在C#中使用Microsoft Graph API移除草稿邮件附件的问题排查

问题背景 #

在C#应用中使用Microsoft Graph API转发邮件时，尝试移除转发生成的草稿邮件附件，遭遇tooManyRetries错误，删除操作长时间挂起后抛出异常。

原代码示例 #

private static async Task<Message> GetDraftMessageAsync(GraphServiceClient client, EmailCreateServiceModel model,
                                                       ComposeMode mode, CancellationToken cancellationToken)
{
    // 其他模式处理逻辑

    var message = await client.Users[model.OriginalEmailAddress]
        .Messages[model.MessageId]
        .CreateForward()
        .Request()
        .Header("Prefer", "IdType=\"ImmutableId\"")
        .PostAsync(cancellationToken)
        .ConfigureAwait(false);

    // 疑问1：能否无需二次调用直接获取带附件的草稿邮件？
    var messageWithAttachments = await client.Users[model.OriginalEmailAddress]
        .Messages[message.Id]
        .Request()
        .Expand(m => m.Attachments)
        .GetAsync(cancellationToken)
        .ConfigureAwait(false);

    return messageWithAttachments;
}

public static async Task<Message> SendEmailAsync(EmailCreateServiceModel model, ComposeMode mode, List<string> keepingAttachments,
                                                  CancellationToken cancellationToken)
{
    var client = await AuthService.GetGraphClientAsync();
    var draftMessage = await GetDraftMessageAsync(client, model, mode, cancellationToken)
                .ConfigureAwait(false);

    if (mode is ComposeMode.Forward && draftMessage.Attachments != null)
    {
        var attachmentsToRemove = draftMessage.Attachments
            .Where(attachment => !keepingAttachments.Contains(attachment.Name))
            .ToList();

        foreach (var attachment in attachmentsToRemove)
        {
            // 疑问2：此代码长时间挂起后抛出异常
            await client.Users[model.OriginalEmailAddress]
                .Messages[draftMessage.Id]
                .Attachments[attachment.Id]
                .Request()
                .DeleteAsync(cancellationToken)
                .ConfigureAwait(false);
        }
    }

    draftMessage.From = string.IsNullOrEmpty(model.From)
        ? null
        : new() { EmailAddress = new() { Address = model.From } };
    draftMessage.ToRecipients = GetRecipients(model.To);
    draftMessage.CcRecipients = GetRecipients(model.Cc);
    draftMessage.BccRecipients = GetRecipients(model.Bcc);
    draftMessage.Subject = model.Subject;
    draftMessage.Body = new ItemBody() { Content = model.Body, ContentType = BodyType.Html, };

    draftMessage = await client.Users[model.OriginalEmailAddress]
            .Messages[draftMessage.Id]
            .Request()
            .UpdateAsync(draftMessage, cancellationToken)
            .ConfigureAwait(false);
            
    return draftMessage;
}

错误详情 #

Microsoft.Graph.ServiceException
  HResult=0x80131500
  Message=Code: tooManyRetries
  Message: More than 3 retries encountered while sending the request.

  Source=Microsoft.Graph.Core
  StackTrace:
   at Microsoft.Graph.HttpProvider.<SendRequestAsync>d__19.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   ...
   at Microsoft.Graph.AttachmentRequest.<DeleteAsync>d__3.MoveNext() in AttachmentRequest.cs
   ...

核心疑问 #

1. 删除草稿邮件附件是否需要特定权限？
2. 获取和修改草稿邮件的方式是否存在问题？
3. 如何正确处理tooManyRetries错误？

更新信息 #

使用Graph Explorer执行删除草稿附件操作时，返回以下错误：

Error code: RequestBroker-ParseUri
Message: Could not parse the URL.
Details: Unrecognized segment 'attachments'

解决方案 #

1. 无需二次调用获取带附件的草稿邮件 #

可以直接在CreateForward请求中添加$expand参数，一次性获取包含附件的草稿邮件，避免二次请求。修改后的代码片段：

var message = await client.Users[model.OriginalEmailAddress]
    .Messages[model.MessageId]
    .CreateForward()
    .Request()
    .Header("Prefer", "IdType=\"ImmutableId\"")
    .Expand(m => m.Attachments) // 添加此行，直接返回附件数据
    .PostAsync(cancellationToken)
    .ConfigureAwait(false);

2. 删除草稿附件的权限要求 #

删除草稿邮件附件需要Mail.ReadWrite权限，需确保应用已正确申请该权限（委托权限或应用权限均可，根据业务场景选择）。

3. 修正草稿邮件的获取与修改方式 #

• 避免替换整个Message对象：原代码中直接更新完整的draftMessage对象，可能覆盖Graph API自动维护的系统字段（如LastModifiedDateTime），建议采用PATCH部分字段的方式更新，仅修改需要变更的属性。
• 避免循环逐个删除附件：循环单次删除会增加请求次数，容易触发Graph API的速率限制，导致重试失败。推荐使用批量请求合并多个删除操作，减少请求频次。

4. 处理tooManyRetries错误 #

• 使用批量请求删除附件：将多个删除附件的请求合并为一个批量请求发送，示例代码如下：

if (attachmentsToRemove.Any())
{
    var batchRequestContent = new BatchRequestContent();
    foreach (var attachment in attachmentsToRemove)
    {
        var deleteRequest = client.Users[model.OriginalEmailAddress]
            .Messages[draftMessage.Id]
            .Attachments[attachment.Id]
            .Request()
            .GetHttpRequestMessage(HttpMethod.Delete);
        
        batchRequestContent.AddBatchRequestStep(deleteRequest);
    }

    var batchResponse = await client.Batch.Request().PostAsync(batchRequestContent, cancellationToken)
        .ConfigureAwait(false);

    // 检查批量请求结果，处理失败的删除操作
    foreach (var stepResponse in batchResponse.BatchResponses)
    {
        if (!stepResponse.Response.IsSuccessStatusCode)
        {
            var errorContent = await stepResponse.Response.Content.ReadAsStringAsync(cancellationToken);
            // 记录日志或执行补偿逻辑
        }
    }

    // 删除后重新获取草稿最新状态，确保后续更新基于最新数据
    draftMessage = await client.Users[model.OriginalEmailAddress]
        .Messages[draftMessage.Id]
        .Request()
        .Expand(m => m.Attachments)
        .GetAsync(cancellationToken)
        .ConfigureAwait(false);
}

• 自定义重试策略：初始化GraphServiceClient时，可自定义重试规则，针对速率限制（429）、服务器错误（5xx）调整重试次数与间隔：

var retryHandler = new RetryHandler(new RetryHandlerOptions
{
    MaxRetry = 5, // 增加重试次数
    RetryDelay = TimeSpan.FromSeconds(2), // 调整重试间隔
    ShouldRetry = (delay, attempt, message) =>
    {
        // 针对429和5xx错误进行重试
        return message.StatusCode == HttpStatusCode.TooManyRequests ||
               (int)message.StatusCode >= 500;
    }
});

var httpClient = HttpClientFactory.Create(retryHandler);
var client = new GraphServiceClient(httpClient, authProvider);

5. 修正Graph Explorer的URL错误 #

确保删除附件的请求路径格式正确：/users/{user-id}/messages/{draft-message-id}/attachments/{attachment-id}，其中draft-message-id是转发生成的草稿邮件ID，而非原邮件ID。

内容的提问来源于stack exchange，提问作者Alireza Noori