Sequence不包含LINQ FirstOrDefault的元素

Question

LINQ FirstOrDefault我得到'Sequence contains no elements'.

int? locationId = _ctx.m_locations.FirstOrDefault(
                       l => l.name.ToLower() == countyOrTown.ToLower()
                  ).location_key;

我认为FirstOrDefault的重点是,如果数据库中没有条目并且只返回null,它不会引发异常？

Answer 1

因为正如你自己所说,.FirstOrDefault()将返回一个NULL值,你需要首先检查该NULL,并且只有当它NOT NULL访问它的.location_key属性时:

int? locationId = null;

var firstOrDefault = _ctx.m_locations.FirstOrDefault(l => l.name.ToLower() == countyOrTown.ToLower());

if(firstOrDefault != null)
    locationId = firstOrDefault.location_key;