1. Add a javascript function to your Web page that sets attributes of the context for the load-on-demand request and assign it as the OnClientItemsRequesting event handler of your combobox.

2. On the RadComboBox, set the WebServiceSettings' Path and Method properties to identify a Web Service method that supplies items to the RadComboBox. The context object is passed to the Web service method, which can use the attributes set in the OnClientItemsRequesting event handler.

CopyASPX

<script type="text/javascript">
    function OnClientItemsRequesting(sender, eventArgs) {
        var context = eventArgs.get_context();
        context["FilterString"] = eventArgs.get_text();
    }
</script>

<telerik:radcombobox 
    runat="server" 
    id="RadComboBox1" 
    width="300px" 
    enableloadondemand="true"
    onclientitemsrequesting="OnClientItemsRequesting">  
    <WebServiceSettings Method="GetProducts" Path="Products.asmx" />
</telerik:radcombobox>

For a live example of this technique, see Populate from WebService.

This example illustrates using the context to pass information about the current value of one combobox to another combobox. Both comboboxes use the load-on-demand mechanism. The value of the second combobox (Cities) is dependant on the value of the first combobox (Countries).

1. Add a javascript function to your Web page that sets attributes of the context for the load-on-demand request and assign it as the OnClientItemsRequesting event handler of your combobox.

   CopyASPX

   <script language="javascript" type="text/javascript">  
   function GetSelectedItem(sender, eventArgs)  
   {     
       //get the text of the selected item in the first (countries) combobox     
       var countriesCombo = $find("<%= Countries.ClientID %>");     
       var countriesText = countriesCombo.get_text();
       var context = eventArgs.get_context();
       // set the text of the countries combobox to
       // the context
       context["FilterString"] = countriesText;
   }
   </script>
   
   <telerik:radcombobox 
       id="Countries" 
       runat="server" 
       enableloadondemand="true" 
       onitemsrequested="Countries_ItemsRequested">
   </telerik:radcombobox>
   <telerik:radcombobox 
       id="Cities" 
       runat="server" 
       enableloadondemand="true" 
       onitemsrequested="Cities_ItemsRequested"
       onclientitemsrequesting="GetSelectedItem">
   </telerik:radcombobox>

2. On the RadComboBox, set the ItemsRequested event handler to read the value from the context object:

   • C#
   • VB.NET

   CopyC#

   protected void Countries_ItemsRequested(object o, RadComboBoxItemsRequestedEventArgs e) 
   { 
       Countries.Items.Add(new RadComboBoxItem("USA", "USA")); 
       Countries.Items.Add(new RadComboBoxItem("UK", "UK")); 
       Countries.Items.Add(new RadComboBoxItem("Germany", "Germany")); 
   }
   
   protected void Cities_ItemsRequested(object o, RadComboBoxItemsRequestedEventArgs e) 
   {    
       /* check which item is selected in the first combobox using the Context object. */    
       if (e.Context["FilterString"].ToString() == "UK") 
       { 
           Cities.Items.Add(new RadComboBoxItem("London", "London")); 
           Cities.Items.Add(new RadComboBoxItem("Oxford", "Oxford")); 
       } 
   }

   CopyVB.NET

   Protected Sub Countries_ItemsRequested(ByVal o As Object, ByVal e As RadComboBoxItemsRequestedEventArgs)
       Countries.Items.Add(New RadComboBoxItem("USA", "USA"))
       Countries.Items.Add(New RadComboBoxItem("UK", "UK"))
       Countries.Items.Add(New RadComboBoxItem("Germany", "Germany"))
   End Sub 'Countries_ItemsRequestedProtected 
   
   
   Sub Cities_ItemsRequested(ByVal o As Object, ByVal e As RadComboBoxItemsRequestedEventArgs)
       'Check which item is selected in the first combobox.   
       'Using the Context object.   
       If e.Context("FilterString").ToString() = "UK" Then
           Cities.Items.Add(New RadComboBoxItem("London", "London"))
           Cities.Items.Add(New RadComboBoxItem("Oxford", "Oxford"))
       End If
   End Sub 'Cities_ItemsRequested