C# 图片下载工具类

写在前面

从浏览器的Html文本中获取图片链接并保存到本地，同时对图片的分辨率和品质进行处理，以满足某些平台的规格需求；可以放到多线程中调用以提高下载效率。

代码实现

    public class ImageDownloader
    {
        private int minImageSize = 1024 * 10;
        private int limitImageSize = 1024 * 1024;

        public Image SaveWebImage(string url, string savePath)
        {
            try
            {
                if (url.EndsWith("gif"))
                    return null;

                using (var c = new WebClient())
                {
                    string imgUrl = url;
                    if (imgUrl.StartsWith("//"))
                    {
                        imgUrl = "http:" + imgUrl;
                    }
                    var img = Image.FromStream(new MemoryStream(c.DownloadData(imgUrl)));
                    // 小图直接略过
                    if (img.Width < 200 || img.Height < 200)
                        return null;
                    var r = Resize(img, 640, 640, true);
                    r.Save(savePath);

                    var fileInfo = new FileInfo(savePath);
                    if (fileInfo.Length < minImageSize) // 如果小于10kb那就是无效图片
                        return r;
                    if (fileInfo.Length > limitImageSize) // 品质可以通过参数来控制
                    {
                        EncoderParameters encoderParams = new EncoderParameters();
                        long[] quality = new long[1];
                        quality[0] = 85; // 品质参数，最大为100
                        EncoderParameter encoderParam = new EncoderParameter(Encoder.Quality, quality);
                        encoderParams.Param[0] = encoderParam;
                        ImageCodecInfo codecInfo = GetEncoder(ImageFormat.Jpeg);
                        r.Save(savePath, codecInfo, encoderParams);
                        encoderParam.Dispose();
                        encoderParams.Dispose();
                    }
                    img.Dispose();
                    return r;
                }
            }
            catch (Exception ex)
            {
                LogHelper.Error(ex);
                LogHelper.Error("error url:" + url);
                return null;
            }
        }

        public Image Resize(Image image, int newWidth, int maxHeight, bool onlyResizeIfWider)
        {
            if (onlyResizeIfWider && image.Width <= newWidth) newWidth = image.Width;

            var newHeight = image.Height * newWidth / image.Width;
            var res = new Bitmap(newWidth, newHeight);

            using (var graphic = Graphics.FromImage(res))
            {
                graphic.InterpolationMode = InterpolationMode.HighQualityBicubic;
                graphic.SmoothingMode = SmoothingMode.HighQuality;
                graphic.PixelOffsetMode = PixelOffsetMode.HighQuality;
                graphic.CompositingQuality = CompositingQuality.HighQuality;
                graphic.DrawImage(image, 0, 0, newWidth, newHeight);
            }

            return res;
        }

        #region Private Methods

        private ImageCodecInfo GetEncoder(ImageFormat format)
        {

            ImageCodecInfo[] codecs = ImageCodecInfo.GetImageDecoders();

            foreach (ImageCodecInfo codec in codecs)
            {
                if (codec.FormatID == format.Guid)
                {
                    return codec;
                }
            }
            return null;
        }

        #endregion
    }