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LINQ to entities : cannot call a method

I'm aware there are alreay a lot of posts concerning this issue, but i can't seem to find a solution for this.
Here's my Ling to Entities query :
IEnumerable<Tblstamp> changes = (from c in userGSN.edb.Tblstamp
where (c.Ts_Date >= userGSN.DateLastCheck &&
TimeSpan.Parse(c.Ts_Time) >= userGSN.TimeLastCheck)
orderby c.Ts_Id ascending
select c);
I want to compare c.Ts_time to userGSN.TimeLastCheck, but for that I have to convert the c.Ts_Time to a timespan (it is a string, and comes from a database I can't modify, tried everything). I also can't do the converting before the query in an other variable since I can't access it outside of the query.
Obviously, I get an error for trying to use the TimeSpan.Parse method in my query, but I can't find any workaround to this. I have tried using LINQ to Object but since I am really not used to it I couldn't make the equivalent query that i have here.
I am aware of the problem, I'm just trying to find a workaround and need some help please !
EDIT :
So I tried the DateDiff function as suggested :
IEnumerable<Tblstamp> changes = (from c in userGSN.edb.Tblstamp
where (c.Ts_Date >= userGSN.DateLastCheck && SqlFunctions.DateDiff("second",userGSN.TimeLastCheck,c.Ts_Time).Value > 0 )
orderby c.Ts_Id ascending
select c
);
but it gives me the same error : "LINQ to Entities does not recognize the method 'System.Nullable1[System.Int32] DateDiff(System.String, System.Nullable1[System.TimeSpan], System.String)' method, and this method cannot be translated into a store expression."
Even though it clearly says here "You cannot call this function directly. This function can only appear within a LINQ to Entities query.", which is exactly what I'm doing ?!

There's no clean way to do this, so you'll have to think outside the box. Try using SqlFunctions.DateDiff to perform the check. You'll need to adapt it for your usage, but:
SqlFunctions.DateDiff("second", c.Ts_Time, userGSN.TimeLastCheck.ToString()) > 0
There are other methods that you can use, if DateDiff is not suitable. See EntityFunctions as well.
Depending on what you're using to query the data, you might want System.Data.Entity.SqlServer.SqlFunctions instead.

Every code you write in LinQ to get data from database must convert to a valid T-SQL query with valid statements. Not all C# methods have a correspondance in T-SQL so whenever you use them in your code while fatching data you will get errors.
You should either do modifications on the SQL side or use C# spesific functions after fething data from database, while working with entities in memory, or use fıunstions which comply with SQL statements.

If you can't get EF to translate your query to SQL, maybe you could do it yourself? See Writing SQL queries for entities

Linq to Entities does not support all functions since the query has to be translated to SQL:
from c in userGSN.edb.Tblstamp.AsEnumerable()
//now you are allready getting the data from the database
//so be carefull with that because you will have bad performance

Steps for a beginner to run very basic linq to sql query using Linqpad

Trying to learn Linq using LinqPad and getting frustated with how to start on it. Let's say I want to write a C# Expression and a C# statment where I have a table in SQL server named Products and I want to pull all rows where price is greater then 50. How would yo write it?

Let's say I want to write a C# Expression and a C# statment where I
have a table in SQL server named Products and I want to pull all rows
where price is greater then 50. How would yo write it?
LINQPad builds the typed DataContext for you automatically, so you don't need to instantiate anything. In C# expression mode, just type the folowing:
Products.Where(p => p.Price > 50)
and press F5. Alternatively, you might prefer to use a query expression:
from p in Products
where p.Price > 50
select p
In C# statements mode, you'll need to call the Dump() method to tell it to write out the results. You'll also need to terminate the expression with a semicolon:
Products.Where(p => p.Price > 50).Dump();
There are more examples in the samples section of LINQPad - look at the 5-minute induction.

Just wanted to add - LINQ pad pluralizes - I did not know this and it drove me crazy for a good fifteen minutes
I was trying to select from a table called DentalApplication
DentalApplication.Where(a=> a.PackageID > 0)
Gave me this error
'LINQPad.User.DentalApplication' does not contain a definition for 'Where'
Changed it to
DentalApplications.Where(a=> a.PackageID > 0)
and it worked

var db = new MyDatabaseContext(); // Your database context.
var result = db.Products.Where(q=>q.Price > 50);
...where db represents your ORM context. Price represents your mapping to the Price field in the database. result represents the result set -- your database rows/entities.

Check out: http://msdn.microsoft.com/en-us/library/bb397933(v=vs.90).aspx which will give you an introduction to Linq and using lambda expressions.
Then have a look at http://msdn.microsoft.com/en-us/library/bb386927.aspx which will give you the basics, but to answer your specific question:
var products = db.Products.Where(prod => prod.Price > 50);
foreach(var product in products)
{
//do something
}

Sort Linq list with one column

I guess it should be really simple, but i cannot find how to do it.
I have a linq query, that selects one column, of type int, and i need it sorted.
var values = (from p in context.Products
where p.LockedSince == null
select Convert.ToInt32(p.SearchColumn3)).Distinct();
values = values.OrderBy(x => x);
SearchColumn3 is op type string, but i only contains integers. So i thought, converting to Int32 and ordering would definitely give me a nice 1,2,3 sorted list of values. But instead, the list stays ordered like it were strings.
199 20 201
Update:
I've done some tests with C# code and LinqPad.
LinqPad generates the following SQL:
SELECT [t2].[value]
FROM (
SELECT DISTINCT [t1].[value]
FROM (
SELECT CONVERT(Int,[t0].[SearchColumn3]) AS [value], [t0].[LockedSince], [t0].[SearchColumn3]
FROM [Product] AS [t0]
) AS [t1]
WHERE ([t1].[LockedSince] IS NULL)
) AS [t2]
ORDER BY [t2].[value]
And my SQL profiler says that my C# code generates this piece of SQL:
SELECT DISTINCT a.[SearchColumn3] AS COL1
FROM [Product] a
WHERE a.[LockedSince] IS NULL
ORDER BY a.[SearchColumn3]
So it look like C# Linq code just omits the Convert.ToInt32.
Can anyone say something useful about this?

[Disclaimer - I work at Telerik]
You can solve this problem with Telerik OpenAccess ORM too. Here is what i would suggest in this case.
var values = (from p in context.Products
where p.LockedSince == null
orderby "cast({0} as integer)".SQL<int>(p.SearchColumn3)
select "cast({0} as integer)".SQL<int>(p.SearchColumn3)).ToList().Distinct();
OpenAccess provides the SQL extension method, which gives you the ability to add some specific sql code to the generated sql statement.
We have started working on improving this behavior.
Thank you for pointing this out.
Regards
Ralph

Same answer as one my other questions, it turns out that the Linq provider i'm using, the one that comes with Telerik OpenAccess ORM does things different than the standard Linq to SQL provider! See the SQL i've posted in my opening post! I totally wasn't expecting something like this, but i seem that the Telerik OpenAccess thing still needs a lot of improvement. So be careful before you start using it. It looks nice, but it has some serious shortcomings.

I can't replicate this problem. But just make sure you're enumerating the collection when you inspect it. How are you checking the result?
values = values.OrderBy(x => x);
foreach (var v in values)
{
Console.WriteLine(v.ToString());
}
Remember, this won't change the order of the records in the database or anywhere else - only the order that you can retrieve them from the values enumeration.

Because your values variable is a result of a Linq expression, so that it doest not really have values until you calling a method such as ToList, ToArray, etc.
Get back to your example, the variable x in OrderBy method, will be treated as p.SearchColumn3 and therefore, it's a string.
To avoid that, you need to let p.SearchColumn3 become integer before OrderBy method.
You should add a let statement in to your code as below:
var values = (from p in context.Products
where p.LockedSince == null
let val = Convert.ToInt32(p.SearchColumn3)
select val).Distinct();
values = values.OrderBy(x => x);
In addition, you can combine order by statement with the first, it will be fine.

date difference in EF4

i need to get a difference of two dates ,one date from a table and one the current date, and the difference should be less than 9 0days. i need to use this as filter in where clause of the linq
i tried doing this
var list = from p in context.persons
where ((p.CreateDT).Subtract(DateTime.Now).Days < 90)
select p;
i get this excpetion :
LINQ to Entities does not recognize the method 'System.TimeSpan Subtract(System.DateTime)' method, and this method cannot be translated into a store expression.
I did research other articles but nothing helped..Any ideas

Trick here is that it can't translate all your fancy oo hoo-ha to plain old sql. Trick is to flip it on it's head:
var before = DateTime.Now.AddDays(-90);
var persons = context.Persons.Where(x => x.CreateDT > before);
EXPLANATION
Remember that everything in the WHERE bit of your LINQ statement must be translated from C# to SQL by the EF. It is very, very capable out of the box and handles most basic tasks, but it has no idea how to understand the most rudimentary method calls, such as DateTime.Subtract(). So the idea here is to let it do what it does best by precalculating a value and then passing that to the data tier.
The first line subtracts 90 days from the current time by adding negative 90 days. The second line passes it off to the database server.
The second line should translate to the SQL WHERE CreateDT > #BEFORETHIS

Update
It seems that EF doesn't support subtracting dates and returning a TimeSpan. Here's one way to solve the problem:
DateTime oldestDate = DateTime.Now.AddDays(-90);
var list = from p in context.persons
where p.CreateDT >= oldestDate
select p;
See this thread on Stackoverflow.

Try doing simply (p.CreateDate - DateTime.Now).Days < 90. Instead of calling DateTime.Subtract(). In some cases the operator overloads are implemented for Entity Framework even when the corresponding named methods are not.
If that doesn't work you could instead use ESQL or a stored procedure. As a final, dirty solution, you could call context.persons.ToList() and then call the DateTime.Subtract().

linq sql - aggregate count

I have some linq that returns the correct data.
var numEmails = (from row in EmailBatchProposal
where row.EmailBatchId == emailBatchId
select row.EmailBatchProposalId).Count();
However, if I understand linq correctly, this does not perform optimally. It grabs all the data and then walks through the list and counts the rows. What I'd really like is for linq (in the background) to use like:
Select count(*) from ...
I trust the performance reasons are obvious.
Does anyone know the proper way to do this?

Actually, if the linq query is used with a collection that implements IQueryable and supports translation into underlying SQL variant, it is quite a basic functionality to translate the Count function from your example correctly.

People generally learn best by practicing. I would suggest you get a copy of LinqPad (free), enter in your Linq query, and see what SQL it generates. Then you can modify the Linq query until you get exactly what you want.

Actually, the LINQ-to-SQL is smart enough to know that it should do a count... For example, if I have the following query:
var v = (from u in TblUsers
select u).Count();
the SQL that actually executes when I run this is:
SELECT COUNT(*) AS [value]
FROM [tblUsers] AS [t0]
Amazing, huh? Another poster made the really good suggestion of getting LinqPad - it is a WONDERFUL tool. It will show you the exact SQL that gets executed. You can always confirm with SQL profiler too.

Check the Log of the SQL used in the query.
using (var dbc = new siteDataContext())
{
dbc.Log = Console.Out;
var bd = (from b in dbc.birthdays
select b).Count();
Console.WriteLine("{0}", bd);
}
gives the query:
SELECT COUNT(*) AS [value]
FROM [dbo].[birthdays] AS [t0]
-- Context: SqlProvider(Sql2008) Model: AttributedMetaModel Build: 3.5.30729.1

You can just use the argument of .Count().
int numEmails = EmailBatchProposal.Count(x => x.EmailBatchId == emailBatchId)
As noted below, this doesn't actually resolve to any different SQL, but I think it's at least a cleaner looking alternative.