Friday, April 1, 2011

Callback Feature in ASP.NET

The callback feature in ASP.NET helps to display content in webpage in an asynchronous way.

Lets go straight to an example and get it working.

1)Create a web application in VS 2008

2)In Page Load of "Default.aspx.cs" , enter the following code

/*Client function to Receive Server Data*/
string strReceiveServerData = "function ReceiveServerData(arg,context){RecieveServerDate2(arg);}";

/*Get Callback Reference*/
string strReceiveCallReference = Page.ClientScript.GetCallbackEventReference(this, "arg", "ReceiveServerData", strReceiveServerData);

/*Client function to Call Server... the reference of "Receive server data" function is added here*/
string strCallTheServer = "function CallServer(arg,context){" + strReceiveCallReference + ";}";

/*Register the script*/
Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "CallServer", strCallTheServer, true);

3) Inherit interface “ICallbackEventHandler” in page

public partial class AsyncServerCall : System.Web.UI.Page,ICallbackEventHandler

4) Add method code for interface "ICallbackEventHandler" in "Default.aspx.cs"

#region ICallbackEventHandler Members

string strVar = "";
public string GetCallbackResult()
return strVar;

public void RaiseCallbackEvent(string eventArgument)
strVar = "Hello World," + eventArgument;


5) Add below Javascript to aspx page. Make sure you add "script" tags.

function RecieveServerDate2(arg) {

6) And the final step... call the javascript function to make server call in body onload

<body onload=" CallServer('Guys', '');" >

Run it! And you will get an alert “Hello World, Guys”. Wasn’t the callback easy in ASP.NET?

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