[System.Web.Services.WebMethod]
[System.Web.Script.Services.ScriptMethod]
public static string GetHtml(string contextKey)
{
    // A little pause to mimic a latent call
    // This is the place to perform server side
    // code like DB access
    System.Threading.Thread.Sleep(350);

    return String.Format("Persona: {0}", contextKey);
}

<%@ Register
    Assembly="AjaxControlToolkit"
    Namespace="AjaxControlToolkit"
    TagPrefix="ajaxToolkit"%>

Add AJAX Script Manager to the page: 
<asp:ScriptManager ID="ScriptManager1"runat="server"/>

Add DynamicPopulateExtender control to the page:

  <ajaxToolkit:DynamicPopulateExtender
          ID="dp"BehaviorID="dp1"runat="server"
          TargetControlID="form1$Label1"
          ClearContentsDuringUpdate="true"
          ServiceMethod="GetHtml" />

Notice TargetControlID is set to "form1$Label1". This is the Label control to be updated with the Html string returned by the server side code I described in Step 2. $ notation means nesting - read "Label1 control inside form1 control". It can be any nesting depth. Save your work and then view in browser to make sure that no exceptions generated.

<script type="text/javascript">
        function updateDateKey(value) {
            var behavior = $find('dp1');
            if(behavior) {
                behavior.populate(value);
            }
        }
        Sys.Application.add_load(function(){updateDateKey('Alik Levin');});
    </script>

Notice initialization function call - the last row. It invokes the client side function to call into server side for the first time when the page is rendered into the browser for the first time.

<input type="radio" 
       name="rbFormat" 
       id="r0" 
       value='alik'
       onclick="updateDateKey(this.value);" 
       checked="checked" />click to set 'alik'<br/>
<input type="radio" 
       name="rbFormat" 
       id="Radio1" 
       onclick="updateDateKey(this.value);"
       value='levin' />click to set 'levin'

• Case Study

I was asked by a customer to offer a solution for very responsive UX [User Experience] while avoiding annoying refresh of the web page. Natural answer was AJAX. The customer also asked to provide the solution for common look and feel. Master pages was my answer. Then another ask came through - content page must update the sidebar that is part of the Master page.... hmm A-ha! Use DynamicPopulateExtender.

My Related posts

• AJAX Security - Client Side Validation Is For Usability Only, Not For Security

• ASP.NET 3.5 Extensions: Basic Steps To Create Dynamic Data Web Application - Focus On Security and Performance

Sample VS2008 project that demonstrates DynamicPopulateExtender can be found on my SkyDrive:

Watch UX [User Experience] in the video below:

Enjoy.