LINQ to SQL details, issues and patterns
LINQ to SQL (formely called DLINQ) is a simple object-relational mapper (ORM) scheduled for .NET Framework 3.5/Visual Studio 2007 (Orcas).
On projects with new data I’m keen on keeping the tables and classes as similar as possible and so the limited functionality of LINQ to SQL really appeals to me.
Sure for larger applications or where legacy data does not inspire great classes then a more advanced ORM with better mapping facilities can outweigh the disadvantages and learning curve imposed by another layer of abstraction. Microsoft have that covered with LINQ to Entities which will ship post-Orcas because of the immaturity of the the designer and advanced scenarios.
Whilst prototyping I’ve come up with a few oddities that deserve documenting somewhere. Here they are in not-quite-FAQ format.
What SQL is this query or operation generating?
Set the Log property of your DataContext object to Console.Out.
What objects are generated and how do I extend them?
MyDataContext : DataContext
Represents a users view of the database and is generated by designer/SQLMetal exposing a property for each table being mapped.
Can be extended by writing your own partial class or sub classing.
MyEntity / T
Represents an entity for each row in the associated table and is generated by designer/SQLMetal exposing a property for each field being mapped which is also decorated with attributes that specify the underlying field name and type.
Can be extended by writing your own partial class, sub classing or providing a superclass.
What patterns does LINQ to SQL use?
Thankfully ActiveRecord is ignored and a more flexible approach is used. It basically provides one class for the database and one for each type of entity (because of inheritance, there could be more than one type of entity per table).
I’ve yet to delve deep enough into the classes and patterns to find good matches but a brief look reveals the following possibilities:
Registry – System.Data.Linq.DataContext
Unit of Work – System.Data.Linq.DataContext
Identity Map – System.Data.Linq.IdentityManager
Table Data Gateway – System.Data.Linq.Table<T>
Query Object – System.Data.Linq.DataQuery<T>
Metadata Mapping – System.Data.Linq.Provider.Meta*, System.Data.Linq.*Attribute, System.Data.Linq.Mapping.*
How do I persist changes?
Call the SubmitChanges method on your DataContext object.
What causes the error “Row not found or changed” when submitting changes?
By default LINQ to SQL creates UPDATE statements that include every field as it was when read in the WHERE clause and not just the primary key as you might expect. The upshot is that if the record has changed since it was loaded it will not find it.
However, there are also a couple of scenarios where it won’t find the record even if it hasn’t changed:
- If the data range/precision of the .NET type can’t exactly hold the value in the SQL table (e.g. DateTime)
- If the table schema doesn’t exactly match the mapping (e.g. a column’s nullability changing)
If in doubt remove the table from your LINQ schema diagram and drag a fresh copy back in from your database using the Server Explorer pane.
Why does the OnPropertyChanging event use the PropertyChanged event handler delegate?
OnPropertyChanging does indeed seem to be using the PropertyChangedEvent and PropertyChangedEventArgs that OnPropertyChanged use.
Being that PropertyChangingEvent and PropertyChangingEventArgs exist I would assume this is a bug within the designer/SQLMetal in beta 2.
What database vendors are supported?
SQL Server 2000, 2005, 2008 and SQLCE are currently included.
Why do queries against a table return entities that do not match the query?
This occurs when entity objects in memory are out-of-step with data in the database.
The query is exectuted against the SQL server and for each matching record it either creates and entity object or uses the cached one it already has.
Therefore it is possible your result set will:
- not include matching entities because the database indicates they do not (database changed or do not exist)
- include entities that do not match because the database indicated they did (database changed or object changed)
Why is memory consumption so high?
LINQ to SQL tends to burn more memory than you might be expecting because of the multiple objects and aggressive caching in place.
Should I share a DataContext between users?
Web applications, web services and middle-tiers often share objects between potential users and requests. It is worth bearing in mind that a DataContext is not well suited to sharing between users because:
- Entities are cached indefinitely
- High memory use as every entity is eventually loaded to memory
- Entities to be out-of-date if the data can be updated elsewhere (triggers, imports, background jobs)
- DataContext.CommitChanges persists all objects changed via that DataContext
- Difficult to determine a safe point to commit User A’s completed entities without committing User B’s incomplete entities
- DataContext’s Table<T> creation is not thread safe
- Accessing a table for the first time on two threads could cause the DataContext to create one instance of a Table<T> whilst creating another for the same <T> and resulting in the latter being kept but the former being returned to one of the callee’s.
A DataContext is therefore best suited for either a single-user application, per-session or per-request.