函数中的实体框架连接语句

Question

I'm trying to create a function to grab a List<>() of type "tableContact" which is an entity (sort of like when you do a simple select statement: query.ToList();)

But this is becoming frustrating because I kept getting "null object reference" errors. For some reason "join" statement doesn't work, so I tried an alternate linq statement.

I have an Object2Contacts table that I want to LEFT JOIN and I'm looking up my Object and trying to see all the contacts for this Object. I also can't figure out how to change an "anonymous type" back into an entity table.

Also not every object has contacts, so sometimes null List<> should be returned.

public List<tableContact> getContactsForObject(int oid)
    {
        if (oid > 0)
        {
            var query = (from s in entities.tableContacts
                         from o in entities.tableObject2Contacts
                         where s.contact_id == o.contact_id
                         where o.object_id == oid                         
                         select new { s });
            if (query != null)
            {
                IEnumerable<tableContact> e = (IEnumerable<tableContact>)query.ToList();
                return (List<tableContact>)e;
            }
        }
        return new List<tableContact>();
    }

I want to then loop through the object returned... eg:

foreach ( tableContact c in MyList){
   WriteLine(c.Name);
}

EDIT I also tried:

List<tableContacts> contacts = (from s in entities.tableContacts
        join o in entities.tableObject2Contacts
        on s.contact_id equals o.contact_id
        where o.object_id == oid                         
        select s).ToList();

Then I can't convert it back into a List and still "null reference".

EDIT 2 Yes the query I am running may definitely bring in an "empty" list. I don't mind empty lists, but it shouldn't give "object null reference."

Answer 1

I would write the join statement a little bit different:

var query = (from s in entities.tableContacts
             join o in entities.tableObject2Contacts
                  on new { ContactID = s.contact_id, ObjectID = oid } 
                  equals new { ContactID = o.contact_id, ObjectID = o.objectID } 
                  into oTemp
             from o in oTemp.DefaultIfEmpty()
             select new { s });

This would be a proper left-join using linq. Maybe your NullReferenceException is caused by a wrong join or something.

Answer 2

Nevermind, The "I also tried this" is actually correct code, but for some reason I hadn't established my entities properly so that was returning null.

Although I'm still super confused about "anonymoustypes" I wouldn't know how to make a function that returns "anonymoustype".