Both the ShowMoreResultsBox and Virtual Scrolling mechanisms fire the ItemsRequested event upon each click on the ShowMoreResultsBox or scrolling down using the vertical scrollbar of the drop-down area.

The ShowMoreResultsBox and the EnableVirtualScrolling properties let you load portions of the combobox items in each ItemsRequested event handler to achieve a faster loading speed.

Note
You can use these approaches separately from one another.

For example, you could add 10 items per a request:

After clicking on the ShowMoreResultsBox or scrolling down via the vertical scrollbar of the dropdown, the next 10 items will be added to the Items list.

To enable load-on-demand with a show more results box and/or virtual scrolling:

1. Set the ShowMoreResultsBox property to True.

2. Set the EnableVirtualScrolling property to True if you need to also enable the virtual scrolling mechanism.

3. Set the EnableLoadOnDemand property to True.

4. Add an ItemsRequested event handler that supplies the next batch of items. On the EventArgs of the event handler, the NumberOfItems property indicates the number of items that have already been loaded and the Message property lets you specify the message that appears in the ShowMoreResults box.

5. • C#
   • VB.NET

   CopyC#

   protected void RadComboBox1_ItemsRequested(object sender, RadComboBoxItemsRequestedEventArgs e)
   {
       string sql = "SELECT * from Customers WHERE CompanyName LIKE '" + e.Text + "%'";
       SqlDataAdapter adapter = new SqlDataAdapter(sql, ConfigurationManager.ConnectionStrings["NorthwindConnectionString"].ConnectionString); 
       DataTable data = new DataTable(); 
       adapter.Fill(data);
       try
       {
           int itemsPerRequest = 10; 
           int itemOffset = e.NumberOfItems; 
           int endOffset = itemOffset + itemsPerRequest; 
           if (endOffset > data.Rows.Count) 
           { 
               endOffset = data.Rows.Count; 
           }
           for (int i = itemOffset; i < endOffset; i++) 
           { 
               RadComboBox1.Items.Add(new RadComboBoxItem(data.Rows[i]["CompanyName"].ToString(), data.Rows[i]["CompanyName"].ToString() + "'")); 
           }
           if (data.Rows.Count > 0) 
           { 
               e.Message = String.Format("Items <b>1</b>-<b>{0}</b> out of <b>{1}</b>", endOffset.ToString(), data.Rows.Count.ToString()); 
           }
           else { e.Message = "No matches"; }
       }
       catch 
       { 
           e.Message = "No matches";
       }
   }

   CopyVB.NET

   Protected Sub RadComboBox1_ItemsRequested(ByVal sender As Object, ByVal e As RadComboBoxItemsRequestedEventArgs) Handles RadComboBox1.ItemsRequested
   
       Dim sql As String = "SELECT * from Customers WHERE CompanyName LIKE '" + e.Text + "%'"
       Dim adapter As New SqlDataAdapter(sql, ConfigurationManager.ConnectionStrings("NorthwindConnectionString").ConnectionString)
       Dim data As New DataTable()
       adapter.Fill(data)
       Try
           Dim itemsPerRequest As Integer = 10
           Dim itemOffset As Integer = e.NumberOfItems
           Dim endOffset As Integer = itemOffset + itemsPerRequest
           If endOffset > data.Rows.Count Then
               endOffset = data.Rows.Count
           End If
           Dim i As Integer = itemOffset
           While i < endOffset
               RadComboBox1.Items.Add(New RadComboBoxItem(data.Rows(i)("CompanyName").ToString(), data.Rows(i)("CompanyName").ToString() + "'"))
               i = i + 1
           End While
           If data.Rows.Count > 0 Then
               e.Message = [String].Format("Items <b>1</b>-<b>{0}</b> out of <b>{1}</b>", endOffset.ToString(), data.Rows.Count.ToString())
           Else
               e.Message = "No matches"
           End If
       Catch
           e.Message = "No matches"
       End Try
   End Sub

In addition you can populate the combobox items via a WebService and still use the ShowMoreResultsBox or the Virtual Scrolling approaches.

You should properly set the WebServiceSettings tag of the combobox to point to the right asmx file, like:

CopyXML

<telerik:radcombobox 
    runat="server" 
    id="RadComboBox1" 
    height="200px" 
    width="300px"
    showmoreresultsbox="True" 
    enablevirtualscrolling="True" 
    enableloadondemand="True">            
    <WebServiceSettings Method="GetCustomers" Path="Products.asmx" />
</telerik:radcombobox>

In the WebService's method, you should pass a parameter (this parameter should be called context) of type RadComboBoxContext:

CopyXML

public RadComboBoxData GetCustomers(RadComboBoxContext context)

The context object is used to obtain:

• The text entered in the input field by the user:context.Text

• The number of items per requestcontext.NumberOfItems

Note
Note that the WebService's method is of type RadComboBoxData unlike the method used in the WebService when the ShowMoreResultsBox is disabled. Then, the WebService's method can be either of type RadComboBoxItemData or RadComboBoxData.

• C#
• VB.NET

CopyC#

public RadComboBoxData GetCustomers(RadComboBoxContext context)
{
    string sql = "SELECT * from Customers WHERE CompanyName LIKE '" + context.Text + "%'";
    SqlDataAdapter adapter = new SqlDataAdapter(sql, ConfigurationManager.ConnectionStrings["NorthwindConnectionString"].ConnectionString); 
    DataTable data = new DataTable(); 
    adapter.Fill(data); 
    List<RadComboBoxItemData> result = new List<RadComboBoxItemData>(context.NumberOfItems); 
    RadComboBoxData comboData = new RadComboBoxData(); 
    try
    {
        int itemsPerRequest = 10;
        int itemOffset = context.NumberOfItems; 
        int endOffset = itemOffset + itemsPerRequest; 
        if (endOffset > data.Rows.Count) 
        { 
            endOffset = data.Rows.Count; 
        } 
        if (endOffset == data.Rows.Count) 
        { 
            comboData.EndOfItems = true; 
        } 
        else 
        { 
            comboData.EndOfItems = false; 
        } 
        result = new List<RadComboBoxItemData>(endOffset - itemOffset); 
        for (int i = itemOffset; i < endOffset; i++) 
        { 
            RadComboBoxItemData itemData = new RadComboBoxItemData(); 
            itemData.Text = data.Rows[i]["CompanyName"].ToString(); 
            itemData.Value = data.Rows[i]["CompanyName"].ToString(); result.Add(itemData); 
        }
        if (data.Rows.Count > 0)
        { 
            comboData.Message = String.Format("Items <b>1</b>-<b>{0}</b> out of <b>{1}</b>", endOffset.ToString(), data.Rows.Count.ToString()); 
        } 
        else 
        {
            comboData.Message = "No matches"; 
        }
    }
    catch 
        (Exception e) 
    { 
        comboData.Message = e.Message; 
    }
    comboData.Items = result.ToArray(); 
    return comboData;
}

CopyVB.NET

Public Function GetCustomers(ByVal context As RadComboBoxContext) As RadComboBoxData

    Dim sql As String = "SELECT * from Customers WHERE CompanyName LIKE '" + Context.Text + "%'"
    Dim adapter As New SqlDataAdapter(sql, ConfigurationManager.ConnectionStrings("NorthwindConnectionString").ConnectionString)
    Dim data As New DataTable()
    adapter.Fill(data)
    Dim result As New List(Of RadComboBoxItemData)(context.NumberOfItems)
    Dim comboData As New RadComboBoxData()
    Try
        Dim itemsPerRequest As Integer = 10
        Dim itemOffset As Integer = context.NumberOfItems
        Dim endOffset As Integer = itemOffset + itemsPerRequest
        If endOffset > data.Rows.Count Then
            endOffset = data.Rows.Count
        End If
        If endOffset = data.Rows.Count Then
            comboData.EndOfItems = True
        Else
            comboData.EndOfItems = False
        End If
        result = New List(Of RadComboBoxItemData)(endOffset - itemOffset)
        Dim i As Integer = itemOffset
        While i < endOffset
            Dim itemData As New RadComboBoxItemData()
            itemData.Text = data.Rows(i)("CompanyName").ToString()
            itemData.Value = data.Rows(i)("CompanyName").ToString()
            result.Add(itemData)
            i = i + 1
        End While
        If data.Rows.Count > 0 Then
            comboData.Message = [String].Format("Items <b>1</b>-<b>{0}</b> out of <b>{1}</b>", endOffset.ToString(), data.Rows.Count.ToString())
        Else
            comboData.Message = "No matches"
        End If
    Catch e As Exception
        comboData.Message = e.Message
    End Try
    comboData.Items = result.ToArray()
    Return comboData
    EndFunction

For a live example, see Load On Demand