工具方法：

 public class ImageToolsB
        /// <summary>
        /// 无损压缩图片
        /// </summary>
        /// <param name="sFile">原图片地址</param>
        /// <param name="dFile">压缩后保存图片地址</param>
        /// <param name="flag">压缩质量（数字越小压缩率越高）1-100</param>
        /// <param name="size">压缩后图片的最大大小</param>
        /// <param name="sfsc">是否是第一次调用</param>
        /// <returns></returns>
        public static bool CompressImage(string sFile, string dFile, int flag = 90, int size = 500, bool sfsc = true)
            //如果是第一次调用，原始图像的大小小于要压缩的大小，则直接复制文件，并且返回true
            FileInfo firstFileInfo = new FileInfo(sFile);
            if (sfsc == true && firstFileInfo.Length < size * 1024)
                firstFileInfo.CopyTo(dFile);
                return true;
            Image iSource = Image.FromFile(sFile);
            ImageFormat tFormat = iSource.RawFormat;
            int dHeight = iSource.Height / 2;
            int dWidth = iSource.Width / 2;
            int sW = 0, sH = 0;
            //按比例缩放
            Size tem_size = new Size(iSource.Width, iSource.Height);
            if (tem_size.Width > dHeight || tem_size.Width > dWidth)
                if ((tem_size.Width * dHeight) > (tem_size.Width * dWidth))
                    sW = dWidth;
                    sH = (dWidth * tem_size.Height) / tem_size.Width;
                    sH = dHeight;
                    sW = (tem_size.Width * dHeight) / tem_size.Height;
                sW = tem_size.Width;
                sH = tem_size.Height;
            Bitmap ob = new Bitmap(dWidth, dHeight);
            Graphics g = Graphics.FromImage(ob);
            g.Clear(Color.WhiteSmoke);
            g.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
            g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
            g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
            g.DrawImage(iSource, new Rectangle((dWidth - sW) / 2, (dHeight - sH) / 2, sW, sH), 0, 0, iSource.Width, iSource.Height, GraphicsUnit.Pixel);
            g.Dispose();
            //以下代码为保存图片时，设置压缩质量
            EncoderParameters ep = new EncoderParameters();
            long[] qy = new long[1];
            qy[0] = flag;//设置压缩的比例1-100
            EncoderParameter eParam = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, qy);
            ep.Param[0] = eParam;
                ImageCodecInfo[] arrayICI = ImageCodecInfo.GetImageEncoders();
                ImageCodecInfo jpegICIinfo = null;
                for (int x = 0; x < arrayICI.Length; x++)
                    if (arrayICI[x].FormatDescription.Equals("JPEG"))
                        jpegICIinfo = arrayICI[x];
                        break;
                if (jpegICIinfo != null)
                    ob.Save(dFile, jpegICIinfo, ep);//dFile是压缩后的新路径
                    FileInfo fi = new FileInfo(dFile);
                    if (fi.Length > 1024 * size)
                        flag = flag - 10;
                        CompressImage(sFile, dFile, flag, size, false);
                    ob.Save(dFile, tFormat);
                iSource.Dispose();
                return true;
            catch (Exception ex)
                return false;
            finally
                iSource.Dispose();
                ob.Dispose();
        #region ImageCompress
        /// <summary>
        /// 图片压缩         ImageCompress.CompressionImage(fileSelect,@"d:\",50);
        /// </summary>
        /// <param name="imagePath">图片文件路径</param>
        /// <param name="targetFolder">保存文件夹</param>
        /// <param name="quality">压缩质量</param>
        /// <param name="fileSuffix">压缩后的文件名后缀（防止直接覆盖原文件）</param>
        public static void CompressionImage(string imagePath, string targetFolder, long quality = 100, string fileSuffix = "compress")
            if (!File.Exists(imagePath))
                throw new FileNotFoundException();
            if (!Directory.Exists(targetFolder))
                Directory.CreateDirectory(targetFolder);
            var fileInfo = new FileInfo(imagePath);
            var fileName = fileInfo.Name.Replace(fileInfo.Extension, "");
            var fileFullName = Path.Combine($"{targetFolder}", $"{fileName}_{fileSuffix}{fileInfo.Extension}");
            var imageByte = CompressionImage(imagePath, quality);
            var ms = new MemoryStream(imageByte);
            var image = Image.FromStream(ms);
            image.Save(fileFullName);
            ms.Close();
            ms.Dispose();
            image.Dispose();
        /// <summary>
        /// 压缩图片
        /// </summary>
        /// <param name="imagePath">压缩前图片全路径</param>
        /// <param name="compressfilefullName">压缩后图片全路径</param>
        /// <param name="quality">压缩质量</param>
        public static void CompressionImage(string imagePath, string compressfilefullName, long quality = 100)
            if (!File.Exists(imagePath))
                throw new FileNotFoundException();
            var imageByte = CompressionImage(imagePath, quality);
            var ms = new MemoryStream(imageByte);
            var image = Image.FromStream(ms);
            image.Save(compressfilefullName);
            ms.Close();
            ms.Dispose();
            image.Dispose();
        private static byte[] CompressionImage(string imagePath, long quality)
            using (var fileStream = new FileStream(imagePath, FileMode.Open))
                using (var img = Image.FromStream(fileStream))
                    using (var bitmap = new Bitmap(img))
                        //var codecInfo = GetEncoder(img.RawFormat);
                        //转成jpg
                        var codecInfo = GetEncoder(ImageFormat.Jpeg);
                        var myEncoder = System.Drawing.Imaging.Encoder.Quality;
                        var myEncoderParameters = new EncoderParameters(1);
                        var myEncoderParameter = new EncoderParameter(myEncoder, quality);
                        myEncoderParameters.Param[0] = myEncoderParameter;
                        using (var ms = new MemoryStream())
                            bitmap.Save(ms, codecInfo, myEncoderParameters);
                            myEncoderParameters.Dispose();
                            myEncoderParameter.Dispose();
                            return ms.ToArray();
        private static ImageCodecInfo GetEncoder(ImageFormat format)
            var codecs = ImageCodecInfo.GetImageDecoders();
            return codecs.FirstOrDefault(codec => codec.FormatID == format.Guid);
        #endregion
在压缩png时，发现图片文件没有变小，反面变大了，
在网上找方法：C# Bitmap/png转成jpg格式，压缩图片
 //转成jpg
                        var codecInfo = GetEncoder(ImageFormat.Jpeg);
 string fileExtension = request.fileFullName.Substring(request.fileFullName.LastIndexOf('.') + 1).ToLower();
                                var fileName = request.fileFullName.Substring(request.fileFullName.LastIndexOf(@"\") + 1);
参考：https://www.cnblogs.com/testsec/p/6095729.html　　　　C# Bitmap/png转成jpg格式，压缩图片
https://blog.csdn.net/nodeman/article/details/80661995　　　　c# 无损高质量压缩图片代码
https://www.cnblogs.com/wdw984/p/13112621.html  C#进行图片压缩（对jpg压缩效果最好）
    
此随笔或为自己所写、或为转载于网络。仅用于个人收集及备忘。