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Nullable object must have a value in join tables query

I have three tables TableTeam, TableUser and TableLink. I want to join them to get a list.
var teams = _db.TableTeam;
var users = _db.TableUser;
var managerIds = teams.Select(x=>x.ManagerId).ToList();
var list = (from user in _db.TableUser
where user.EndDate == null && user.ManagerId != null && managerIds.Contains(user.ManagerId.Value)
let r = _db.TableLink.FirstOrDefault(l => l.ProjectId == projectId && l.UserId == user.UserId)
let t = _db.TableTeam.FirstOrDefault(m => m.ManagerId == user.ManagerId)
select new MyDto
{
ProjectId = projectId,
UserId = user.UserId,
IsAssigned = r.AssignedId > 0 ?true : false,
AssignedId = r.AssignedId,
Percent = r.Percent.HasValue ? (r.Percent.Value).ToString() + "%": ""
}).GroupBy(d => d.UserId).ToList();
For the table TableLink, the corresponding dataSet class in entity framework is:
public LinkDto
{
public int AssignedId {get;set;}
public short ProjectId {get;set;}
public short UserId {get;set;}
public decimal? Percent {get;set;}
// ...
}
However I got the error:
Nullable object must have a value
I think that it may be caused by decimal? Percent. But not sure how to fix it. Also I can use the store d procedure as well, which means if you can solve it by SQL query, that is also great.

It turns out I have to move let codes before where clause then add the condition to where.
Such as
var list = (from user in _db.TableUser
let r = _db.TableLink.FirstOrDefault(l => l.ProjectId == projectId && l.UserId == user.UserId)
let t = _db.TableTeam.FirstOrDefault(m => m.ManagerId == user.ManagerId)
where user.EndDate == null && user.ManagerId != null && managerIds.Contains(user.ManagerId.Value)
&& r != null && t != null
select new MyDto
{
ProjectId = projectId,
UserId = user.UserId,
IsAssigned = r.AssignedId > 0 ?true : false,
AssignedId = r.AssignedId,
Percent = r.Percent.HasValue ? (r.Percent.Value).ToString() + "%": ""
}).GroupBy(d => d.UserId).ToList();

Union Two Linq Queries

I have two LinqToSql queries that return result sets:
var grResults = (from g in ctx.GeneralRequests
join rt in ctx.RequestTypes on g.RequestTypeId equals rt.RequestTypeId
join sub in ctx.Logins on g.SubmitterStaffId equals sub.LoginId
join onb in ctx.Logins on g.OnBehalfOfStaffId equals onb.LoginId
where sub.GadId == gadId
select new
{
Status = "Submitted",
RequestId = g.GeneralRequestId,
Submitter = sub.UserName,
OnBehalf = (onb == null ? string.Empty : onb.UserName),
RequestType = (rt == null ? string.Empty : rt.Description),
ProjectName = (g == null ? string.Empty : g.ProjectName) ,
Comments = (g == null ? string.Empty : g.Comments),
LastUpdate = g.LastUpdateDate
});
var grdResults = (from gd in ctx.GeneralRequestDrafts
join rt in ctx.RequestTypes on gd.RequestTypeId equals rt.RequestTypeId
into tempRequestTypes
from rt1 in tempRequestTypes.DefaultIfEmpty()
join onb in ctx.Logins on gd.OnBehalfOfStaffId equals onb.LoginId
into tempOnBehalf
from onb1 in tempOnBehalf.DefaultIfEmpty()
join sub in ctx.Logins on gd.SubmitterStaffId equals sub.LoginId
where sub.GadId == gadId
select new
{
Status = "Draft",
RequestId = gd.GeneralRequestDraftId,
Submitter = sub.UserName,
OnBehalf = (onb1 == null ? string.Empty : onb1.UserName),
RequestType = (rt1 == null ? string.Empty : rt1.Description),
ProjectName = (gd.ProjectName == null ? string.Empty : gd.ProjectName),
Comments = (gd.Comments == null ? string.Empty : gd.Comments),
LastUpdate = gd.LastUpdateDate
});
The problem is when I try to Union them.
var results = grResults.Union(grdResults).OrderByDescending(r => r.LastUpdate);
This returns no records even though the two individual queries do.

Since the 2 queries don't appear to rely on each other just execute both and union the results of each if you are just trying to get a single list.
var results = grResults.ToList().Union(grdResults.ToList())
.OrderByDescending(r => r.LastUpdate);

linq subselect causing out of memory error

The Version field subselect in the where caluse is causing an out of memory error. If I remove it then the query works fine. The user profile table has multiple entries for each version and I only want the most recent in the final query results. Any ideas on how to make this query work without getting an out of memory error?
var x = (from p in userProfileRepo.All
join u1 in userRepo.All on p.USERID equals u1.USERID
join u2 in userRepo.All on p.LAST_MODIFIED_BY equals u2.USERID
join a in attrRepo.All on new { p.GROUP_NAME, p.PROFILE_NAME } equals new { a.GROUP_NAME, a.PROFILE_NAME } into attrJoin
from sub_a in attrJoin.DefaultIfEmpty()
where p.VERSION == (from up in userProfileRepo.All
where up.GROUP_NAME == p.GROUP_NAME
&& up.PROFILE_NAME == p.PROFILE_NAME
&& up.USERID == p.USERID
group up by up.VERSION into g
select g.Max(t => t.VERSION)).FirstOrDefault()
select new
{
GroupName = p.GROUP_NAME,
Author = u1.FIRSTNAME + #"\" + u1.LASTNAME,
LastModifiedBy = u2.FIRSTNAME + #"\" + u2.LASTNAME,
ProfileName = p.PROFILE_NAME,
Version = p.VERSION,
GroupName2 = (sub_a == null ? "" : sub_a.GROUP_NAME),
AttrName = (sub_a == null ? "" : sub_a.ATTR_NAME),
AttrDisplayName = (sub_a == null ? "" : sub_a.ATTR_DISPLAY_NAME),
AttrValue = (sub_a == null ? "" : sub_a.ATTR_VALUE),
Visible = (sub_a == null ? "" : sub_a.VISIBLE)
}).ToList();

Entity Framework - Handle null value in Linq

I'm writing two LINQ queries where I use my first query's result set in my second query.
But in some cases when there is no data in the database table my first query returns null,
and because of this my second query fails since wsdetails.location and wsdetails.worklocation are null causing an exception.
Exception:
Object reference not set to an instance of an object
My code is this:
var wsdetails = (from assetTable in Repository.Asset
join userAsset in Repository.UserAsset on
assetTable.Asset_Id equals userAsset.Asset_Id
join subLocationTable in Repository.SubLocation on
assetTable.Sub_Location_Id equals subLocationTable.Sub_Location_ID
where userAsset.User_Id == userCode
&& assetTable.Asset_TypeId == 1 && assetTable.Asset_SubType_Id == 1
select new { workstation = subLocationTable.Sub_Location_Name, location = assetTable.Location_Id }).FirstOrDefault();
result = (from emp in this.Repository.Employee
join designation in this.Repository.Designation on
emp.DesignationId equals designation.Id
where emp.Code == userCode
select new EmployeeDetails
{
firstname = emp.FirstName,
lastname = emp.LastName,
designation = designation.Title,
LocationId = wsdetails.location,
WorkStationName = wsdetails.workstation
}).SingleOrDefault();
As a workaround I can check
if wsdetails == null
and change my second LINQ logic, but I believe there are some ways to handle null values in LINQ itself like the ?? operator.
But I tried this and it didn't work for me.
Any help?

The problem is EF can't translate the null-coalescing operator to SQL. Personally I don't see what's wrong with checking the result with an if statement before executing the next query. However, if you don't want to do that, then because your result is always going to be a single query why not do something like:
var wsdetails = (from assetTable in Repository.Asset
join userAsset in Repository.UserAsset on
assetTable.Asset_Id equals userAsset.Asset_Id
join subLocationTable in Repository.SubLocation on
assetTable.Sub_Location_Id equals subLocationTable.Sub_Location_ID
where userAsset.User_Id == userCode
&& assetTable.Asset_TypeId == 1 && assetTable.Asset_SubType_Id == 1
select new { workstation = subLocationTable.Sub_Location_Name, location = assetTable.Location_Id }).FirstOrDefault();
result = (from emp in this.Repository.Employee
join designation in this.Repository.Designation on
emp.DesignationId equals designation.Id
where emp.Code == userCode
select new EmployeeDetails
{
firstname = emp.FirstName,
lastname = emp.LastName,
designation = designation.Title
}).SingleOrDefault();
result.LocationId = wsdetails != null ? wsdetails.location : "someDefaultValue";
result.WorkStationName = wsdetails != null ? wsdetails.workstation ?? "someDefaultValue";

Instead of the "binary" operator ?? maybe you should use the good old ternary one, ? :, like in
wsdetails != null ? wsdetails.location : null

Try not evaluating the first query, and use it in the second query. This should result in a single SQL statement.
var wsdetails = (from assetTable in Repository.Asset
join userAsset in Repository.UserAsset on
assetTable.Asset_Id equals userAsset.Asset_Id
join subLocationTable in Repository.SubLocation on
assetTable.Sub_Location_Id equals subLocationTable.Sub_Location_ID
where userAsset.User_Id == userCode
&& assetTable.Asset_TypeId == 1 && assetTable.Asset_SubType_Id == 1
select new { workstation = subLocationTable.Sub_Location_Name, location = assetTable.Location_Id });
// wsdetails is still an IEnumerable/IQueryable
result = (from emp in this.Repository.Employee
join designation in this.Repository.Designation on
emp.DesignationId equals designation.Id
where emp.Code == userCode
select new EmployeeDetails
{
firstname = emp.FirstName,
lastname = emp.LastName,
designation = designation.Title,
LocationId = wsdetails.First().location,
WorkStationName = wsdetails.First().workstation
}).SingleOrDefault();

Get linq to return IEnumerable<DataRow> result

How can I convert following SQL query to LINQ in C#?
I don't need all columns from both tables and the result set should be
IEnumerable<DataRow>
Query:
select
c.level, cl.name, c.quantity
from
dbo.vw_categories c
left join
dbo.vw_categoriesLocalization cl on c.level = cl.level
and cl.language = 'en'
where
c.level like '000%'
and c.level <> '000';
Expected:
IEnumerable<DataRow> result = ????
Thanks in advance..

Here's how you would write the query:
var query =
from c in db.vw_categories
join cl in db.vw_categoriesLocalization on c.level equals cl.level into clo
from cl in clo.DefaultIfEmpty()
where (cl == null || cl.language == "en")
&& c.level.StartsWith("000")
&& c.level != "000"
select new
{
c.level,
name = cl == null ? null : cl.name,
c.quantity
}
To convert that to IEnumerable<DataRow> maybe you could do something like this:
var datarows = query.Select(r => {
var row = dataTable.NewRow();
row["level"] = r.level;
row["name"] = r.name;
row["quantity"] = r.quantity;
return row;
});

If using LINQ to SQL:
var ctx = new SomeDataContext();
from c in ctx.vw_categories
join cl in ctx.vw_categoriesLocation
on c.level equals cl.level into g
from clv in g.DefaultIfEmpty()
where (clv == null || clv.language == "en")
&& c.level.Contains("000%")
&& c.level != "000"
select new
{
c.level,
name = (clv != null) ? clv.name : default(string),
quantity = c.quantity
};
More info here: http://codingsense.wordpress.com/2009/03/08/left-join-right-join-using-linq/