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Retrieving non-duplicates from 2 Collections using LINQ

Background: I have two Collections of different types of objects with different name properties (both strings). Objects in Collection1 have a field called Name, objects in Collection2 have a field called Field.
I needed to compare these 2 properties, and get items from Collection1 where there is not a match in Collection2 based on that string property (Collection1 will always have a greater or equal number of items. All items should have a matching item by Name/Field in Collection2 when finished).
The question: I've found answers using Lists and they have helped me a little(for what it's worth, I'm using Collections). I did find this answer which appears to be working for me, however I would like to convert what I've done from query syntax (if that's what it's called?) to a LINQ query. See below:
//Query for results. This code is what I'm specifically trying to convert.
var result = (from item in Collection1
where !Collection2.Any(x => x.ColumnName == item.FieldName)
select item).ToList();
//** Remove items in result from Collection1**
//...
I'm really not at all familiar with either syntax (working on it), but I think I generally understand what this is doing. I'm struggling trying to convert this to LINQ syntax though and I'd like to learn both of these options rather than some sort of nested loop.
End goal after I remove the query results from Collection1: Collection1.Count == Collection2 and the following is true for each item in the collection: ItemFromCollection1.Name == SomeItemFromCollection2.Field (if that makes sense...)

You can convert this to LINQ methods like this:
var result = Collection1.Where(item => !Collection2.Any(x => x.ColumnName == item.FieldName))
.ToList();

Your first query is the opposite of what you asked for. It's finding records that don't have an equivalent. The following will return all records in Collection1 where there is an equivalent:
var results=Collection1.Where(c1=>!Collection2.Any(c2=>c2.Field==c1.Name));
Please note that this isn't the fastest approach, especially if there is a large number of records in collection2. You can find ways of speeding it up through HashSets or Lookups.

if you want to get a list of non duplicate values to be retained then do the following.
List<string> listNonDup = new List<String>{"6","1","2","4","6","5","1"};
var singles = listNonDup.GroupBy(n => n)
.Where(g => g.Count() == 1)
.Select(g => g.Key).ToList();
Yields: 2, 4, 5
if you want a list of all the duplicate values then you can do the opposite
var duplicatesxx = listNonDup.GroupBy(s => s)
.SelectMany(g => g.Skip(1)).ToList();

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);

Parameterise LINQ GroupBy

The following C# code takes a large datatable with many columns and an array of 2 column names. It will give a new datatable with two rows where there are duplicate rows for the two fields supplied staff no & skill.
This is too specific and I need to supply any number of fields as the groupby.
can someone help me?
string[] excelField = new string[0]; // contains a list of field name for uniquness
excelField[0] = "staff No";
excelField[1] = "skill";
DataTable dataTableDuplicateRows = new DataTable();
dataTableDuplicateRows.Clear();
dataTableDuplicateRows.Columns.Clear();
foreach (string fieldName in excelField)
{
dataTableDuplicateRows.Columns.Add(fieldName);
}
var duplicateValues = dataTableCheck.AsEnumerable()
.GroupBy(row => new { Field0 = row[excelField[0]], Field1 = row[excelField[1]] })
.Where(group => group.Count() > 1)
.Select(g => g.Key);
foreach (var duplicateValuesRow in duplicateValues)
{
dataTableDuplicateRows.Rows.Add(duplicateValuesRow.Field0, duplicateValuesRow.Field1);
}

I think what you require is something make the linq more dynamic, even though you could achieve it by using expression tree, the DynamicLinq library would appear to solve your issue in an easier way.
For you case, with the library, just use the GroupBy extension method with a string value.
More info about DynamicLinq library:
Scott Gu's blog

extract one column from a var using linq

i have something like this
var emp = db.Employees.toList();
in my employees table i have emp name, emp id and salary, using linq with lambda expressions, how do i access emp id in some another variable.
I have tried looking up for it, couldn't find a solution which does it using linq with lambda expressions
var employeeLeaves = db.Leaves
.Include("Employee")
.Include("Employee.Manager")
.Where(l => l.Status.Id == 1)
.GroupBy(l => l.Employee.Manager.Id)
.Select(l => l.GroupBy(k=>k.Employee.Id).Select(j=>j.GroupBy(‌​p=>p.GroupId)))
.ToList();
this is the actual query i have,dont ask me how i wrote it..:P now i want to get id column from employeeLeaves and store it in some another variable

It's not entirely clear what you're trying to do, which is why you've got so many answers with different approaches. If you're trying to turn the "collection of employees" into a "collection of IDs" then you want something like this:
var ids = emp.Select(x => x.Id);
Or more directly:
var ids = db.Employees.Select(x => x.Id);
(Optionally with ToList at the end of each of these.)
I would strongly advise you to learn LINQ (and the somewhat-related languages features such as var) thoroughly, from scratch, with the help of a good book or tutorial. Learning bits piecemeal by just finding samples which do something a bit like what you want is not a good approach, IMO.

var empId = db.Employees.Single(x => x.id == 5).Id;

You can use the select. Maybe something like this:
var result=db.Employees
.Select(s=>s.emp_id);

Try this syntax:
var empIds = db.Employees.Select(e=>e.EmpID)

Database.Tablename.Single(x => x.id == anyId)
This should select single row

Join between in memory collection and EntityFramework

Is there any mechanism for doing a JOIN between an in-memory collection and entity framework while preserving the order.
What I am trying is
var itemsToAdd =
myInMemoryList.Join(efRepo.All(), listitem => listitem.RECORD_NUMBER,
efRepoItem => efRepoItem.RECORD_NUMBER, (left, right) => right);
which gives me the rather curiously titled "This method supports the LINQ to Entities infrastructure and is not intended to be used directly from your code." error.
Now of course I can do this iteratively with something like
foreach (var item in myInMemoryList)
{
var ho = efRepo.Where(h => h.RECORD_NUMBER == item.RECORD_NUMBER).FirstOrDefault();
tmp.Add(ho);
}
but this is an N+1 query. Which is nasty as myInMemoryList might be quite large!
Resharper can refactor that for me to
tmp = (from TypeOfItemInTheList item in myInMemoryList
select efRepo.Where(h => h.RECORD_NUMBER == item.RECORD_NUMBER)
.FirstOrDefault());
which I suspect is still doing N+1 queries. So any ideas for a better approach to getting ef entities that match (on key field) with an in-memory collection. The resulting set must be in the same order as the in-memory collection was.

No you cannot join in-memory collection with database result set without loading whole result set to the memory and performing the join with linq-to-objects. Try using contains instead of join:
var myNumbers = myInMemoryList.Select(i => i.RECORD_NUMBER);
var itemsToAdd = efRepo.Where(e => myNumbers.Contains(e.RECORD_NUMBER));
This will generate query with IN operator

You can read how you can do this with the PredicateBuilder from the LINQKit or Stored Procedures in my blog post.
http://kalcik.net/2014/01/05/joining-data-in-memory-with-data-in-database-table/

try this:
var list = (from n in efRepo
where myInMemoryList.Select(m=>m.RECORD_NUMBER).Contains(n.RECORD_NUMBER)
select n).ToList();
Contains will be translated to IN operator in SQL (only if your RECORD_NUMBER member is a primitive type like int, string, Guid, etc)

What about loading the whole efRepo? I mean something like this (ToArray()):
var itemsToAdd = myInMemoryList.Join(
efRepo.ToArray(),
listitem => listitem.RECORD_NUMBER, efRepoItem => efRepoItem.RECORD_NUMBER, (left, right) => right);