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Asked: 2026-05-15T20:14:47+00:00

After a fair amount of research and some errors , I modified my code

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After a fair amount of research and some errors, I modified my code so that it creates a new DataContext each time the database is queried or data is inserted. And the database is queried frequently – for each of 250k transactions that are processed, the database is queried to obtain a customer id, department id, and category before the transaction is inserted.

So now I’m trying to optimize the code as it was only processing around 15 transactions a second. I removed some extraneous queries and added some indexes and got it up to 30/sec. I then figured that even though everyone says a DataContext is lightweight, it’s got to cost something to create a new one 4 times per transaction, so I tried reusing the DataContext. I found, much to my surprise, that reusing the context caused performance to degrade to 10 transactions a second!

Why would this be the case? Is it because the DataContext caches the entities in memory and first searches through its in-memory list before querying the database? So that if, for example, I’m looking for the customer id (primary key) for the customer with name ‘MCS’ and the customer name column has a clustered index on it so that the database query is fast, the in-memory lookup will be slower?

And is it true that creating/disposing so many db connections could slow things down, or is this just another premature optimization? And if it is true, is there a way to reuse a DataContext but have it perform an actual database query for each linq-to-sql query?

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1 Answer

  1. Editorial Team

    Here’s why re-using a DataContext is not a best practice, from the MSDN DataContext documentation:

    The DataContext is the source of all
    entities mapped over a database
    connection. It tracks changes that you
    made to all retrieved entities and
    maintains an “identity cache” that
    guarantees that entities retrieved
    more than one time are represented by
    using the same object instance.

    In general, a DataContext instance is
    designed to last for one “unit of
    work” however your application defines
    that term. A DataContext is
    lightweight and is not expensive to
    create. A typical LINQ to SQL
    application creates DataContext
    instances at method scope or as a
    member of short-lived classes that
    represent a logical set of related
    database operations.

    If you’re re-using a DataContext for a large number of queries, your performance will degrade for a couple of possible reasons:

    1. If DataContext’s in-memory identity cache becomes so large that it has to start writing to the pagefile then your performance will be bound to the HD’s read-head speed and effectively there won’t be a reason to use a cache at all.

    2. The more identity objects there are in memory, the longer each save operation takes.

    Essentially what you’re doing is violating the UoW principle for the DataContext class.

    Opening database connections does have some overhead associated with it, but keeping a connection open for a long period of time (which often also means locking a table) is less preferable than opening and closing them quickly.

    Another link which may or may not help you from MSDN:

    How to: Reuse a Connection Between an ADO.NET Command and a DataContext (LINQ to SQL)

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