Detecting Changes with SqlDependency
A SqlDependency object can be associated with a SqlCommand in order to detect when query results differ from those originally retrieved. You can also assign a delegate to the OnChange event, which will fire when the results change for an associated command. You must associate the SqlDependency with the command before you execute the command. The HasChanges property of the SqlDependency can also be used to determine if the query results have changed since the data was first retrieved.
The dependency infrastructure relies on a SqlConnection that is opened when Start is called in order to receive notifications that the underlying data has changed for a given command. The ability for a client to initiate the call to SqlDependency.Start is controlled through the use of SqlClientPermission and code access security attributes. For more information, see Enabling Query Notifications and Code Access Security and ADO.NET.
The following steps illustrate how to declare a dependency, execute a command, and receive a notification when the result set changes:
Initiate a SqlDependency connection to the server.
Create a new SqlDependency object, or use an existing one, and bind it to the SqlCommand object. Internally, this creates a SqlNotificationRequest object and binds it to the command object as needed. This notification request contains an internal identifier that uniquely identifies this SqlDependency object. It also starts the client listener if it is not already active.
Subscribe an event handler to the OnChange event of the SqlDependency object.
Execute the command using any of the Execute methods of the SqlCommand object. Because the command is bound to the notification object, the server recognizes that it must generate a notification, and the queue information will point to the dependencies queue.
Stop the SqlDependency connection to the server.
If any user subsequently changes the underlying data, Microsoft SQL Server detects that there is a notification pending for such a change, and posts a notification that is processed and forwarded to the client through the underlying SqlConnection that was created by calling SqlDependency.Start. The client listener receives the invalidation message. The client listener then locates the associated SqlDependency object and fires the OnChange event.
The following code fragment shows the design pattern you would use to create a sample application.
Sub Initialization() ' Create a dependency connection. SqlDependency.Start(connectionString, queueName) End Sub Sub SomeMethod() ' Assume connection is an open SqlConnection. ' Create a new SqlCommand object. Using command As New SqlCommand( _ "SELECT ShipperID, CompanyName, Phone FROM dbo.Shippers", _ connection) ' Create a dependency and associate it with the SqlCommand. Dim dependency As New SqlDependency(command) ' Maintain the refence in a class member. ' Subscribe to the SqlDependency event. AddHandler dependency.OnChange, AddressOf OnDependencyChange ' Execute the command. Using reader = command.ExecuteReader() ' Process the DataReader. End Using End Using End Sub ' Handler method Sub OnDependencyChange(ByVal sender As Object, _ ByVal e As SqlNotificationEventArgs) ' Handle the event (for example, invalidate this cache entry). End Sub Sub Termination() ' Release the dependency SqlDependency.Stop(connectionString, queueName) End Sub