Question

我有一个模型，其中Person有0,1或2个Contact个对象联系信息主页（contact_type为“h”）和工作联系信息（contact_type为“w”）。

选择街道名称时，有没有办法检查Contact contact_type == "h"对象是否存在？现在，如果Contact对象不存在，我将得到一个null异常。

from m in persons
select new
{
    Id = m.id,
    Name = m.surname,
    Address = m.Contacts.Where(c => c.contact_type == "H").SingleOrDefault().streetname
};

Answer 1

怎么样：

var rows = from m in persons
    let h = m.Contacts.Where(c => c.contact_type == "H").SingleOrDefault()
    select new {
        Id = m.id,
        Name = m.surname,
        Address = (h == null ? null : h.streetname)
    };

或：

var rows = from m in persons
    select new {
        Id = m.id,
        Name = m.surname,
        Address = m.Contacts.Where(c => c.contact_type == "H")
                 .Select(c => c.streetname).SingleOrDefault()
    };

也可以写（也许更清楚）：

var rows = from m in persons
    select new {
        Id = m.id,
        Name = m.surname,
        Address = (from c in m.Contacts
                   where c.contact_type == "H"
                   select c.streetname).SingleOrDefault()
    };

Answer 2

您可以拥有多个Where子句：

Address = m.Where( person => person.Contacts != null ).Contacts.Where( ...

选择语句以防止选择null

2 个答案: