使用Microsoft Graph API发送带附件邮件报错及批量发送异常排查

报错Cannot process input of abstract type 'microsoft.graph.attachment'以及批量发送时仅第一个收件人成功的核心原因是：重复复用同一个SendMailPostRequestBody及内部的Message对象。Graph SDK在第一次请求序列化后，会修改对象的内部元数据状态，导致后续请求时无法正确识别具体的FileAttachment类型，转而尝试序列化抽象的Attachment基类，触发API报错。

解决步骤 #

• 每次循环生成全新的请求体对象，包括Message和附件集合，避免状态残留
• 把GraphServiceClient的创建移到循环外，减少重复初始化开销
• 简化MemoryStream的使用，利用using自动释放资源

修改后的代码示例 #

private async Task ExecuteSendEmailAsync()
{
    try
    {
        // 获取收件人列表
        List<Model.User> receivers = new();
        receivers.Add(new Model.User { EMAIL = "email@email.com" });
        List<Reservation> reservation = new();

        // 生成PDF附件字节（只做一次，复用字节数组）
        string contenuto = "<h1 style='text-align:center; font-family: Arial, Helvetica, sans-serif; font-size: 24px;' >Daily report</h1><br>";
        contenuto += "<br>";
        // 此处添加其他内容...

        byte[] pdfBytes;
        using (MemoryStream memoryStream = new MemoryStream())
        {
            Document document = new(PageSize.A4, 10f, 10f, 10f, 0f);
            HtmlWorker htmlparser = new(document);
            PdfWriter writer = PdfWriter.GetInstance(document, memoryStream);
            
            document.Open();
            htmlparser.Parse(new StringReader(contenuto));
            document.Close();
            
            pdfBytes = memoryStream.ToArray();
            // using块会自动关闭memoryStream，无需手动调用Close
        }

        // 只创建一次GraphClient，避免重复初始化
        GraphServiceClient graphClient = GraphHelper.GetAuthenticatedGraphClient("*tenant*");
        string fromEmail = "from@email.com";

        // 循环发送邮件，每次创建全新的请求体
        foreach (Model.User rec in receivers)
        {
            var requestBody = new Microsoft.Graph.Users.Item.SendMail.SendMailPostRequestBody
            {
                Message = new Message
                {
                    Subject = "test",
                    Body = new ItemBody
                    {
                        ContentType = BodyType.Text,
                        Content = "Hello, look at the attachment"
                    },
                    ToRecipients = new List<Recipient>
                    {
                        new Recipient { EmailAddress = new EmailAddress { Address = rec.EMAIL } }
                    },
                    Attachments = new List<Microsoft.Graph.Models.Attachment>
                    {
                        new FileAttachment
                        {
                            OdataType = "#microsoft.graph.fileAttachment",
                            Name = "Daily_report.pdf",
                            ContentType = "application/pdf",
                            ContentBytes = pdfBytes,
                        },
                    }
                }
            };

            await graphClient.Users[fromEmail].SendMail.PostAsync(requestBody);
        }
    }
    catch (Exception e)
    {
        string msg = "Errore durante l'invio di una mail";
        ManageExceptions.ManageLog(e, msg, "ExecuteSendEmail - ReportMensaEmailJob", "Error");
        throw;
    }
}

关键修改点 #

1. PDF字节复用：把PDF生成逻辑移到循环外，只生成一次字节数组，提升性能
2. 请求体循环内创建：每次循环都新建SendMailPostRequestBody和Message，确保对象状态干净
3. GraphClient复用：避免在循环内重复创建认证客户端，减少资源消耗
4. 简化流操作：移除手动的memoryStream.Close()，依赖using自动释放

内容的提问来源于stack exchange，提问作者Croicco