1.
OrderByDescending(c => c.ModifyTime).Select(c => c.ModifyTime).FirstOrDefault() Select(c => c.ModifyTime).OrderByDescending(c => c.HasValue).FirstOrDefault()
linq语句的顺序查询，查询结果不同
原文：https://www.cnblogs.com/carlpeng/p/14276224.html
