C# 4 and .NET 4

626 ❘ ChaPTer 23 system.trAnsActiOns
isolaTion leVel dirTy reads nonrePeaTable reads PhanTom reads
Read Uncommitted Y Y Y
Read Committed N Y Y
Repeatable Read N N Y
Serializable N N N
The following code segment shows how the isolation level can be set with the TransactionScope
class. With the constructor of TransactionScope, you can set the TransactionScopeOption that was
discussed earlier and the TransactionOptions. The TransactionOptions class allows you to define the
IsolationLevel and the Timeout:
var options = new TransactionOptions();
options.IsolationLevel = IsolationLevel.ReadUncommitted;
options.Timeout = TimeSpan.FromSeconds(90);
using (TransactionScope scope =
new TransactionScope(TransactionScopeOption.Required,
options))
{
// Read data without waiting for locks from other transactions,
// dirty reads are possible.
}
CusTom resourCe managers
One of the biggest advantages of the new transaction model is that it is relatively easy to create custom
resource managers that participate in the transaction. A resource manager does not manage only durable
resources but can also manage volatile or in-memory resources — for example, a simple int and a generic list.
Figure 23-4 shows the relationship between a resource manager and transaction classes. The resource
manager implements the interface IEnlistmentNotification that defines the methods Prepare(),
InDoubt(), Commit(), and Rollback(). The resource manager implements this interface to manage
transactions for a resource. To be part of a transaction, the resource manager must enlist with the
Transaction class. Volatile resource managers invoke the method EnlistVolatile(); durable resource
managers invoke EnlistDurable(). Depending on the transaction’s outcome, the transaction manager
invokes the methods from the interface IEnlistmentNotification with the resource manager.
TransactionManager
IEnlistmentNotification
Transaction
EnlistVolatile()
EnlistDurable()
figure 23-4
ResourceManager
Prepare()
InDoubt()
Commit()
Rollback()
Resource
The next table explains the methods of the IEnlistmentNotification interface that you must implement
with resource managers. As you review the table, recall the active, prepared, and committing phases
explained earlier in this chapter in the “Transaction Phases” section.
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