调用Microsoft Graph API发邮件遇HTML内容解析错误求助

调用graphClient.Users["UserID"].SendMail.PostAsync(requestBody)接口时触发错误：

"Content type text/html does not have a factory registered to be parsed"

相关代码如下：

var scopes = new[] { "https://graph.microsoft.com/.default" };
var tenantId = "XXX";
var clientId = "XXX";
var clientSecret = "XXX";
var options = new TokenCredentialOptions
{
    AuthorityHost = AzureAuthorityHosts.AzurePublicCloud,
};
var clientSecretCredential = new ClientSecretCredential(tenantId, clientId, clientSecret, options);
var graphClient = new GraphServiceClient(clientSecretCredential, scopes);
var requestBody = new Microsoft.Graph.Users.Item.SendMail.SendMailPostRequestBody
{
    Message = new Message
    {
        Subject = "Meet for lunch?",
        Body = new ItemBody
        {
            ContentType = BodyType.Html,
            Content = "The new cafeteria is open.",
        },
        ToRecipients = new List<Recipient>
        {
            new Recipient
            {
                EmailAddress = new EmailAddress
                {
                    Address = "xxx@xxx.com",
                },
            },
        },
        CcRecipients = new List<Recipient>
        {
            new Recipient
            {
                EmailAddress = new EmailAddress
                {
                    Address = "xxx@xxx.com",
                },
            },
        },
    },
    SaveToSentItems = false,
};

await graphClient.Users["UserID"].SendMail.PostAsync(requestBody);

已对照微软邮件权限文档检查并授予了委派权限与应用权限，但问题仍未解决。

可能的解决方法 #

• 更新Graph SDK版本：该错误多由SDK版本不兼容或缺少序列化工厂导致，确保安装最新稳定版的Microsoft.Graph和Microsoft.Graph.Core NuGet包，旧版本存在HTML内容解析的已知bug。
• 手动配置序列化器：若更新SDK后问题依旧，可自定义客户端序列化选项，确保HTML类型被正确处理：

var serializerOptions = new JsonSerializerOptions
{
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
    Converters =
    {
        new JsonStringEnumConverter(),
        new ItemBodyConverter() // 自定义转换器处理ItemBody的HTML类型解析
    }
};

var graphClient = new GraphServiceClient(
    clientSecretCredential,
    scopes,
    serializer: new SystemTextJsonSerializer(serializerOptions));

• 排查内容格式：暂时将ContentType改为BodyType.Text测试能否正常发送，排除HTML内容本身的格式问题（如未转义的特殊字符）。
• 确认权限类型：当前使用客户端凭据流（ClientSecretCredential），需确保应用已授予Mail.Send应用权限并完成管理员同意——委派权限仅适用于用户交互场景，客户端凭据流不支持。

内容的提问来源于stack exchange，提问作者Ankush Daga