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how to use session variable in Linq

How to get Session["UserId"] value in Linq Query
var result = (from c in db.UserMaster
where c.UserID == Session["vUSerID"].ToString()
select C ).ToList();
but when i execute it gives me error as follows
LINQ to Entities does not recognize the method 'System.String ToString()' method, and this method cannot be translated into a store expression.
Please guide me how to works with ToString()

You need to materialize the value before you send it off to be translated to SQL:
var userId = Session["vUSerID"].ToString();
var result = (from c in db.UserMaster
where c.UserID == userId
select C ).ToList();

Alternatively, you could try bringing entity into memory first, and then apply the condition to get the filtered result, although not recommended because of the potential performance issue.
var result = db.UserMaster.Select(x => x).ToList().Where(x => x.UserId == Session["vUSerID"].ToString()).ToList();

ASP.NET Entity Framework .Where() with Lesser than

How can I make this lesser than or equal work in my .Where() clause? I am getting an error.
var filteredProducts = Products.Where(p => p.State.Contains("Bruikbaar"))
.Where(p => p.Privilege <= ui.GetPrivilegeNumber())
.ToList();
Error:
LINQ to Entities does not recognize the method 'Int32 GetPrivilegeNumber()' method, and this method cannot be translated into a store expression.
I hope this question is never asked before. Googled couldn't find it either or I am using the wrong words to express my problem.

ui.GetPrivilegeNumber() is not a recognized method.
Use this:
var uiPrivilege = ui.GetPrivilegeNumber();
var filteredProducts = Products.Where(p => p.State.Contains("Bruikbaar"))
.Where(p => p.Privilege <= uiPrivilege)
.ToList();
And as other users mentionted, you can optimize your Where.

EF does not execute method calls which you use in predicates. It stores them as expression (i.e. syntax tree) and then analyzes this tree to build SQL query by translating C# code to SQL code. It cannot translate GetPrivilegeNumber() method call into SQL, because there is no appropriate SQL code for that. So all you need is move this method call out of expression and pass only result of method call instead:
var privilegeNumber = ui.GetPrivilegeNumber();
var filteredProducts = Products.Where(p => p.State.Contains("Bruikbaar"))
.Where(p => p.Privilege <= privilegeNumber)
.ToList();
Now privilegeNumber is just an integer variable which is translated into SQL parameter
SELECT * FROM Products p
WHERE p.State LIKE '%Bruikbaar%' AND p.Privilege <= #privilegeNumber

You need to move ui.GetPrivilegeNumber() outside of the query. You can also merge those Where queries into a single one:
var privilegeNumber = ui.GetPrivilegeNumber();
var filteredProducts = Products.Where(p =>
p.State.Contains("Bruikbaar")
&& p => p.Privilege <= privilegeNumber)
.ToList();

You can use other evaluation method inside LinQ. To simplified the code, you can use it in little old way of writing LinQ.
var uiPrivilege = ui.GetPrivilegeNumber();
var filteredProducts =(from p in Products
where p.State.Contains("Bruikbaar") && p.Privilege <= uiPrivilege
select p).ToList();
The above query generate same output but easy to understood.

how to compare values from a list in Linq to Entity (any / contains)

There is a list, policiesToDelete of entity class, MonitoringRelations. Out of this list I have selected two elements and construed a new list:
var policyKeysToDelete = policiesToDelete
.Select(r => new {r.PolicyId, r.GroupId})
.ToList();
Now, I have a query where I want to compare elements from policyKeysToDelete list.
var objectsToDelete = (from p in storageContext.MonitoringRelations
where policyKeysToDelete
.Any(x => x == new {p.PolicyId, p.GroupId})
select p)
.ToList();
The problem: the query above throws this exception:
NotSupportedException: Unable to create a constant value of type 'Anonymous type'. Only primitive types or enumeration types are supported in this context.
I have tried changing the anonymous list to a list<tuple<PolicyId, GroupId>> , but that also didn't help, throwing the almost same exception. I tried using Contains in place of Any but that also didn't help.
Any idea how can I solve this problem?

EF cannot translate a list of complex objects into the SQL query. What EF can do, is translate a list of simple values into SQL when you use it with the .Contains method.
So if you extract a list of PolicyId's and a list of GroupId's from the policyKeysToDelete, and use it to select as much as you can with EF, then you can do the full check in the resultset which is then in-memory, using Linq-to-objects.
Warning: you are extracting too much from the database, so depending on the amount of data, a different solution might be better.
var policyKeysToDelete = policiesToDelete
.Select(r => new {r.PolicyId, r.GroupId})
.ToList();
// List of values types, which can be translated to SQL
var policyIds = policyKeysToDelete.Select(x => x.PolicyId).ToList();
var groupIds = policyKeysToDelete.Select(x => x.GroupId).ToList();
var objectsToDelete = storageContext.MonitoringRelations
// Do the part that we can do in the database, which is select the records
// which have an corresponding PolicyId or GroupId
.Where(x => policyIds.Contains(x.PolicyId) || groupIds.Contains(x.GroupId))
// Use this method to indicate that whatever follows after should not be
// translated to SQL
.AsEnumerable()
// Do the full check in-memory
.Where(x => policyKeysToDelete
.Any(y => x.PolicyId == y.PolicyId && x.GroupId == y.GroupId)
)
.ToList();

Subquery in Where Clause of LINQ statement

So I tried to follow this example to have a sub-query in the where clause of this LINQ query.
var innerquery =
from app in context.applications
select new { app.app_id };
IEnumerable<postDatedCheque> _entityList = context.postDatedCheques
.Where(e => innerquery.Contains(e.appSancAdvice.application.app_id));
The objective was to select those records from postDatedCheques that have app_id in applications table.
But I am getting following erros inside the where clause:
Delegate 'System.Func' does not
take 1 arguments
Cannot convert lambda expression to type 'string' because it is not
a delegate type
'System.Linq.IQueryable' does not contain a
definition for 'Contains' and the best extension method overload
'System.Linq.ParallelEnumerable.Contains(System.Linq.ParallelQuery,
TSource)' has some invalid arguments
Instance argument: cannot convert from
'System.Linq.IQueryable' to
'System.Linq.ParallelQuery'
What am I coding incorrect?

I think a simple join would do the job. It will filter out the 'cheques' that have no relative 'app':
var _entitylist =
from cheque in context.postDatedCheques
join app in context.applications on cheque.appSancAdvice.application equals app
select cheque;
Edit:
Solutions using a .Contains(...) will be translated into a SQL IN statement. Which will be very inefficient. Linq join is translated into SQL INNER JOIN which is very efficient if your DB schema is well trimmed (FKs, index)

What about?
IEnumerable<postDatedCheque> _entityList = context.postDatedCheques.Where(
e => context.applications.Any(
x => e.appSancAdvice.application.app_id == x.app_id));
And if you want to use two statements, set the first as an expression function.
Expression<Func<string, bool>> innerQuery =
x => context.applications.Any(y => y.app_id == x);
IEnumerable<postDatedCheque _entityList =
context.postDatedCheques.Where(
x => innerQuery(x.appSancAdvice.application.app_id));

innerquery is a IQueryable of anonymous type that contains an app_id.
The line Contains(e.appSancAdvice.application.app_id) doesn't make sense since e.appSancAdvice.application.app_id and the anonymous type are not the same type.
Simply do:
var _entityList = context.postDatedCheques
.Where(e =>
context.applications
.Select(a => a.app_id)
.Contains(e.appSancAdvice.application.app_id));

Try this instead:
var innerquery =
from app in context.applications
select new { app.app_id };
IEnumerable<postDatedCheque> _entityList = context.postDatedCheques
.Where(e => innerquery.Any(a => a.app_id == e.appSansAdvice.application.app_id));

Linq IN in C# error

I'm trying to do this:
myList = (from c in ipE.Cars
where (someListOfIntegers.Contains(idCar) && c.name == someName)
select c).ToList();
I'm getting the eror LINQ to Entities does not recognize the method 'Boolean Contains(Int32)' method, and this method cannot be translated into a store expression.
I tried with ANY, it still doesn't work. I got the error Unable to create a constant value of type 'Closure type'. Only primitive types ('such as Int32, String, and Guid') are supported in this context.
How to resolve this?

Unfortunately, you cant't use local collections in an EF query. You can work around it, by using the PredicateBuilder and translate the Contains operation manually in a sequence of ORs:
var predicate = PredicateBuilder.False<Car>();
foreach (int id in someListOfIntegers)
{
int temp = id;
predicate = predicate.Or (car => car.idCar == id);
}
myList = (from c in ipE.Cars
where predicate && c.name == someName)
select c).ToList();
Sidenote: I assume, that you have a typo in your question, because neither someListOfIntegers nor idCar are properties of your EF model you query. So i guess you want to query for Car.idCar?.
Otherwise you just could refactor the Contains completely out of the EF query.

You seem to be suggesting (comments) that idCar does not relate to any entities - in which case, don't do that in the query:
List<Car> list;
if(someListOfIntegers.Contains(idCar)) {
myList = (from c in ipE.Cars
where c.name == someName
select c).ToList();
} else {
myList = new List<Car>(); // no need to touch the DB
}

Seen the same myself: L2E uses a small set of commands - and there isn't a "Contains(Int32)" command.
If your lists are fairly small, then try adding ".AsEnumerable()", which will execute the T-SQL select statements and leave you with POCO objects which CAN use Contains.
e.g.
myList = (from c in ipE.Cars.AsEnumerable
where (someListOfIntegers.AsEnumerable().Contains(idCar)
&& c.name == someName)
select c)
.ToList();
Not sure if you need both AsEnumerables...try it out!