LINQ查询加入

Question

当我使用以下代码检索信息时,它显示错误..

var mails = from m in entity.mailboxes
            join p in entity.ordinary_user_profile_info on m.from_id equals p.user_id
            select new MailList
            {
                mid = m.m_id,
                mfrom = m.**from_id,** // Error occours here
                mfomname = p.username,
                msubject = m.subject
            };

错误是:

"int？mailbox.from_id"

无法隐式转换类型'int？' 'int'.存在显式转换(您是否错过了演员？)

我在DB和MailList类中声明了m_id和from_idint.

Answer 1

我猜这应该解决它.

那么int？是一个空类型,你需要要么

(1)定义MailList.mfrom为int？ 或
(2)从int转换？到int,如下所示:

var mails = from m in entity.mailboxes
            join p in entity.ordinary_user_profile_info on m.from_id equals p.user_id
            select new MailList
            {
                mid = m.m_id,
                **mfrom = m.from_id.HasValue ? m.from_id.Value : 0**
               //this is saying is, since the int is nullable, 
               //if it has a value, take it, or return 0
                mfomname = p.username,
                msubject = m.subject
            };

更新

经过多一点研究后,似乎@abatishchev解决方案与null-coalescing运算符是正确的方法,根据msdn,和@Konstantin一样,提到的注释Nullable.GetValueOrDefault(T)也更正确.