LINQ query Joins

问题

When i use the below code to retrieve information, it shows an error..

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
            };

Error is:

"int? mailbox.from_id "

Cannot implicitly convert type 'int?' to 'int'. An explicit conversion exists (are you missing a cast?)

I have declared m_id and from_id as int in DB as well as in MailList class.

回答1:

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
            };

回答2:

I'm guessing this should fix it.

so int? is a Nullable type, you need to either

(1) Define MailList.mfrom as an int? OR
(2) Convert from int? to int, like below:

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
            };

Update

---

After a little more research, seems like @abatishchev solution with the null-coalescing operator is the correct way to go, according to msdn, and like @Konstantin on the comments mentioned Nullable.GetValueOrDefault(T) is also more correct.

回答3:

Try this:

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
            };

来源：https://stackoverflow.com/questions/4813968/linq-query-joins