害我找半天原因
https://github.com/TencentCloud/tencentcloud-sdk-dotnet/blob/master/TencentCloudExamples/UploadLog.cs

log.Contents.Add(content);

LogGroup group = new LogGroup();

LogGroupList list = new LogGroupList();

应该是：
log.Contents.Add(content);

LogGroup group = new LogGroup();
group.Logs.Add(log);//少了此行

LogGroupList list = new LogGroupList();