根据路径将图片转base64，base64转Bitmap ，图片压缩，图片转base64

string path = "D:\\img\\xiedl\\2020-11-20\\1 (" + (z - 43210) + ").png";
  string base64Str = ReadFromFile(path);
  Bitmap bitmap = Base64StringToImage(base64Str);
  Bitmap bitmap2 = ZoomImage(bitmap, 300, 300);
  base64Str = ToBase64(bitmap2);


  private static string ReadFromFile(string path)
        {
            FileStream fsForRead = new FileStream(path, FileMode.Open);
            string base64Str = "";
            try
            {
                //读入一个字节
                Console.Write("文件的第一个字节为：" + fsForRead.ReadByte().ToString());
                Console.ReadLine();
                //读写指针移到距开头10个字节处
                fsForRead.Seek(0, SeekOrigin.Begin);
                byte[] bs = new byte[fsForRead.Length];
                int log = Convert.ToInt32(fsForRead.Length);
                //从文件中读取10个字节放到数组bs中
                fsForRead.Read(bs, 0, log);
                base64Str = Convert.ToBase64String(bs);
            }
            catch (Exception ex)
            {
                Console.Write(ex.Message);
                Console.ReadLine();
            }
            finally
            {
                fsForRead.Close();
            }
            return base64Str;
        }
        
        //base64编码的文本转为图片  
        private Bitmap Base64StringToImage(string inputStr)
        {
            try
            {
                byte[] arr = Convert.FromBase64String(inputStr);
                MemoryStream ms = new MemoryStream(arr);
                Bitmap bmp = new Bitmap(ms);
                ms.Close();
                return bmp;
            }
            catch (Exception ex)
            {
                MessageBox.Show("Base64StringToImage 转换失败/nException：" + ex.Message);
                return null;
            }
        }
        //图片转为base64编码的文本   
        private string ToBase64(Bitmap bmp)
        {
            try
            {
                MemoryStream ms = new MemoryStream();
                bmp.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
                byte[] arr = new byte[ms.Length];
                ms.Position = 0;
                ms.Read(arr, 0, (int)ms.Length);
                ms.Close();
                String strbaser64 = Convert.ToBase64String(arr);
                return strbaser64;
            }
            catch (Exception ex)
            {
                MessageBox.Show("ImgToBase64String 转换失败 Exception:" + ex.Message);
                return "";
            }
        }

        private Bitmap ZoomImage(Bitmap bitmap, int destHeight, int destWidth)
        {
            try
            {
                System.Drawing.Image sourImage = bitmap;
                int width = 0, height = 0;
                //按比例缩放             
                int sourWidth = sourImage.Width;
                int sourHeight = sourImage.Height;
                if (sourHeight > destHeight || sourWidth > destWidth)
                {
                    if ((sourWidth * destHeight) > (sourHeight * destWidth))
                    {
                        width = destWidth;
                        height = (destWidth * sourHeight) / sourWidth;
                    }
                    else
                    {
                        height = destHeight;
                        width = (sourWidth * destHeight) / sourHeight;
                    }
                }
                else
                {
                    width = sourWidth;
                    height = sourHeight;
                }
                Bitmap destBitmap = new Bitmap(destWidth, destHeight);
                Graphics g = Graphics.FromImage(destBitmap);
                g.Clear(System.Drawing.Color.White);
                //设置画布的描绘质量           
                g.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
                g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
                g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
                g.DrawImage(sourImage, new System.Drawing.Rectangle((destWidth - width) / 2, (destHeight - height) / 2, width, height), 0, 0, sourImage.Width, sourImage.Height, GraphicsUnit.Pixel);
                g.Dispose();
                //设置压缩质量       
                System.Drawing.Imaging.EncoderParameters encoderParams = new System.Drawing.Imaging.EncoderParameters();
                long[] quality = new long[1];
                quality[0] = 100;
                System.Drawing.Imaging.EncoderParameter encoderParam = new System.Drawing.Imaging.EncoderParameter(System.Drawing.Imaging.Encoder.Quality, quality);
                encoderParams.Param[0] = encoderParam;
                sourImage.Dispose();
                return destBitmap;
            }
            catch
            {
                return bitmap;
            }
        }