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List<T> joins DataTable

I have a List of objects (lst) and DataTable (dt). I want to join the lst and dt on the common field (code as string) and need to return all matching rows in the lst.
My List contains two columns i.e code and name along with values below:
code name
==== ====
1 x
2 y
3 z
The DataTable contains two columns i.e code and value along with values below:
code value
==== =====
3 a
4 b
5 c
The result is:
3 z
Below is my code; but I know it is not a correct statement and thus seeking your advice here. I would be much appreciated if you could guide me on how to write the correct statement.
var ld = from l in lst
join d in dt.AsEnumerable() on l.code equals d.code
select new { l.code, l.name };

You can use Linq query or Join extension method to join the collection on code. Just that when you select data from datatable, you need to use dt.Field method. Please use either of the following code.
Query1:
var ld = lst.Join(dt.AsEnumerable(),
l => l.code,
d => d.Field<string>("code"),
(l, d) => new
{
l.code,
l.name,
value = d.Field<string>("value")
}).ToList();
Query2:
var ld = (from l in lst
join d in dt.AsEnumerable()
on l.code equals d.Field<string>("code")
select new
{
l.code,
l.name,
value = d.Field<string>("value")
}).ToList();
Query3:
var ld = (from l in lst
join d in dt.AsEnumerable()
on l.code equals d.Field<string>("code")
let value = d.Field<string>("value")
select new
{
l.code,
l.name,
value
}).ToList();

You can try any of the below.
var ld = from l in lst
join d in dt.AsEnumerable() on l.code equals d.Field<int>("code")
select new { l.code, l.name };
var ld = lst.Join(dt.AsEnumerable(), l => l.code, d => d.Field<int>("code"), (l,d) => new { l.code, l.name });

It's not clear what your required output is but it looks as if you are correctly getting the only common records. You could extend your select to
select new { l.code, l.name, d.value }
Which would give all the data/columns from both tables.
code name value
==== ==== =====
3 z a

Try this:
var ld = from l in lst
join d in dt.Cast <DataRow>() on l.code equals d["code"].ToString()
select new { l.code, l.name };

SO you have a List and a DataTable. You don't plan to use the Values of the DataTable, only the Codes.
You want to keep those List items, that have a Code that is also a code in the DataTable.
If you plan to use your DataTable for other things than just for this problem, My advice would be to first create a procedure to convert your DataTable into an enumerable sequence.
This way you can add LINQ statements, not only for this problem, but also for other problems.
Let's create an extension method for your DataTable that converts the data into the items that are in the DataTable. See extension methods demystified.
Alas, I don't know what's in your DataTable, let's assume that your DataTable contains Orders
class CustomerOrder
{
public int Id {get; set;}
public int CustomerId {get; set;}
public int Code {get; set;}
public string Value {get; set;}
...
}
The extension method that extends functionality of class DataTable:
public static IEnumerable<Order> ToCustomerOrders(this DataTable table)
{
return table.AsEnumerable().Select(row => new CustomerOrder
{
Id = ...
CustomerId = ...
Code = ...
Value = ...
};
}
I'm not really familiar with DataTables, but you know how to convert the cells of the row into the proper value.
Usage:
DataTable table = ...
Int customerId = 14;
var ordersOfThisCustomer = table.ToCustomerOrders
.Where(customerOrder => customerOrder.CustomerId == customerId)
.FirstOrDefault();
In words: convert the datatable into CustomerOrders, row by row, and check for every converted CustomerOrder whether it has a CustomerId equal to 14. Stop if found. return null if there is no such row.
Now that you've got a nice reusable procedure that is also easy to test, debug and change, we can answer your question.
Given a DataTable with CustomerOrders, and a sequence of items that contain Code and Name, keep only those items from the sequence that have a Code that is also a Code in the DataTable.
var dataTable = ... // your DataTable, filled with CustomerOrders.
var codeNames = ... // your list with Codes and Names
var codesInDataTable = dataTable.ToCustomerOrders
.Select(customerOrder => customerOrder.Code)
.Distinct();
This will create an enumerable sequence that will convert your DataTable row by row and extract property Code. Duplicate Code values will be removed.
If Codes are unique, you don't need Distinct.
Note: the enumerable sequence is not enumerated yet!
var result = codeNames
.Where(codeName => codesInDataTable.Contains(codeName.Code))
.ToList();
In words: for every [Code, Name] combination in your list, keep only those [Code, Name] combinations that have a value for Code that is also in codesInDataTable.

c# store two vars objects in one var object

In the past, i was reading data from one excel sheet named "On leave" and do operation on the result like this:
var resultNew1 = (from x in dataNew.Worksheet<Employee>("On Leave")
select x).ToList();
but now I have got another sheet which is "Working". so I did this:
var resultNew1 = (from x in dataNew.Worksheet<Employee>("On Leave")
select x);
var resultNew2 = (from x in dataNew.Worksheet<Employee>("Working")
select x);
please notice that the first one was ToList()
but now I didn't make ToLists() because I want to ask you if there is a way to store these two vars which are resultsNew1 and resultsNew2 in one var and then make that var ToList()
If you need more infomration, please tell me

You can use Concat
var employees = dataNew.Worksheet<Employee>("On Leave")
.Concat(dataNew.Worksheet<Employee>("Working"))
.ToList();

You can use List<T>.AddRange:
List<Employee> employees = new List<Employee>();
employees.AddRange(resultNew1);
employees.AddRange(resultNew2);

How to convert to int and then compare in linq query c#

IEnumerable<classB> list = getItems();
//dt is datatable
list = list.Where(x => Convert.ToInt32( !dt.Columns["Id"]) == (x.Id));
I want to only keep the items in the list which match in datatable id column. The rest are removed. I m not doing it right.
The datatable can have: ID - 1,3,4,5,7
The list can have: ID - 1,2,3,4,5,6,7,8,9,10
I want the output list to have: ID - 1,3,4,5,7

Your code won't work because you're comparing a definition of a column to an integer value. That's not a sensible comparison to make.
What you can do is put all of the values from the data table into a collection that can be effectively searched and then get all of the items in the list that are also in that collection:
var ids = new HashSet<int>(dt.AsEnumerable()
.Select(row => row.Field<int>("Id"));
list = list.Where(x => ids.Contains(x.Id));

Try this one
var idList = dt.AsEnumerable().Select(d => (int) d["Id"]).ToList();
list = list.Where(x => idList.Contains(x.Id));

You can't do it like that. Your dt.Columns["Id"] returns the DataColumn and not the value inside that column in a specific datarow. You need to make a join between two linq query, the first one you already have, the other you need to get from the DataTable.
var queryDt = (from dtRow in dt
where !dtRow.IsNull("Id")
select int.Parse(dtRow["Id"])).ToList();
Now the join
var qry = from nonNull in queryDt
join existing in list on nonNull equals list.id

select values from DataTable column where not in a list

Here is my code :
List<long> LevelsArray = CardServiceDetailsDt.AsEnumerable()
.Select(r => r.Field<long("LEVEL_ID"))
.ToList<long>();
DataTable leveldt =ClientsDataHelper.GetServicePackagesLevel(new WebSiteDataHelper()
.SelectedStoreID.Value);
how can I select the LEVEL_ID from which are not in the list ?

Asssuming that the second table also has the column LEVEL_ID:
IEnumerable<DataRow> filteredRows = leveldt.AsEnumerable()
.Where(r => !LevelsArray.Contains(r.Field<long>("LEVEL_ID")));
Here's a probably more efficient Linq Left-Outer-Join appproach
filteredRows = from row in leveldt.AsEnumerable()
join levelID in LevelsArray
on row.Field<long>("LEVEL_ID") equals levelID into rowLevelID
from subrow in rowLevelID.DefaultIfEmpty()
where subrow == null
select row;
If you only need the ID's which are not in the list, use Enumerable.Except:
List<long> notInList = leveldt.AsEnumerable()
.Select(r => r.Field<long>("LEVEL_ID"))
.Except(LevelsArray)
.ToList();
which is the most efficient approach since it's using a set. It select only the ID's from the datarows which are not in the array.

List<long> LevelsArray = (from r in leveldt.AsEnumerable()
let LEVEL_ID = r.Field<long>("LEVEL_ID")
where !list.Contains(LEVEL_ID)
select LEVEL_ID).ToList<long>();

LINQ to SQL omit field from results while still including it in the where clause

Basically I'm trying to do this in LINQ to SQL;
SELECT DISTINCT a,b,c FROM table WHERE z=35
I have tried this, (c# code)
(from record in db.table
select new table {
a = record.a,
b = record.b,
c = record.c
}).Where(record => record.z.Equals(35)).Distinct();
But when I remove column z from the table object in that fashion I get the following exception;
Binding error: Member 'table.z' not found in projection.
I can't return field z because it will render my distinct useless. Any help is appreciated, thanks.
Edit:
This is a more comprehensive example that includes the use of PredicateBuilder,
var clause = PredicateBuilder.False<User>();
clause = clause.Or(user => user.z.Equals(35));
foreach (int i in IntegerList) {
int tmp = i;
clause = clause.Or(user => user.a.Equals(tmp));
}
var results = (from u in db.Users
select new User {
a = user.a,
b = user.b,
c = user.c
}).Where(clause).Distinct();
Edit2:
Many thanks to everyone for the comments and answers, this is the solution I ended up with,
var clause = PredicateBuilder.False<User>();
clause = clause.Or(user => user.z.Equals(35));
foreach (int i in IntegerList) {
int tmp = i;
clause = clause.Or(user => user.a.Equals(tmp));
}
var results = (from u in db.Users
select u)
.Where(clause)
.Select(u => new User {
a = user.a,
b = user.b,
c = user.c
}).Distinct();
The ordering of the Where followed by the Select is vital.

problem is there because you where clause is outside linq query and you are applying the where clause on the new anonymous datatype thats y it causing error
Suggest you to change you query like
(from record in db.table
where record.z == 35
select new table {
a = record.a,
b = record.b,
c = record.c
}).Distinct();

Can't you just put the WHERE clause in the LINQ?
(from record in db.table
where record.z == 35
select new table {
a = record.a,
b = record.b,
c = record.c
}).Distinct();
Alternatively, if you absolutely had to have it the way you wrote it, use .Select
.Select(r => new { a = r.a, b=r.b, c=r.c }).Distinct();
As shown here LINQ Select Distinct with Anonymous Types, this method will work since it compares all public properties of anonymous types.
Hopefully this helps, unfortunately I have not much experience with LINQ so my answer is limited in expertise.