LINQ question

[Nick]

Hello,

I'm trying to figure out if I'm going to use LINQ on a new project and I've
got a dumb question. I ran sqlmetal against my database to generate the
classes I need. I setup a class (like an "entity access layer" class) to run
the actual queries. So I've got a method that returns a list of contacts:

public static IEnumerable<Contact> GetContacts

I've got a datagrid where I want to display the contacts, but I need to join
the contacts with another table (OtherData, for example). How can I do a join
and then return all of the data back from this method? Would I need to create
a class that contains all of the fields from the contacts and otherdata
classes (so I could have a method like below)?

public static IEnumerable<ContactsWithOtherData> GetContactsWithOtherData

I hope I'm making sense. Thanks for any help, I really appreciate it.

Thanks,
Nick

 

[Nicholas Paldino [.NET/C# MVP]]

Nick,

Yes, if you want to flatten the data into one "table" (by that, I mean a
type which has the attributes for both the parent and the child), you need
to create a new type, and use a join operation to perform your select into
the new type.

 

[Marc Gravell]

public static IEnumerable<ContactsWithOtherData> GetContactsWithOtherData

Minor thing - but you might want to make the IQueryable<Contract> etc
so that it is composable - i.e. if you do use GetContacts() in a
further query (even if just to order it, or take the first page, or
apply an additional "where" filter), then the combined query happens
at the database. Of course, this only applies if using direct SELECT;
if you are using SPs it can't inject into the middle.

Marc

 

[Nick]

Thanks for your help. Pardon my ignorance again, but if I setup my class and
I want to use LINQ to do the query, how do I cast what is returned to my new
class (or what would be the preferred method for this)? This is what I have
in a method:

var q =
from c in db.Contacts
from od in c.OtherData
where od.someid=x
select new { c, od };

return q;

How do I cast q as a ContactOtherData type?

Thanks again for your help, I really appreciate it.

Nick

 

[Jon Skeet [C# MVP]]

Thanks for your help. Pardon my ignorance again, but if I setup my class and
I want to use LINQ to do the query, how do I cast what is returned to my new
class (or what would be the preferred method for this)? This is what I have
in a method:

var q =
from c in db.Contacts
from od in c.OtherData
where od.someid=x
select new { c, od };

return q;

How do I cast q as a ContactOtherData type?

Thanks again for your help, I really appreciate it.

q isn't a ContactOtherData type unless you've got some interesting
logic somewhere. It's more likely to be IEnumerable<SomeType> or
IQueryable<SomeType> where SomeType is an anonymous type consisting of
the contact and the other data. Hover over "var" to see the actual type
of q.

 

[Nick]

Jon,

Thanks for your help. You're right about q, it's an anonymous type (I don't
have any interesting logic). I haven't dealt much with anonymous types. Is it
possible to cast it to the ContractOtherData type?

Thanks,
Nick

 

[Jon Skeet [C# MVP]]

Thanks for your help. You're right about q, it's an anonymous type (I don't
have any interesting logic).

I haven't dealt much with anonymous types. Is it
possible to cast it to the ContactOtherData type?

No - but if you want to use a particular type, put that in the
"select" clause instead of using an anonymous type there. For instance:

var q =
from c in db.Contacts
from od in c.OtherData
where od.someid=x
select new ContactOtherData (c, od);

(assuming you've got a ContactOtherData constructor taking a Contact
parameter and an OtherData parameter).

 

[Nick]

Ahh, now the light bulb comes on. Thanks for your help, I really appreciate it.

 

[Jon Skeet [C# MVP]]

Ahh, now the light bulb comes on. Thanks for your help, I really appreciate it.

No problem. With all the new syntax in C# 3, it's easy to follow a
pattern but not really understand all the different bits. Once you pick
it apart, it gradually makes sense