ObjectDataSourceDisposingEventHandler Delegate

 

Represents the method that will handle the ObjectDisposing event of the ObjectDataSource control.

Namespace:   System.Web.UI.WebControls
Assembly:  System.Web (in System.Web.dll)

Public Delegate Sub ObjectDataSourceDisposingEventHandler (
	sender As Object,
	e As ObjectDataSourceDisposingEventArgs
)

Parameters

sender
Type: System.Object

The source of the event, the ObjectDataSource.

e
Type: System.Web.UI.WebControls.ObjectDataSourceDisposingEventArgs

An ObjectDataSourceDisposingEventArgs that contains the event data.

When you create an ObjectDataSourceDisposingEventHandler delegate, you identify the method that will handle the event. To associate the event with your event handler, add an instance of the delegate to the event. The event handler is called whenever the event occurs, unless you remove the delegate. For more information about event handler delegates, see NIB: Consuming Events.

This section contains two code examples. The first code example demonstrates how to use an ObjectDataSource control with a business object and a GridView control to display information. The second code example provides an example middle-tier business object that the first code example uses.

The following code example demonstrates how to use an ObjectDataSource control with a business object and a GridView control to display information. You might work with a business object that is very expensive (in terms of time or resources) to create for every data operation that your Web page performs. One way to work with an expensive object might be to create an instance of it once, and then cache it for subsequent operations instead of creating and destroying it for every data operation. This example demonstrates this pattern. You can handle the ObjectCreating event to check the cache first for an object, and then create an instance, only if one is not already cached. Then, handle the ObjectDisposing event to cache the business object for future use, instead of destroying it. In this example, the CancelEventArgs.Cancel property of the ObjectDataSourceDisposingEventArgs class is set to true, to direct the ObjectDataSource to not call Dispose on the instance.

<%@ Import namespace="Samples.AspNet.VB" %>
<%@ Page language="vb" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server">

' Instead of creating and destroying the business object each time, the 
' business object is cached in the ASP.NET Cache.
Sub GetEmployeeLogic(sender As Object, e As ObjectDataSourceEventArgs)

    ' First check to see if an instance of this object already exists in the Cache.
    Dim cachedLogic As EmployeeLogic 

    cachedLogic = CType( Cache("ExpensiveEmployeeLogicObject"), EmployeeLogic)

    If (cachedLogic Is Nothing) Then
            cachedLogic = New EmployeeLogic            
    End If

    e.ObjectInstance = cachedLogic

End Sub ' GetEmployeeLogic

Sub ReturnEmployeeLogic(sender As Object, e As ObjectDataSourceDisposingEventArgs)

    ' Get the instance of the business object that the ObjectDataSource is working with.
    Dim cachedLogic  As EmployeeLogic  
    cachedLogic = CType( e.ObjectInstance, EmployeeLogic)

    ' Test to determine whether the object already exists in the cache.
    Dim temp As EmployeeLogic 
    temp = CType( Cache("ExpensiveEmployeeLogicObject"), EmployeeLogic)

    If (temp Is Nothing) Then
        ' If it does not yet exist in the Cache, add it.
        Cache.Insert("ExpensiveEmployeeLogicObject", cachedLogic)
    End If

    ' Cancel the event, so that the object will 
    ' not be Disposed if it implements IDisposable.
    e.Cancel = True
End Sub ' ReturnEmployeeLogic
</script>

<html xmlns="http://www.w3.org/1999/xhtml" >
  <head>
    <title>ObjectDataSource - VB Example</title>
  </head>
  <body>
    <form id="Form1" method="post" runat="server">

        <asp:gridview
          id="GridView1"
          runat="server"          
          datasourceid="ObjectDataSource1">
        </asp:gridview>

        <asp:objectdatasource 
          id="ObjectDataSource1"
          runat="server"          
          selectmethod="GetCreateTime"          
          typename="Samples.AspNet.VB.EmployeeLogic"
          onobjectcreating="GetEmployeeLogic"
          onobjectdisposing="ReturnEmployeeLogic" >
        </asp:objectdatasource>        

    </form>
  </body>
</html>

The following code example provides an example middle-tier business object that the preceding code example uses. The code example consists of a basic business object, defined by the EmployeeLogic class, which is a class that maintains state and encapsulates business logic. For a complete working example, you must compile this code as a library, and then use these classes from an ASP page.

Imports System
Imports System.Collections
Imports System.Web.UI
Imports System.Web.UI.WebControls

Namespace Samples.AspNet.VB

  Public Class EmployeeLogic


    Public Sub New() 
        MyClass.New(DateTime.Now)

    End Sub 'NewNew


    Public Sub New(ByVal creationTime As DateTime) 
        _creationTime = creationTime

    End Sub 'NewNew

    Private _creationTime As DateTime


    ' Returns a collection of NorthwindEmployee objects.
    Public Function GetCreateTime() As ICollection 
        Dim al As New ArrayList()

        ' Returns creation time for this example.      
        al.Add("The business object that you are using was created at " + _creationTime)

        Return al

    End Function 'GetCreateTime
  End Class 'EmployeeLogic
End Namespace ' Samples.AspNet.VB

.NET Framework
Available since 2.0
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