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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.


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.

Security noteSecurity 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

Supported in: 4, 3.5, 3.0, 2.0

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.