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Events Overview

Updated on Jun 24, 2026

Use RadComboBox events to react when the drop-down opens, closes, or the selection changes. This article shows how to subscribe to the most common events and how to use DropDownOpened to reset the drop-down scroll position each time the list opens.

Subscribing to Event

Subscribe to a RadComboBox event in XAML or in code-behind.

Example 1: Subscribe to an event in XAML

xaml

<telerik:RadComboBox x:Name="radComboBox" DropDownOpened="RadComboBox_DropDownOpened" />

Example 2: Subscribe to an event in code-behind

csharp

this.radComboBox.DropDownOpened += RadComboBox_DropDownOpened;

Events

Start with the following events when you customize RadComboBox behavior:

DropDownOpened

Use DropDownOpened when you need to change the drop-down state after the popup becomes visible. Typical scenarios include loading data on demand, focusing an element in the popup, or resetting the scroll position.

Example 3: Handle the DropDownOpened event

csharp

private void RadComboBox_DropDownOpened(object sender, EventArgs e)
{
	var radComboBox = (RadComboBox)sender;
}

DropDownClosed

Use DropDownClosed when you need to react after the user dismisses the drop-down. This event is useful when you want to commit UI changes or trigger cleanup logic after the popup closes.

Example 4: Handle the DropDownClosed event

csharp

private void RadComboBox_DropDownClosed(object sender, EventArgs e)
{
	var radComboBox = (RadComboBox)sender;
}

SelectionChanged

Use SelectionChanged when you need to inspect the selected and unselected items. The SelectionChangedEventArgs type exposes the AddedItems and RemovedItems collections.

Example 5: Handle the SelectionChanged event

csharp

private void RadComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
	var radComboBox = (RadComboBox)sender;
	IList selectedItems = e.AddedItems;
	IList unselectedItems = e.RemovedItems;
}

SelectionChanged is inherited from the Selector class.

Reset the Drop-Down Scroll Position on Open

By default, RadComboBox keeps the last scroll position when the user closes and reopens the drop-down. This behavior is the same when AllowMultipleSelection is True.

RadComboBox does not expose a built-in property that resets the drop-down list to the first item every time it opens. To enforce that behavior, handle DropDownOpened and scroll the popup ScrollViewer in code.

To reset the drop-down scroll position each time the list opens:

Subscribe to the DropDownOpened event.
Find the ScrollViewer inside the popup after the drop-down opens.
Call ScrollToTop() on that ScrollViewer.

Example 6: Subscribe to DropDownOpened in a multiple-selection combo box

xaml

<telerik:RadComboBox x:Name="radComboBox"
					 AllowMultipleSelection="True"
					 DropDownOpened="CountriesComboBox_DropDownOpened" />

Example 7: Reset the drop-down scroll position in code-behind

csharp

        private void CountriesComboBox_DropDownOpened(object sender, EventArgs e)
        {
            if (sender is RadComboBox comboBox && comboBox.Items.Count > 1)
            {
                // VisualTreeHelper cannot cross Popup boundaries because the Popup
                // opens in a separate PresentationSource. We find the Popup first,
                // then search within popup.Child. The Dispatcher delay ensures the
                // popup layout is complete before we try to access its visual tree.
                Dispatcher.BeginInvoke(() =>
                {
                    var popup = FindChildElement<Popup>(comboBox);
                    if (popup?.Child != null)
                    {
                        var scrollViewer = FindChildElement<ScrollViewer>(popup.Child);
                        scrollViewer?.ScrollToTop();
                    }
                }, DispatcherPriority.Loaded);
            }
        }

        private static T FindChildElement<T>(DependencyObject parent) where T : DependencyObject
        {
            for (int i = 0; i < VisualTreeHelper.GetChildrenCount(parent); i++)
            {
                var child = VisualTreeHelper.GetChild(parent, i);
                if (child is T match)
                    return match;

                var result = FindChildElement<T>(child);
                if (result != null)
                    return result;
            }
            return null;
        }

Add the required using directives (for example System.Windows, System.Windows.Controls, System.Windows.Controls.Primitives, System.Windows.Media, and System.Windows.Threading) if they are not already present in the code-behind file.

Because the code runs after DropDownOpened fires, the popup is already created and the ScrollViewer is available to scroll back to the top.