Question

当我使用以下代码检索信息时，它会显示错误..

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？）

我已在数据库和MailList类中将m_id和from_id声明为int。

Answer 1

我猜这应该解决它。

所以 int？是Nullable type，你需要

（1）将MailList.mfrom定义为int？或
（2）从int转换？到int，如：

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

根据msdn的说法，经过一番研究，似乎 @abatishchev 解决方案与null-coalescing operator是正确的方法，并且喜欢 @Konstantin 关于提到的评论Nullable.GetValueOrDefault(T)也更正确。

Answer 2

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 ?? 0,
                mfomname = p.username,
                msubject = m.subject
            };

Answer 3

试试这个：

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 = (int)m.from_id,  // Error occours here
                mfomname = p.username,
                msubject = m.subject
            };

LINQ查询加入

3 个答案: