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LINQ Query Returns Cached Data

Posted on 2013-11-01
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Last Modified: 2016-02-10
Working on a vb.net web form that uses LINQ-to-SQL. I have a function similar to the following:

   Function GetSubjectResults(ByVal Id As Integer) As String
    Dim strReturn As String = Nothing
    Dim dc As New myDataContext

    Dim q = From iterVar In dc.Subjects _
      Where iterVar.ID = Id
      Select iterVar.Results

    If q.Count > 0 Then 
      If q.First Is Nothing = False Then
        strReturn = q.First
      End If
    End If

    Return strReturn
  End Function

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The first time it runs, it gets the correct Results data (e.g. for Subject.Id = 1). However, when running it later, for a different subject (Subject.Id = 2), it returns the Results data for Subject.Id = 1 instead of 2.

I've debugged the function to confirm that the Id truly has changed to 2, so I'm pretty sure that the 2nd query is using a cache and not actually re-querying the database.

How do I force it to actually re-query the database?
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Question by:cdakz
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10 Comments
 
LVL 16

Expert Comment

by:DcpKing
ID: 39618273
I don't work with Linq (thanks be!) but you'll find an explanation of the caching on this MS page,  In Step 2 it refers to refreshing your data context, which is probably what you need to do in order to get linq to stop using old data.

hth

Mike
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LVL 62

Expert Comment

by:Fernando Soto
ID: 39618299
Hi cdakz;

Because the DataContext is created and is disposed of when it leaves the function because it goes out of scope I doubt highly that it is a caching issue caused by linq, a new DataContext is created each time the function is called.

Can you set a breakpoint at the first if statement and view the variable q in the watch window and expand the results to see what value it holds?
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LVL 8

Expert Comment

by:Eric Flamm
ID: 39618357
Also, have you checked your database to make sure it's the results for ID=2 should not be the same as the results for ID=1? Maybe the code is working properly, and you have a data problem. It's not an issue of caching the datacontext; in that case, the query would still return different results for ID=1 and ID=2.
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Author Comment

by:cdakz
ID: 39618815
Yes I've set a breakpoint and examined that the ID truly changed and also am viewing the database directly with SQL server management studio before and after each line of code. The code is definitely not issuing a new data query.
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Author Comment

by:cdakz
ID: 39618825
Oh yeah,  I also did look at q in the watch Window and it is showing the result from the previous query.
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Accepted Solution

by:
Fernando Soto earned 500 total points
ID: 39618845
Hi cdakz;

Try modifying your code as follows. This should make sure that the myDataContext is disposed of.

Function GetSubjectResults(ByVal Id As Integer) As String

    Dim strReturn As String = Nothing
    
    Using dc As New myDataContext
		Dim q = From iterVar In dc.Subjects _
		              Where iterVar.ID = Id
			      Select iterVar.Results
				
		If q.Count > 0 Then 
		  If q.First Is Nothing = False Then
		    strReturn = q.First
		  End If
		End If
    End Using

    Return strReturn
    
  End Function

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LVL 8

Expert Comment

by:Eric Flamm
ID: 39618851
Just a thought: What if you pull the New from the Dim statement for dc and make it a separate statement:
Dim dc as myDataContext
dc=new myDataContext()

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or
You could wrap the query in a Using container for the datacontext instance, to make sure it's destroyed at the end of the loop:
dim dc as myDataContext
Using  dc as new myDataContext()
   ...
End Using

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Author Comment

by:cdakz
ID: 39619110
eflamm, regarding suggestion #1: I'm already using the New keyword, it's just that it's on the same line as the assignment.

Regarding suggestion #2, that causes the error: "Variable 'dc' hides a variable in an enclosing block." during compilation.
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Author Closing Comment

by:cdakz
ID: 39619114
FernandoSoto, That did the trick. Not sure why it had to be explicitly disposed of, but thanks (gets me past this roadblock while meeting my deadline).
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LVL 62

Expert Comment

by:Fernando Soto
ID: 39619144
Hi cdakz;

What was most likely causing the issues was that because the DataContext was not explicitly  disposed of when leaving the function call it needed to wait for garbage collection to find out if any object had a reference to the DataContext object and if there were none then it could dispose of the object. But if there were  a call to the object in this state then it is possible to get the value that it holds, the old value as you were seeing. It is always a good idea to use a Using statement because of this.

Not a problem and glad to help.
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