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getting a formatted nullable date string in a LINQ query

This really seems like it should be working:
var result = (from x in Db.RebateDetail
where batch != null && batch != "" ? x.BatchNumber.Value == Convert.ToInt32(batch) : x.DatetimeCreated.Date >= Convert.ToDateTime(fromDate).Date && x.DatetimeCreated.Date <= Convert.ToDateTime(toDate).Date
select new
{
id = x.Id,
batchNumber = x.BatchNumber,
firstName = x.FirstName,
checkDate = x.CheckDate.Value.ToString("MM/dd/yy") ?? "",
checkNumber = x.CheckNumber
}).ToList();
The checkDate errors out with 'Nullable object must have a value.'
Is that not what the ?? should be doing?
I have tried several variations on this but cannot seem to make it happy.
Update: This is using Entity Framework and LinqToEF

Currently, "if CheckDate is null" it would yield a System.InvalidOperationException rather than take the value on the RHS of the ?? operator hence the error "Nullable object must have a value".
You'll need to ensure that either the LHS returns a value or the RHS does.
What you're looking for is the conditional operator ?:
checkDate = x.CheckDate.HasValue ?
x.CheckDate.Value.ToString("MM/dd/yy") : string.Empty

LINQ: Set a default value if navigation property is null

This is my current LINQ statement:
var results = from l in leads
select new MyObject
{
LeadID = l.LeadID,
SelectedProposalEngineerID = l.LeadContacts.Where(contact => contact.LeadContactTypeID == LeadContactType.ProposalEngineer).FirstOrDefault().ContactID
};
The trouble I'm having is that the last item is often null. So when I try to convert "results" to a List, I get
{"The cast to value type 'System.Int32' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type."}
I don't want to make SelectedProposalEngineerID a nullable int, for problems that would cause down stream. How would I give it a value of 0 when it's null?
I have seen a LOT of other threads about this, but I can't seem to adapt any of their answers to this case.

Use DefaultIfEmpty extension method.
var results = from l in leads
select new MyObject
{
LeadID = l.LeadID,
SelectedProposalEngineerID =
l.LeadContacts.Where(contact => contact.LeadContactTypeID == LeadContactType.ProposalEngineer)
.Select(contact => contact.ContactID)
.DefaultIfEmpty(0)
.FirstOrDefault()
};

Nullable<int> ID;
var results = from l in leads
select new MyObject
{
LeadID = l.LeadID,
SelectedProposalEngineerID = (ID = l.LeadContacts.Where(contact => contact.LeadContactTypeID == LeadContactType.ProposalEngineer).FirstOrDefault().ContactID).HasValue ? ID.Value : 0;
};
A ternary operator should do the job. Assign the result to a variable, then if it's not null, cast the variable to int and return it, else return 0.

LINQ and nullable parameter

I have a nullable field in DB and nullable parameter in model. I try the following LINQ to Entities query:
EditPersonViewModel model = (from i in db.Person
where i.PersonID == id.Value
select new EditPersonViewModel()
{
PersonID = i.PersonID,
Fullname = i.Fullname,
Comment = i.Comment,
Username = (i.UserId != null) ? i.AspNetUsers.UserName : String.Empty,
// this is parameter has type "int?"
PersonStatusID = (i.PersonStatus!=null) ? i.PersonStatus.PersonStatusID : null
}).FirstOrDefault();
I get the compilation error:
Error 1 Type of conditional expression cannot be determined because
there is no implicit conversion between 'int' and ''
it works fine with
Username = (i.UserId != null) ? i.AspNetUsers.UserName : String.Empty,
but does not work with "int?" type. Why and how to do it correctly?

Documentation of Conditional Operator says:-
Either the type of first_expression and second_expression must be the
same, or an implicit conversion must exist from one type to the other.
Since i.AspNetUsers.UserName & String.Empty are string types its working fine for you. Now, your problem is self explanatory because null cannot be casted to integer type, you need this instead:-
PersonStatusID = (i.PersonStatus!=null) ? i.PersonStatus.PersonStatusID : 0;
Or the other way around if you need that as Nullable of integer:-
PersonStatusID = (i.PersonStatus!=null) ? (int?)i.PersonStatus.PersonStatusID : null;
PersonStatusID should be of type int? in this case.

The property 'x' on 'tblX' could not be set to a 'null' value. You must set this property to a non-null value of type 'Int16'

List<tblX> messages = (from x in db.tblX where x.msg_id == id_id
|| x.name == firstName select x).ToList();
I get the error:
The property 'x' on 'tblX' could not be set to a 'null' value. You must set this property to a non-null value of type 'Int16'.
I have a property msg_blocked in db, which is nullable and integer. I know that I need to make a conversion, but I don't use it or need it anywhere in my linq.

Seems like your class definition for tblX doesn't match the database representation, so either modify your class to accept a nullable value, or just project out the required fields:
List<tblX> messages = (from x in db.tblX
where (x.msg_id == id_id || x.name == firstName)
select new tblX
{
//required fields
msg_id = x.msg_id,
name = x.name,
...
}).ToList();
Addendum: The reason you run into this problem is behind the scenes when you
select x
this is translated into a
select new tblX
which projects into all its available fields. The code provided is more explicit and specifies which fields to query for and then project into.

List<tblX> messages = (from x in db.tblX
where (x.msg_id == id_id || x.name == firstName) &&
x.msg_blocked != null
select x).ToList();

Problem with LINQ to Entities query using Sum on child object property

Given this query:
from s in services
select new
{
s.Id,
s.DateTime,
Class = s.Class.Name,
s.Location,
s.Price,
HeadCount = s.Reservations.Sum(r => r.PartySize), // problem here. r.PartySize is int
s.MaxSeats
}
If the service doesn't have any reservations, this exception is thrown:
System.InvalidOperationException: The cast to value type 'Int32' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type.
I get it, but how should I deal with it? My intention is if there are no reservations, then HeadCount be assigned 0.

There's an even simpler solution:
from s in services
select new
{
s.Id,
s.DateTime,
Class = s.Class.Name,
s.Location,
s.Price,
HeadCount = (int?)s.Reservations.Sum(r => r.PartySize),
s.MaxSeats
}
Note the cast. This may also produce simpler SQL than #Ahmad's suggestion.
Essentially, you're just helping out type inference.

You should check for it:
HeadCount = s.Reservations != null ? s.Reservations.Sum(r => r.PartySize) : 0,

This should resolve your problem:
Try to cost the int to int?
from s in services
select new
{
s.Id,
s.DateTime,
Class = s.Class.Name,
s.Location,
s.Price,
HeadCount = s.Reservations.Sum(r => (int?) r.PartySize),
s.MaxSeats
};
HeadCount = HeadCount ?? 0;

A simple ternary operator should fix the problem nicely...
something like this:
HeadCount = (s.Reservations != null && s.Reservations.Any()) ? s.Reservations.Sum(r => r.PartySize) : 0;
This will handle for both null and empty situations