ObjectDataSource.ObjectCreating 事件

参考

定义

命名空间:: System.Web.UI.WebControls

程序集:: System.Web.dll

重要

一些信息与预发行产品相关，相应产品在发行之前可能会进行重大修改。对于此处提供的信息，Microsoft 不作任何明示或暗示的担保。

在创建由 TypeName 属性标识的对象之前发生。

public:
 event System::Web::UI::WebControls::ObjectDataSourceObjectEventHandler ^ ObjectCreating;

public event System.Web.UI.WebControls.ObjectDataSourceObjectEventHandler ObjectCreating;

member this.ObjectCreating : System.Web.UI.WebControls.ObjectDataSourceObjectEventHandler

Public Custom Event ObjectCreating As ObjectDataSourceObjectEventHandler

事件类型

ObjectDataSourceObjectEventHandler

示例

本部分包含两个代码示例。第一个 ObjectDataSource 代码示例演示如何将对象与业务对象和 GridView 控件结合使用来显示信息。第二个代码示例提供在第一个代码示例中使用的中间层业务对象。

下面的代码示例演示如何将 ObjectDataSource 控件与业务对象和 GridView 控件一起使用来显示信息。你可能使用一个业务对象，该业务对象在创建 (方面非常昂贵，) 网页执行的每个数据操作) 。使用成本高昂的对象的方法之一可能是创建一次实例，然后缓存该实例以用于后续操作，而不是针对每个数据操作创建和销毁它。

注意

在生产应用程序中，多个请求最终可能会同时使用同一实例。因此，需要以线程安全的方式实现对象。

此代码示例演示了此模式。可以处理 ObjectCreating 事件以首先检查对象的缓存，并且仅创建对象的实例（如果尚未缓存该实例）。然后，处理 ObjectDisposing 事件以缓存业务对象以供将来使用，而不是销毁它。在此代码示例中 CancelEventArgs.Cancel ，对象的 ObjectDataSourceDisposingEventArgs 属性设置为， true 以指示 ObjectDataSource 不对对象调用 Dispose 方法。

<%@ Import namespace="Samples.AspNet.CS" %>
<%@ Page language="c#" %>
<!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.
private void GetEmployeeLogic(object sender, ObjectDataSourceEventArgs e)
{
    // First check to see if an instance of this object already exists in the Cache.
    EmployeeLogic cachedLogic;
    
    cachedLogic = Cache["ExpensiveEmployeeLogicObject"] as EmployeeLogic;
    
    if (null == cachedLogic) {
            cachedLogic = new EmployeeLogic();            
    }
        
    e.ObjectInstance = cachedLogic;     
}

private void ReturnEmployeeLogic(object sender, ObjectDataSourceDisposingEventArgs e)
{    
    // Get the instance of the business object that the ObjectDataSource is working with.
    EmployeeLogic cachedLogic = e.ObjectInstance as EmployeeLogic;        
    
    // Test to determine whether the object already exists in the cache.
    EmployeeLogic temp = Cache["ExpensiveEmployeeLogicObject"] as EmployeeLogic;
    
    if (null == temp) {
        // If it does not yet exist in the Cache, add it.
        Cache.Insert("ExpensiveEmployeeLogicObject", cachedLogic);
    }
    
    // Cancel the event, so that the object will 
    // not be Disposed if it implements IDisposable.
    e.Cancel = true;
}
</script>

<html xmlns="http://www.w3.org/1999/xhtml" >
  <head>
    <title>ObjectDataSource - C# 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.CS.EmployeeLogic"
          onobjectcreating="GetEmployeeLogic"
          onobjectdisposing="ReturnEmployeeLogic" >
        </asp:objectdatasource>        

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

<%@ 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>

下面的代码示例提供了前面的代码示例使用的示例中间层业务对象。该代码示例由类定义 EmployeeLogic 的基本业务对象组成，该类是封装业务逻辑的有状态类。对于完整的工作示例，必须将此代码编译为库，并从 ASP.NET 页 (.aspx 文件) 使用这些类。

namespace Samples.AspNet.CS {

using System;
using System.Collections;
using System.Web.UI;
using System.Web.UI.WebControls;
  //
  // EmployeeLogic is a stateless business object that encapsulates
  // the operations you can perform on a NorthwindEmployee object.
  //
  public class EmployeeLogic {

    public EmployeeLogic () : this(DateTime.Now) {        
    }
    
    public EmployeeLogic (DateTime creationTime) { 
        _creationTime = creationTime;
    }

    private DateTime _creationTime;
    
    // Returns a collection of NorthwindEmployee objects.
    public ICollection GetCreateTime () {
      ArrayList al = new ArrayList();
      
      // Returns creation time for this example.      
      al.Add("The business object that you are using was created at " + _creationTime);
      
      return al;
    }
  }
}

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
    
    
    Public Sub New(ByVal creationTime As DateTime) 
        _creationTime = creationTime
    
    End Sub
    
    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
End Namespace ' Samples.AspNet.VB

注解

如果在 Visual Basic) 中 (Shared 用于执行数据操作static的方法，ObjectCreating则永远不会引发和 ObjectCreated 事件。

控件 ObjectDataSource 自动调用业务对象的无参数构造函数，以使用反射创建它的实例。 ObjectCreating处理事件以显式调用另一个构造函数，并将结果对象的实例设置为ObjectInstance关联ObjectDataSourceEventArgs对象的属性。

有关如何处理事件的详细信息，请参阅处理和引发事件。

适用于

通过

ObjectDataSource.ObjectCreating 事件

定义

事件类型

示例

注解

适用于

另请参阅

其他资源