AddOnPreRenderCompleteAsync Method (BeginEventHandler, EndEventHandler)

Page.AddOnPreRenderCompleteAsync Method (BeginEventHandler, EndEventHandler)


Registers beginning and ending event handler delegates that do not require state information for an asynchronous page.

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

public void AddOnPreRenderCompleteAsync(
	BeginEventHandler beginHandler,
	EndEventHandler endHandler


Type: System.Web.BeginEventHandler

The delegate for the BeginEventHandler method.

Type: System.Web.EndEventHandler

The delegate for the EndEventHandler method.

Exception Condition

The <async> page directive is not set to true.

- or -

The AddOnPreRenderCompleteAsync method is called after the PreRender event.


The BeginHandler or EndHandler is null.

Use the AddOnPreRenderCompleteAsync method to add handlers to an asynchronous Web page.

You can register multiple asynchronous handlers; however, only one handler runs at a time. If you want to process multiple asynchronous methods simultaneously, you should use a single BeginEventHandler method and launch multiple asynchronous operations from that handler.

The asynchronous handlers are called between the PreRender and PreRenderComplete events.

First, all Page events (through the PreRender event) are run, and then each registered BeginEventHandler method is called. When the handler completes, the corresponding EndEventHandler method is called. If there are multiple asynchronous handlers, the next handler is called.

After the registered asynchronous event handlers have been called, the rest of the page events are called, beginning with the PreRenderComplete event.

The following code example uses an asynchronous request to display the HTML source code of the local Web server's default page in a TextBox control.

System_CAPS_security Security Note

This example has a text box that accepts user input, which is a potential security threat. By default, ASP.NET Web pages validate that user input does not include script or HTML elements. For more information, see Script Exploits Overview.

<%@ page language="C#" Async="true"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
<script runat="server">
  System.Net.WebRequest myRequest;

  void Page_Load(object sender, EventArgs e)
    Label1.Text = "Page_Load: thread #" + System.Threading.Thread.CurrentThread.GetHashCode();

    BeginEventHandler bh = new BeginEventHandler(this.BeginGetAsyncData);
    EndEventHandler eh = new EndEventHandler(this.EndGetAsyncData);

    AddOnPreRenderCompleteAsync(bh, eh);

    // Initialize the WebRequest.
    string address = "http://localhost/";

    myRequest = System.Net.WebRequest.Create(address);

  IAsyncResult BeginGetAsyncData(Object src, EventArgs args, AsyncCallback cb, Object state)
    Label2.Text = "BeginGetAsyncData: thread #" + System.Threading.Thread.CurrentThread.GetHashCode();
    return myRequest.BeginGetResponse(cb, state);

  void EndGetAsyncData(IAsyncResult ar)
    Label3.Text = "EndGetAsyncData: thread #" + System.Threading.Thread.CurrentThread.GetHashCode();

    System.Net.WebResponse myResponse = myRequest.EndGetResponse(ar);

    result.Text = new System.IO.StreamReader(myResponse.GetResponseStream()).ReadToEnd();

<html xmlns="" >
  <head runat="server">
      Page.AddOnPreRenderCompleteAsync Example</title>
    <form id="form1" runat="server">
      <asp:label id="Label1" runat="server">
        Label 1</asp:label><br />
      <asp:label id="Label2" runat="server">
        Label 2</asp:label><br />
      <asp:label id="Label3" runat="server">
        Label 3</asp:label><br />
      <asp:textbox id="result" runat="server" textMode="multiLine" ReadOnly="true" columns="80" rows="25" />

.NET Framework
Available since 2.0
Return to top
© 2015 Microsoft