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NOT IN Condition in Linq

I have a simple scenario.I want to list out all the employees except the logged in user.
Similar SQL Condition is
select * from employee where id not in(_loggedUserId)
How can I acheive the above using LINQ.I have tried the following query but not getting the desired list
int _loggedUserId = Convert.ToInt32(Session["LoggedUserId"]);
List<int> _empIds = _cmn.GetEmployeeCenterWise(_loggedUserId)
.Select(e => e.Id)
.Except(_loggedUserId)
.ToList();

Except expects argument of type IEnumerable<T>, not T, so it should be something like
_empIds = _cmn.GetEmployeeCenterWise(_loggedUserId)
.Select(e => e.Id)
.Except(new[] {_loggedUserId})
.ToList();
Also note, this is really redundant in the case when exclusion list contains only one item and can be replaces with something like .Where(x => x != _loggedUserId)

Why not use a very simple Where condition?
_empIds = _cmn.GetEmployeeCenterWise(_loggedUserId).Where(e=>e.Id != _loggedUserId).ToList();

The title of your question is how to perform a not in query against a database using LINQ. However, as others have pointed out your specific problem is better solved by a using users.Where(user => user.Id != loggedInUserId).
But there is still an answer on how to perform a query against a database using LINQ that results in NOT IN SQL being generated:
var userIdsToFilter = new[] { ... };
var filteredUsers = users.Where(user => !userIdsToFilter.Contains(user.Id));
That should generate the desired SQL using either Entity Framework or LINQ to SQL.
Entity Framework also allows you to use Except but then you will have to project the sequence to ID's before filtering them and if you need to original rows you need to fetch them again from the filtered sequence of ID's. So my advice is use Where with a Contains in the predicate.

Use LINQ without filtering. This will make your query execute much faster:
List<int> _empIds = _cmn.GetEmployeeCenterWise(_loggedUserId)
.Select(e => e.Id).ToList();
Now use List.Remove() to remove the logged-in user.
_empIds.Remove(_loggedUserId);

C# and LINQ - arbitrary statement instead of let

Let's say I'm doing a LINQ query like this (this is LINQ to Objects, BTW):
var rows =
from t in totals
let name = Utilities.GetName(t)
orderby name
select t;
So the GetName method just calculates a display name from a Total object and is a decent use of the let keyword. But let's say I have another method, Utilities.Sum() that applies some math on a Total object and sets some properties on it. I can use let to achieve this, like so:
var rows =
from t in totals
let unused = Utilities.Sum(t)
select t;
The thing that is weird here, is that Utilities.Sum() has to return a value, even if I don't use it. Is there a way to use it inside a LINQ statement if it returns void? I obviously can't do something like this:
var rows =
from t in totals
Utilities.Sum(t)
select t;
PS - I know this is probably not good practice to call a method with side effects in a LINQ expression. Just trying to understand LINQ syntax completely.

No, there is no LINQ method that performs an Action on all of the items in the IEnumerable<T>. It was very specifically left out because the designers actively didn't want it to be in there.

Answering the question
No, but you could cheat by creating a Func which just calls the intended method and spits out a random return value, bool for example:
Func<Total, bool> dummy = (total) =>
{
Utilities.Sum(total);
return true;
};
var rows = from t in totals
let unused = dummy(t)
select t;
But this is not a good idea - it's not particularly readable.
The let statement behind the scenes
What the above query will translate to is something similar to this:
var rows = totals.Select(t => new { t, unused = dummy(t) })
.Select(x => x.t);
So another option if you want to use method-syntax instead of query-syntax, what you could do is:
var rows = totals.Select(t =>
{
Utilities.Sum(t);
return t;
});
A little better, but still abusing LINQ.
... but what you should do
But I really see no reason not to just simply loop around totals separately:
foreach (var t in totals)
Utilities.Sum(t);

You should download the "Interactive Extensions" (NuGet Ix-Main) from Microsoft's Reactive Extensions team. It has a load of useful extensions. It'll let you do this:
var rows =
from t in totals.Do(x => Utilities.Sum(x))
select t;
It's there to allow side-effects on a traversed enumerable.

Please, read my comment to the question. The simplest way to achieve such of functionality is to use query like this:
var rows = from t in totals
group t by t.name into grp
select new
{
Name = t.Key,
Sum = grp.Sum()
};
Above query returns IEnumerable object.
For further information, please see: 101 LINQ Samples

LINQ flavored IS IN Query

There are quite a few other questions similiar to this but none of them seem to do what I'm trying to do. I'd like pass in a list of string and query
SELECT ownerid where sysid in ('', '', '') -- i.e. List<string>
or like
var chiLst = new List<string>();
var parRec = Lnq.attlnks.Where(a => a.sysid IN chiList).Select(a => a.ownerid);
I've been playing around with a.sysid.Contains() but haven't been able to get anywhere.

Contains is the way forward:
var chiLst = new List<string>();
var parRec = Lnq.attlnks.Where(a => chiList.Contains(a.sysid))
.Select(a => a.ownerid);
Although you'd be better off with a HashSet<string> instead of a list, in terms of performance, given all the contains checks. (That's assuming there will be quite a few entries... for a small number of values, it won't make much difference either way, and a List<string> may even be faster.)
Note that the performance aspect is assuming you're using LINQ to Objects for this - if you're using something like LINQ to SQL, it won't matter as the Contains check won't be done in-process anyway.

You wouldn't call a.sysid.Contains; the syntax for IN (SQL) is the reverse of the syntax for Contains (LINQ)
var parRec = Lnq.attlnks.Where(a => chiList.Contains(a.sysid))
.Select(a => a.ownerid);

In addition to the Contains approach, you could join:
var parRec = from a in Lnq.attlnks
join sysid in chiLst
on a.sysid equals sysid
select a.ownerid
I'm not sure whether this will do better than Contains with a HashSet, but it will at least have similar performance. It will certainly do better than using Contains with a list.

How can I set properties on all items from a linq query with values from another object that is also pulled from a query?

I have a query pulling from a database:
List<myClass> items = new List<myClass>(from i in context
select new myClass
{
A = i.A,
B = "", // i doesn't know this, this comes from elsewhere
C = i.C
}
I also have another query doing a similar thing:
List<myClass2> otherItems = new List<myClass2>(from j in context
select new myClass2
{
A = j.A, // A is the intersection, there will only be 1 A here but many A's in items
B = j.B
}
In reality these classes are much larger and query data that is separated not only by database but by server as well. Is it possible to use a LINQ query to populate the property B for all items where items.A intersect? All of the built in LINQ predicates appear only to do aggregates, selections or bool expressions.
In my brain I had something like this, but this is all off:
items.Where(x => x.B = (otherItems.Where(z => z.A == x.A).Single().B));
Or am I being ridiculous with trying to make this work in LINQ and should just abandon it in favor of a for loop where the actual setting becomes trivial? Because of deadlines I will be resorting to the for loop (and it's probably going to end up being a lot more readable in the long run anyway), but is it possible to do this? Would an extension method be necessary to add a special predicate to allow this?

LINQ is designed for querying. If you're trying to set things, you should definitely use a loop (probably foreach). That doesn't mean you won't be able to use LINQ as part of that loop, but you shouldn't be trying to apply a side-effect within LINQ itself.

Query the OtherItems first. Do a ToDictionary() on the result. Then, when querying the database, do this:
var items = from i in context
select new myClass
{ A = i.A,
B = otherItems[i.A],
C = i.C
}

How to perform group by in LINQ and get a Iqueryable or a Custom Class Object?

Here is my query -
var data = Goaldata.GroupBy(c => c.GoalId).ToList();
This returns a Igrouping object and I want an Iqueryable object which I can directly query to get the data while in this case I have to loop through using a foreach() and then get the data. Is there another way to group by in LINQ which returns directly as a list of Iqueryable or a List as similar to what happens for order by in LINQ.

The easiest way is probably
var data = Goaldata.GroupBy(c => c.GoalId).SelectMany(c => c).ToList();
In the OO sense they aren't really grouped, but they are ordered with the groups together.

Whilst the accepted answer is correct, it seems to be unnecessarily complicated. Assuming GoalId is an int you can just use OrderBy:
var data = Goaldata.OrderBy(c => c.GoalId).ToList();

Or .GroupBy(c => c.GoalId).AsQueryable()...