){
                        link = "https://img.nga.178.com/attachments" + link.Substring(1);
                    }
                    
                    images.Add(name, link);
                    
                }
                
            }
            return images;
        }
        
        public Dictionary<string, string> refine_audio_links(string[] pages){
            Dictionary<string, string> audios = new Dictionary<string, string>();
            //string bb = "[flash=audio]./mon_202010/22/-7Q5-ab07Zo.mp3?duration=1′2″[/flash]<br/>拙作半首，列位见笑。";
            Regex audio_re = new Regex(@"\[flash=audio\](.*?)\[\/flash\]", RegexOptions.Multiline);
            int special = 1;
            foreach (string page in pages){
                // 提取回复内容
                
                
                
                MatchCollection m = audio_re.Matches(page);
            
                for (int r = 0; r < m.Count; r += 1){
                    int c = 1;
                    string link = m[r].Groups[1].Value;
                    if (link.Contains("[")){
                        // 地址最后的[/flash]文本被截断，地址中会含有其他bbcode或者中文字符,因此舍弃
                        continue;
                    }
                    if (link.Contains("?duration")){
                        link = link.Substring(0, link.IndexOf("?duration"));
                    }
                    string name = link.Substring(link.LastIndexOf("/")+1);
                    if (audios.ContainsKey(name)){ continue; }
                    if (link.StartsWith("./")){
                        link = "https://img.nga.178.com/attachments" + link.Substring(1);
                    }
                    if (name.Contains("nga_audio")){
                        name.Replace("nga_audio", String.Format("audio_{0}", special));
                        special += 1;
                    }
                    audios.Add(name, link);
                    
                }
                
            }
            return audios;
        }
        
        /// <summary>
        /// 下载具体图片
        /// </summary>
        /// <param name="images"></param>
        public int download_media(string tid, Dictionary<string, string> media){
            string path = String.Format("./{0}", tid);
            if (!Directory.Exists(path)){
                Directory.CreateDirectory(path);
            }
            int count = 0;
            foreach (string name in media.Keys){
                string link = media[name];
                byte[] body;
                lblProgress.Text = name;
                Application.DoEvents();
                
                string media_path = String.Format("./{0}/{1}", tid, name);
                
                if (File.Exists(media_path)){
                    continue;
                }
                using (WebClient dl = new WebClient()){
                    dl.Headers.Add("User-agent", MainForm.user_agent);
                    dl.Headers.Add("Cookie", MainForm.default_cookie);
                    body = dl.DownloadData(link);
                }
                if (body.Length <= 12000){
                    // 排除过小的图片，可能是表情包
                    continue;
                }
                using (BinaryWriter bw = new BinaryWriter(File.Open(media_path, FileMode.Create))){
                    bw.Write(body);
                }
                count += 1;
                Thread.Sleep(1000);
            }
            return count;
        }
        
        void BtnImageClick(object sender, EventArgs e)
        {
            string tid = inputTid.Text;
            string[] pages = scan_pages(tid);
            Dictionary<string, string> images = refine_image_links(pages);
            int count = download_media(tid, images);
            lblProgress.Text = String.Format("共下载{0}张图片", count);
        }

        
        void MainFormFormClosing(object sender, FormClosingEventArgs e)
        {
            Environment.Exit(0);
        }
        
        void BtnAudioClick(object sender, EventArgs e)
        {
            string tid = inputTid.Text;
            string[] pages = scan_pages(tid);
            Dictionary<string, string> audios = refine_audio_links(pages);
            int count = download_media(tid, audios);
            lblProgress.Text = String.Format("共下载{0}个音频", count);
        }
        

    }
}