使用Microsoft Graph发送带大小附件的邮件时程序无限运行无报错

核心问题：同步阻塞引发的死锁 #

在.NET 4.7.2中，若在带有同步上下文的线程（如UI线程）里使用.GetAwaiter().GetResult()阻塞等待异步操作完成，极易触发死锁。原因是异步方法会捕获当前同步上下文，而同步阻塞会占用该上下文，导致异步回调无法执行，最终程序陷入无限等待状态。

其他潜在问题 #

• 文件流位置未重置：打开大附件文件流后，流指针默认停留在文件末尾，直接上传会导致无数据传输，可能引发上传停滞。
• 小附件ContentBytes设置不规范：Graph SDK 5中FileAttachment提供了直接的ContentBytes属性，无需通过AdditionalData字典间接设置。
• ContentType匹配错误：固定使用application/octet-stream不够规范，建议根据文件实际类型设置对应MIME类型。

修正后的代码 #

方案1：异步到底（推荐） #

将方法改为异步模式，全程使用await避免阻塞，这是.NET异步编程的最佳实践：

public async Task<string> SendEmailWithAttachmentsAsync(GraphServiceClient graphClient, string objectID, string recipient, FileInfo littleAttachment, FileInfo bigAttachment)
{
    var _Msg = string.Empty;

    // 创建邮件草稿
    var draftMessage = new Microsoft.Graph.Models.Message
    {
        Subject = "Large attachment test",
        ToRecipients = new List<Recipient>
        {
            new Recipient { EmailAddress = new EmailAddress { Address = recipient } }
        }
    };

    // 添加小附件
    byte[] littleStream = File.ReadAllBytes(littleAttachment.FullName);
    draftMessage.Attachments = new List<Microsoft.Graph.Models.Attachment>
    {
        new FileAttachment
        {
            OdataType = "#microsoft.graph.fileAttachment",
            Name = littleAttachment.Name,
            ContentBytes = littleStream
        }
    };

    // 保存草稿
    var savedDraft = await graphClient.Users[objectID].Messages.PostAsync(draftMessage);

    // 处理大附件上传
    using (var fileStream = System.IO.File.OpenRead(bigAttachment.FullName))
    {
        // 重置流指针到文件开头
        fileStream.Seek(0, SeekOrigin.Begin);

        var uploadRequestBody = new Microsoft.Graph.Users.Item.Messages.Item.Attachments.CreateUploadSession.CreateUploadSessionPostRequestBody
        {
            AttachmentItem = new AttachmentItem
            {
                AttachmentType = AttachmentType.File,
                Name = bigAttachment.Name,
                Size = fileStream.Length,
                // 自动获取文件对应MIME类型，需引用System.Web程序集
                ContentType = MimeMapping.GetMimeMapping(bigAttachment.FullName)
            }
        };

        var uploadSession = await graphClient.Users[objectID]
            .Messages[savedDraft.Id]
            .Attachments
            .CreateUploadSession
            .PostAsync(uploadRequestBody);

        // 最大分片大小必须是320KiB的倍数
        int maxSliceSize = 320 * 1024;
        var fileUploadTask = new LargeFileUploadTask<FileAttachment>(uploadSession, fileStream, maxSliceSize);

        var totalLength = fileStream.Length;
        IProgress<long> progress = new Progress<long>(prog =>
        {
            _Msg = $"Uploaded {prog} bytes of {totalLength} bytes";
        });

        try
        {
            var uploadResult = await fileUploadTask.UploadAsync(progress);
            _Msg = uploadResult.UploadSucceeded ? "Upload complete" : "Upload failed";
        }
        catch (ServiceException ex)
        {
            _Msg = $"Error uploading: {ex.ToString()}";
        }
    }

    // 发送草稿
    await graphClient.Users[objectID].Messages[savedDraft.Id].Send.PostAsync();

    return _Msg + " send: succeed";
}

方案2：强制同步调用（仅当无法改为异步时使用） #

若必须保持同步方法，需在每个异步调用后添加.ConfigureAwait(false)避免捕获同步上下文，同时修正流位置：

public string SendEmailWithAttachments(GraphServiceClient graphClient, string objectID, string recipient, FileInfo littleAttachment, FileInfo bigAttachment)
{
    var _Msg = string.Empty;

    var draftMessage = new Microsoft.Graph.Models.Message
    {
        Subject = "Large attachment test",
        ToRecipients = new List<Recipient>
        {
            new Recipient { EmailAddress = new EmailAddress { Address = recipient } }
        }
    };

    byte[] littleStream = File.ReadAllBytes(littleAttachment.FullName);
    draftMessage.Attachments = new List<Microsoft.Graph.Models.Attachment>
    {
        new FileAttachment
        {
            OdataType = "#microsoft.graph.fileAttachment",
            Name = littleAttachment.Name,
            ContentBytes = littleStream
        }
    };

    // 避免捕获同步上下文，防止死锁
    var savedDraft = graphClient.Users[objectID].Messages.PostAsync(draftMessage).ConfigureAwait(false).GetAwaiter().GetResult();

    using (var fileStream = System.IO.File.OpenRead(bigAttachment.FullName))
    {
        fileStream.Seek(0, SeekOrigin.Begin);

        var uploadRequestBody = new Microsoft.Graph.Users.Item.Messages.Item.Attachments.CreateUploadSession.CreateUploadSessionPostRequestBody
        {
            AttachmentItem = new AttachmentItem
            {
                AttachmentType = AttachmentType.File,
                Name = bigAttachment.Name,
                Size = fileStream.Length,
                ContentType = MimeMapping.GetMimeMapping(bigAttachment.FullName)
            }
        };

        var uploadSession = graphClient.Users[objectID]
            .Messages[savedDraft.Id]
            .Attachments
            .CreateUploadSession
            .PostAsync(uploadRequestBody)
            .ConfigureAwait(false)
            .GetAwaiter()
            .GetResult();

        int maxSliceSize = 320 * 1024;
        var fileUploadTask = new LargeFileUploadTask<FileAttachment>(uploadSession, fileStream, maxSliceSize);

        var totalLength = fileStream.Length;
        IProgress<long> progress = new Progress<long>(prog =>
        {
            _Msg = $"Uploaded {prog} bytes of {totalLength} bytes";
        });

        try
        {
            var uploadResult = fileUploadTask.UploadAsync(progress).ConfigureAwait(false).GetAwaiter().GetResult();
            _Msg = uploadResult.UploadSucceeded ? "Upload complete" : "Upload failed";
        }
        catch (ServiceException ex)
        {
            _Msg = $"Error uploading: {ex.ToString()}";
        }
    }

    graphClient.Users[objectID].Messages[savedDraft.Id].Send.PostAsync().ConfigureAwait(false).GetAwaiter().GetResult();

    return _Msg + " send: succeed";
}

关键说明 #

• 异步优先：方案1的异步写法能彻底避免死锁问题，同时提升程序响应性，是.NET官方推荐的写法。
• 流管理：使用using语句确保文件流被正确释放，上传前重置流指针到起始位置，避免无数据上传。
• ContentType优化：使用MimeMapping.GetMimeMapping自动匹配文件MIME类型，若无法引用System.Web，可使用第三方库如MimeKit替代。

内容的提问来源于stack exchange，提问作者CDVS