问证明一个文件的存在？另外，如何停止你的程序冻结时美化网站

EN

if (!File.Exists(@"Res\" + pokeName + ".jpg"))
        {
            var item = doc.DocumentNode.SelectNodes("//a[@class='image']//img")[0];
            if (doc.DocumentNode.SelectNodes("//a[@class='image']//img") != null)
            {
                getImage(item.GetAttributeValue("src", ""));
                pictureBox1.Load(@"Res\" + pokeName + ".jpg");
            }
        }
        else
        {
            pictureBox1.Load(@"Res\" + pokeName + ".jpg");
        }

        string imageUrl = @""+ url;
        string saveLocation = @"Res\"+ pokeName + ".jpg";

        byte[] imageBytes;
        HttpWebRequest imageRequest = (HttpWebRequest)WebRequest.Create(imageUrl);
        WebResponse imageResponse = imageRequest.GetResponse();

        Stream responseStream = imageResponse.GetResponseStream();

        using (BinaryReader br = new BinaryReader(responseStream ))
        {
            imageBytes = br.ReadBytes(500000);
            br.Close();
        }
        responseStream.Close();
        imageResponse.Close();

        FileStream fs = new FileStream(saveLocation, FileMode.Create);
        BinaryWriter bw = new BinaryWriter(fs);
        try
        {
            bw.Write(imageBytes);
        }
        finally
        {
            fs.Close();
            bw.Close();
        }
    }