C# 4 and .NET 4

linQ to entities ❘ 883
event of the ObjectContext class. This event is fired before the data is written to the store, so you can add
some verification logic to see if the changes should be really done. SaveChanges() returns the number of
entity objects that have been written.
What happens if the records in the database that are represented by the entity classes have been changed
after reading the record The answer depends on the ConcurrencyMode property that is set with the
model. With every property of an entity object, you can configure the ConcurrencyMode to Fixed or
None. The value Fixed means that the property is validated at write time to determine if the value was
not changed in the meantime. None — which is the default — ignores any change. If some properties
are configured to the Fixed mode, and data changed between reading and writing the entity objects, an
OptimisticConcurrencyException occurs.
You can deal with this exception by invoking the Refresh() method to read the actual information
from the database into the object context. This method accepts two refresh modes configured by a
RefreshMode enumeration value: ClientWins or StoreWins. StoreWins means that the actual information
is taken from the database and set to the current values of the entity objects. ClientWins means that the
database information is set to the original values of the entity objects, and thus the database values will
be overwritten with the next SaveChanges. The second parameter of the Refresh() method is either a
collection of entity objects or a single entity object. You can decide the refresh behavior entity by entity:
private static void ChangeInformation()
{
//...
int changes = 0;
try
{
changes += data.SaveChanges();
}
catch (OptimisticConcurrencyException ex)
{
data.Refresh(RefreshMode.ClientWins, ex.StateEntries);
changes += data.SaveChanges();
}
Console.WriteLine("{0} entities changed", changes);
//...
code snippet Formula1Demo/Program.cs
linq To enTiTies
In several chapters of this book, you’ve seen LINQ to Query objects, databases, and XML. Of course,
LINQ is also available to query entities.
With LINQ to Entities, the source for the LINQ query is ObjectQuery. Because ObjectQuery
implements the interface IQueryable, the extension methods selected for the query are defined with the
class Queryable from the namespace System.Linq. The extension methods defined with this class have a
parameter Expression; that’s why the compiler writes an expression tree to the assembly. You can read
more about expression trees in Chapter 11. The expression tree is then resolved from the ObjectQuery
class to the SQL query.
You can use a simple LINQ query as shown here to return the racers that won more than 40 races:
using (var data = new Formula1Entities())
{
var racers = from r in data.Racers
where r.Wins > 40
orderby r.Wins descending
select r;
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