Image Lazy Loading

Updated on May 13, 2026

As of 2026 Q2, the RadListBox control supports the Image Lazy Loading feature. The following article demonstrates how to use this feature to improve performance when items contain images.

Overview

When a RadListBox contains many items with images, all images are downloaded on initial page load, regardless of whether the items are visible. This can cause significant latency and degrade user experience, particularly with long lists or items with sub-choices that also have images.

The EnableImageLazyLoading property defers image loading until items scroll into the visible viewport. Images are rendered with a data-src attribute instead of src, and are activated only when they enter the visible area.

Enabling Image Lazy Loading

1. Set the EnableImageLazyLoading property to true.

2. Set Height to RadListBox. A fixed height is required so that the control has a scrollable viewport.

3. Assign images to items via the ImageUrl property.

Here is a basic example:

ASP.NET

<telerik:RadListBox runat="server" ID="RadListBox1" Height="300px" Width="350px" EnableImageLazyLoading="true" />

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        string[] colors = { "E74C3C", "3498DB", "2ECC71", "9B59B6", "F39C12", "1ABC9C", "E67E22", "34495E", "16A085", "D35400" };

        for (int i = 1; i <= 500; i++)
        {
            RadListBoxItem item = new RadListBoxItem
            {
                Text = "Product " + i,
                ImageUrl = "https://placehold.co/32x32/" + colors[(i - 1) % colors.Length] + "/FFFFFF?text=" + i
            };

            RadListBox1.Items.Add(item);
        }
    }
}

VB

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not IsPostBack Then
        Dim colors() As String = {"E74C3C", "3498DB", "2ECC71", "9B59B6", "F39C12", "1ABC9C", "E67E22", "34495E", "16A085", "D35400"}

        For i As Integer = 1 To 300
            Dim item As New RadListBoxItem()
            item.Text = "Product " & i
            item.ImageUrl = "~/Images/product_" & i & ".png"
            RadListBox1.Items.Add(item)
        Next
    End If
End Sub