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"Atlas" Foundations: ASP.NET 2.0 Callback

  • April 27, 2006
  • By Thiru Thangarathinam
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Implementing the Server-Side Event for Callback

At the top of page, you import the required namespaces by using the Import directive. After that we use the implements directive to implement the ICallbackEventHandler interface. This interface has a method named RaiseCallbackEvent that must be implemented to make the callback work.

<%@ implements interface="System.Web.UI.ICallbackEventHandler" %>

The signature of the RaiseCallbackEvent method is as follows.

void ICallbackEventHandler.RaiseCallbackEvent(string eventArgs)

As you can see from the above, the RaiseCallbackEvent method takes an argument of type string. If you need to pass values to the server-side method, you should use this string argument. Inside the RaiseCallbackEvent method, you store the supplied event argument in a local private variable for future use.

void ICallbackEventHandler.RaiseCallbackEvent(string eventArgument)
  _callbackArg = eventArgument;

After that, you override the GetCallbackResult() method as shown below.

string ICallbackEventHandler.GetCallbackResult()
  int value = Int32.Parse(_callbackArg);
  return GetEmployeeDetails(value);

The GetCallbackResult() method is the one that is responsible for returning the output of the server-side execution to the client. Inside the GetCallbackResult() method, you first convert the supplied employee ID into an integer type and then invoke a function named GetEmployeeDetails passing in the employee ID as an argument.

int value = Int32.Parse(eventArgs);
return GetEmployeeDetails(value);

As the name suggests, the GetEmployeeDetails() method retrieves the details of the employee and returns that information in the form of an XML string. This method starts by retrieving the connection string from the web.config file by using the following line of code.

string connString = WebConfigurationManager.

The above line of code retrieves the connection string from the connectionStrings section of the web.config file. The connection string is stored in the web.config as follows.

  <add name="adventureWorks" 
    connectionString="server=localhost;Integrated Security=true;


Once the connection string is retrieved, you then create an instance of the SqlConnection object passing in the connection string as an argument. Then you create instances of DataSet, SqlDataAdapter, and SqlCommand objects passing in the appropriate parameters to their constructors. Then you execute the sql query by invoking the Fill() method of the SqlDataAdapter object. Once the query is executed and the results available in the DataSet object, you then invoke the GetXml() method of the DataSet object to return the XML representation of the DataSet to the caller. The GetCallbackResult() method receives the output xml string and simply returns it back to the caller.

Generating the Client-Side Script for Callback

This section will look at the Page_Load event of the page. In the beginning of the Page_Load event, you check to see if the browser supports callback by examining the SupportsCallback property of the HttpBrowserCapabilities object.

if (!Request.Browser.SupportsCallback)
  throw new ApplicationException

Then you invoke the Page.ClientScript.GetCallbackEventReference() method to implement the callback in client-side. You can use this method to generate client-side code, which is required to initiate the asynchronous call to server.

string src = Page.ClientScript.GetCallbackEventReference(this, "arg",    
  "DisplayResultsCallback", "ctx", "DisplayErrorCallback", false);

The arguments passed to the GetCallbackEventReference method are as follows:

  • this — Control that implements ICallbackEventHandler(Current Page)
  • arg — String to be passed to server-side as argument
  • DisplayResultsCallback — Name of the client-side function that will receive the result from server-side event
  • ctx — String to be passed from one client-side function to other client-side function through context parameter
  • DisplayErrorCallback — Name of the client-side function that will be called if there is any error during the execution of the code
  • false — Indicates that the server-side function to be invoked asynchronously

When you execute this page from the browser and view the HTML source code, you will see that the following callback code is generated due to the above GetCallbackEventReference method call.

WebForm_DoCallback('__Page', arg, DisplayResultsCallback,
  ctx,DisplayErrorCallback, false)

WebForm_DoCallback is a JavaScript function (introduced with ASP.NET 2.0) that in turn invokes the XmlHttp class methods to actually perform the callback. Then you embed the callback code inside a function by concatenating the callback generated code with a JavaScript function named GetEmployeeDetailsUsingPostback using the following line of code.

string mainSrc = @"function " + 
  "GetEmployeeDetailsUsingPostback(arg, ctx)" + "{ " + src + "; }";

Finally you register the client script block through the Page.ClientScript.RegisterClientScriptBlock() method call. Note that in ASP.NET 2.0, the Page.RegisterClientScriptBlock and Page.RegisterStartupScript methods are obsolete. That's why you had to take the help of Page.ClientScript to render client-side script to client browser. Page.ClientScript property returns an object of type ClientScriptManager type, which is used for managing client scripts.

Implementing Client Callback Method

In the client side, you have a method named GetEmployeeDetails, which is invoked when the Get Employee Details command button is clicked.

function GetEmployeeDetails()
  var n = document.forms[0].txtEmployeeID.value;
  GetEmployeeDetailsUsingPostback(n, "txtNumber");

From within the GetEmployeeDetails() method, you invoke a method named GetEmployeeDetailsUsingPostback() and pass in the required parameters. Note that the definition of the GetEmployeeDetailsUsingPostback() method is added in the Page_Load event in the server side (through the RegisterClientScriptBlock method call). Once the server-side function is executed, the callback manager automatically calls the DisplayResultsCallback() method.

The code of the DisplayResultsCallback() method is shown below. In this example, because the value returned from the server-side page is an XML string, you load the returned XML into an XMLDOM parser and then parse its contents.

objXMLDoc = new ActiveXObject("Microsoft.XMLDOM");

//Load the returned XML string into XMLDOM Object


Then you get reference to the Employees node by invoking the selectSingleNode method of the MSXML DOM object.

objEmployee = objXMLDoc.selectSingleNode("EmployeesRoot").

If a valid Employees element is returned from the function call, you display its contents. You display this information in a div tag by setting the innerHTML property of the div element to the dynamically constructed HTML.

if (objEmployee != null)
  //Dynamically generate HTML and append the contents
  strHTML += "<br><br>Employee ID :<b>" + 
    objEmployee.selectSingleNode("EmployeeID").text + "</b><br><br>";
  strHTML += "Title:<b>" + 
    objEmployee.selectSingleNode("Title").text + "</b><br><br>";
  strHTML += "Hire Date :<b>" + 
    objEmployee.selectSingleNode("HireDate").text + "</b><br><br>";
  strHTML += "Gender:<b>" + 
    objEmployee.selectSingleNode("Gender").text + "</b><br><br>";
  strHTML += "Birth Date:<b>" + 
    objEmployee.selectSingleNode("BirthDate").text + "</b><br><br>";	

When you browse to Listing 1 using the browser and search for an employee with employee ID of 1, the page will display the employee attributes such as title, hire date, gender, and birth date.

Figure 1
Figure 1

When you click on the Get Employee Details button in Figure 1, you will notice that the employee information is retrieved from the server and displayed in the browser; all without refreshing the page.


The callback capability in ASP.NET 2.0 is an extremely useful feature that can go a long way in increasing the usability of a Web application by obviating the need to perform post backs. You can implement callbacks to retrieve lookup data, perform validation, execute backend code, and so on. Callbacks can also play an important role in increasing the performance of a Web application by fetching only relevant data asynchronously. Microsoft's "Atlas" tool for ASP.NET 2.0, currently available in pre-release format, provides libraries of ASP.NET Server Controls, Web services, and JavaScript libraries to simplify and enhance application creation by providing in-built controls and components that can be used in traditional JavaScript script and event or through ASP.NET Atlas script.

This article is adapted from Professional ASP.NET 2.0 XML by Thiru Thangarathinam (Wrox, 2006, ISBN: 0-7645-9677-2), from Chapter 9 "XML Data Display." Copyright 2006 by Wiley Publishing, Inc. All rights reserved. Reproduced here by permission of the publisher.

Thiru works for Intel Corporation where he focuses on enterprise applications and service oriented architecture. He is a Microsoft Certified Application Developer (MCAD) specializing in architecting and building distributed n-tier applications using ASP.NET, C#, VB, ADO.NET, and SQL Server..

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