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PostBackTrigger Class

Defines a control inside a UpdatePanel control as a postback control.

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

[AspNetHostingPermissionAttribute(SecurityAction.LinkDemand, Level = AspNetHostingPermissionLevel.Minimal)]
[AspNetHostingPermissionAttribute(SecurityAction.InheritanceDemand, Level = AspNetHostingPermissionLevel.Minimal)]
public class PostBackTrigger : UpdatePanelControlTrigger

Use the PostBackTrigger control to enable controls inside an UpdatePanel to cause a postback instead of performing an asynchronous postback.

Use the RegisterPostBackControl method of the ScriptManager control to programmatically register a postback control. You can then call the Update method of the UpdatePanel control when the trigger control performs a postback.


Programmatically adding PostBackTrigger controls is not supported.

If a control is set as both a PostBackTrigger and AsyncPostBackTrigger control, an exception is thrown.

The following example shows how to declaratively define a PostBackTrigger control for an UpdatePanel control. In the panel, a FileUpload control enables users to upload a file. Users must first check whether the file to upload exists. The Button control that calls the event handler to check the file name causes an asynchronous postback. However, the Button control that uploads the file is registered as a PostBackTrigger, because files cannot be uploaded asynchronously.

<%@ Page Language="C#" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"  

<script runat="server">

    private string saveDir = @"Uploads\";

    protected void UploadButton_Click(object sender, EventArgs e)
        if (FileUpload1.HasFile && FileUpload1.FileBytes.Length < 10000 &&
            string savePath = Request.PhysicalApplicationPath + saveDir +
            //Remove comment from the next line to upload file. 
            UploadStatusLabel.Text = "The file was processed successfully.";
            UploadStatusLabel.Text = "You did not specify a file to upload, or a file name, or the file was too large. Please try again.";

    protected void CheckButton_Click(object sender, EventArgs e)
        if (FileName.Text.Length > 0)
            string s = CheckForFileName() ? "exists already." : "does not exist.";
            UploadStatusLabel.Text = "The file name choosen " + s;
            UploadStatusLabel.Text = "Specify a file name to check.";
    private Boolean CheckForFileName()
        System.IO.FileInfo fi = new System.IO.FileInfo(Request.PhysicalApplicationPath + 
            saveDir + Server.HtmlEncode(FileName.Text));
            return fi.Exists;


<html xmlns="" >
<head runat="server">
    <title>PostBackTrigger Example</title>
    <form id="form1" runat="server">
    <asp:ScriptManager ID="ScriptManager1" runat="server" />
    The upload button is defined as a PostBackTrigger.<br/>
    <asp:UpdatePanel ID="UpdatePanel1" UpdateMode="Conditional" runat="server">
    <legend>FileUpload in an UpdatePanel</legend>
       First, enter a file name to upload your file to: 
       <asp:TextBox ID="FileName" runat="server" />
       <asp:Button ID="CheckButton" Text="Check" runat="server" OnClick="CheckButton_Click" />
       <br />
       Then, browse and find the file to upload:
       <asp:FileUpload id="FileUpload1"                 
       <br />
       <asp:Button id="UploadButton" 
           Text="Upload file"
       <br />
       <asp:Label id="UploadStatusLabel"
           runat="server" style="color:red;">
    <asp:PostBackTrigger ControlID="UploadButton" />

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003

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

.NET Framework

Supported in: 3.5