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Demonstrates an ASP.NET Web page that implements a client callback. For more information, see Implementing Client Callbacks Without Postbacks in ASP.NET Web Pages.


The following code example is in two parts. The first part of the example shows an ASP.NET Web page (the .aspx page). The second part shows the corresponding code-behind file (the .aspx.vb file).

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' ClientCallback.aspx page
<%@ Page Language="VB" AutoEventWireup="false" 
  CodeFile="ClientCallback.aspx.vb" Inherits="ClientCallback" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" 
<html xmlns="" >
<head runat="server">
  <title>Untitled Page</title>
  <script type="text/javascript">
    function LookUpStock()
        var lb = document.forms[0].ListBox1;
        var product = lb.options[lb.selectedIndex].text 
        CallServer(product, "");

    function ReceiveServerData(rValue)
        Results.innerText = rValue;
  <form id="form1" runat="server">
      <asp:ListBox ID="ListBox1" Runat="server"></asp:ListBox>
      <br />
      <br />
      <button onclick="LookUpStock()">Look Up Stock</button>
      <br />
      <br />
      Items in stock: <span ID="Results"></span>
      <br />

' ClientCallback.aspx.vb code-behind file
Partial Class ClientCallback
    Inherits System.Web.UI.Page
    Implements System.Web.UI.ICallbackEventHandler

    Protected catalog As ListDictionary
    Sub Page_Load(ByVal sender As Object, ByVal e As _
        System.EventArgs) Handles Me.Load
        Dim cbReference As String
        cbReference = Page.ClientScript.GetCallbackEventReference(Me, _
            "arg", "ReceiveServerData", "context")
        Dim callbackScript As String = ""
        callbackScript &= "function CallServer(arg, context) { " & _
            cbReference & "} ;"
        Page.ClientScript.RegisterClientScriptBlock(Me.GetType(), _
            "CallServer", callbackScript, True)

        ' Populate List Dictionary with invented database data
        catalog = New ListDictionary()
        catalog.Add("monitor", 12)
        catalog.Add("laptop", 10)
        catalog.Add("keyboard", 23)
        catalog.Add("mouse", 17)

        ListBox1.DataSource = catalog
        ListBox1.DataTextField = "key"
    End Sub

    Public Function RaiseCallbackEvent(ByVal eventArgument _
        As String) As String Implements _
        Dim returnValue As String
        If catalog(eventArgument) Is Nothing Then
            returnValue = "-1"
            returnValue = catalog(eventArgument).ToString()
        End If
        Return returnValue
    End Function
End Class

The Web page emulates a database lookup to determine the number of items that are available, or in stock, for a series of products (monitors, keyboards, and so on). To simplify this code example, the database is represented by a dictionary list that contains a small set of items. For each item in the table, the key is the item name (such as monitor) and the value is the number of items that are in stock. In a production application, a database would be used instead.

When the page runs, a ListBox control is bound to the hash table so that the ListBox control displays the list of products. The page also contains a button element (not a button Web server control), whose onclick event is bound to a client function named LookUpStock. When users click the button, the button executes the LookUpStock function, which gets the current selection from the list box and then performs the client callback by calling the CallServer function.

The code-behind page adds client-side script to the page via the RegisterClientScriptBlock method. The script that is added to the page includes a function called CallServer, which gets the name of the method that will post back to the server from the GetCallbackEventReference method.

The client callback invokes the RaiseCallbackEvent method, which returns the available stock for the product passed to it. Note that the arguments sent between the client script and the server code can only be strings.

Security noteSecurity Note

When you use this feature, there are potential security threats. Callback arguments are not validated and therefore should be considered unsafe. You should always check the contents of the arguments before using them. For details, see Script Exploits Overview.

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