Added new kb article combobox-aria-attribute-value-error (#613)

Co-authored-by: KB Bot <kb-bot@telerik.com>

knowledge-base/combobox-aria-attribute-value-error.md

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+---
+title: Resolving Incorrect ARIA Attribute Value in RadComboBox with Load-on-Demand
+description: This article provides a solution to the ARIA attribute value issue in RadComboBox with Load-on-Demand functionality, ensuring WAI-ARIA compliance.
+type: troubleshooting
+page_title: Fix ARIA Attribute Value Error in RadComboBox for ASP.NET AJAX
+slug: combobox-aria-attribute-value-error
+tags: dropdown, combobox, web forms, asp.net ajax, aria, load-on-demand, accessibility, wai-aria
+res_type: kb
+ticketid: 1667614
+---
+
+## Environment
+
+<table>
+<tbody>
+<tr>
+<td>Product</td>
+<td>RadComboBox for ASP.NET AJAX</td>
+</tr>
+<tr>
+<td>Version</td>
+<td>2024.3.805</td>
+</tr>
+</tbody>
+</table>
+
+## Description
+When using the [RadComboBox](https://docs.telerik.com/devtools/aspnet-ajax/controls/combobox/overview) with `EnableLoadOnDemand="true"` and subscribing to the `OnItemsRequested` event, an error notification from ARC Toolkit Chrome add-on indicates an error: `ARIA attribute value is incorrect`. This issue occurs on the first load when the listbox is not pre-loaded. However, after the listbox is displayed, the error does not recur.
+
+## Cause
+The problem stems from the `Load-on-Demand` functionality, where the dropdown list is not initially rendered. Consequently, the `aria-controls` attribute references an ID for a component that does not exist at the moment, leading to the error.
+
+## Solution
+To fix this issue, dynamically set the `id` attribute for the listbox in the `OnClientLoad` event and remove it in the `OnClientDropDownOpened` event. This approach ensures effective management of the `aria-controls` attribute, aligning it with the `Load-on-Demand` behavior and maintaining WAI-ARIA compliance.
+
+```aspx
+<telerik:RadComboBox ID="RadComboBox1" runat="server" RenderMode="Lightweight"
+    EnableAriaSupport="true"
+    AllowCustomText="true"
+    CausesValidation="false"
+    EnableLoadOnDemand="true" OnItemsRequested="RadComboBox1_ItemsRequested"
+    OnClientLoad="OnClientLoad"
+    OnClientDropDownOpened="OnClientDropDownOpened"
+    EmptyMessage="RadComboBox1 Empty Message">
+</telerik:RadComboBox>
+
+<script>
+    function OnClientLoad(sender, args) {
+        var listboxId = sender.get_id() + "_listbox";
+        $telerik.$(".rcbSlide").attr("id", listboxId);
+    }
+
+    function OnClientDropDownOpened(sender, args) {
+        var listboxId = sender.get_id() + "_listbox";
+        $telerik.$(".rcbSlide").removeAttr("id");
+    }
+</script>
+```
+```C#
+protected void RadComboBox1_ItemsRequested(object sender, Telerik.Web.UI.RadComboBoxItemsRequestedEventArgs e)
+{
+    RadComboBox combo = sender as RadComboBox;
+    combo.Items.Add(new RadComboBoxItem("Item1", "1"));
+    combo.Items.Add(new RadComboBoxItem("Item2", "2"));
+    combo.Items.Add(new RadComboBoxItem("Item3", "3"));
+}
+```
+
+This solution ensures the `aria-controls` attribute correctly references the listbox when it becomes available, preventing accessibility issues related to non-existent IDs.
+
+## See Also
+- [RadComboBox Overview](https://docs.telerik.com/devtools/aspnet-ajax/controls/combobox/overview)
+- [WAI-ARIA Basics](https://www.w3.org/WAI/standards-guidelines/aria/)
+