image-processing - 如何处理嵌入在电子邮件中的图像并调整大小

Question

我们的代码在 Xamarin（和 .Net Standard 2.0）之外运行良好，但在尝试在 Xamarin Forms 应用程序中执行时失败并出现平台不受支持的错误（在 Image.FromStream(...)）行。此代码处理保存在数据库中的电子邮件并尝试调整图像大小，然后重新输出消息正文以供 Xamarin WebView 控件使用。如果 Image.FromStream() 不适合我们，我们如何解决这个问题？

这是执行处理的代码（可以假设比率为 50 作为示例）：

        /// <summary>
        /// Converts images in an email to Base64 imbedded source
        /// </summary>
        /// <param name="newMessage">MimeMessage that represents the loaded email message</param>
        /// <param name="bodyHtml">Body property of that message</param>
        /// <param name="imageResizeRatio">New ratio for image</param>
        /// <returns>String used for new body for UI to display</returns>
        private static string PopulateInlineImages(MimeMessage newMessage, string bodyHtml, double imageResizeRatio = 1)
        {
            // encode as base64 for easier downloading/storage 
            if (bodyHtml != null)
            {
                foreach (MimePart att in newMessage.BodyParts)
                {
                    if (att.ContentId != null && att.Content != null && att.ContentType.MediaType == "image" && (bodyHtml.IndexOf("cid:" + att.ContentId, StringComparison.Ordinal) > -1))
                    {
                        byte[] b;
                        using (var mem = new MemoryStream())
                        {
                            att.Content.DecodeTo(mem);
                            b = ReduceSize(mem, imageResizeRatio);
                            //b = mem.ToArray();
                        }
                        string imageBase64 = "data:" + att.ContentType.MimeType + ";base64," + System.Convert.ToBase64String(b);
                        bodyHtml = bodyHtml.Replace("cid:" + att.ContentId, imageBase64);
                    }
                }
            }
            return bodyHtml;
        }

        private static byte[] ReduceSize(Stream stream, double imageResizeRatio = 1)
        {
            Image source = Image.FromStream(stream);
            Image thumbnail = source.GetThumbnailImage((int)(source.Width * imageResizeRatio), (int)(source.Height * imageResizeRatio), AbortCallback, IntPtr.Zero);

            using (var memory = new MemoryStream())
            {
                thumbnail.Save(memory, source.RawFormat);
                return memory.ToArray();
            }
        }

        private static bool AbortCallback()
        {
            return false;
        }