Progress® Telerik® UI for WPF Feedback Portal

Unplanned

Last Updated: 21 Mar 2025 13:05 by Stenly

ComboBox: Selecting an item from a RadComboBox in a touch scenario causes RadComboBox below the item to open

Created by: Stenly

Comments: 0

Category: ComboBox

Type: Bug Report

ComboBox: Selecting an item from a RadComboBox in a touch scenario causes RadComboBox below the item to open

In a touch scenario, when there are RadComboBox instances one below the other, selecting an item from a RadComboBox opens the RadComboBox instance, which is below the selected item.

This is present when there is a global Style that targets the native WPF ScrollViewer element, which sets its PanningMode property to Both.

To work this around, set the TouchManager.TouchMode to HitTestHidden, in order to exclude the RadComboBox from the TouchManager's events propagation.

Unplanned

Last Updated: 19 Mar 2025 15:11 by Martin Ivanov

ComboBox: ArgumentOutOfRangeException on ItemsSource reset when AllowMultipleSelection is True

Created by: Martin Ivanov

Comments: 0

Category: ComboBox

Type: Bug Report

ComboBox: ArgumentOutOfRangeException on ItemsSource reset when AllowMultipleSelection is True

ArgumentOutOfRangeException occurs when the ItemsSource of RadComboBox gets reset. To recreate this the IsSelected property of RadComboBoxItem should be bound too and multiple selection should be allowed.

To work this around, you can disable the autiomation peers globally:

AutomationManager.AutomationMode = AutomationMode.Disabled;

Or you disable them only for the RadComboBox:

public class CustomComboBox : RadComboBox
{
    protected override AutomationPeer OnCreateAutomationPeer()
    {
        return null;
    }
}

The stacktrace:

System.ArgumentOutOfRangeException: 'Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index')'
  System.Private.CoreLib.dll!System.ThrowHelper.ThrowArgumentOutOfRange_IndexMustBeLessException() Unknown
  System.Private.CoreLib.dll!System.Collections.Generic.List<System.__Canon>.this[int].get(int index) Unknown  Telerik.Windows.Controls.Input.dll!Telerik.Windows.Automation.Peers.RadComboBoxAutomationPeer.RaiseSelectionEvents(System.Windows.Controls.SelectionChangedEventArgs e) Line 202 C#  Telerik.Windows.Controls.Input.dll!Telerik.Windows.Controls.RadComboBox.OnSelectionChanged(System.Windows.Controls.SelectionChangedEventArgs e) Line 1927 C#
  PresentationFramework.dll!System.Windows.Controls.Primitives.Selector.SetSelectedHelper(object item, System.Windows.FrameworkElement UI, bool selected) Unknown
  PresentationFramework.dll!System.Windows.Controls.Primitives.Selector.NotifyIsSelectedChanged(System.Windows.FrameworkElement container, bool selected, System.Windows.RoutedEventArgs e) Unknown
  PresentationCore.dll!System.Windows.EventRoute.InvokeHandlersImpl(object source, System.Windows.RoutedEventArgs args, bool reRaised) Unknown
  PresentationCore.dll!System.Windows.UIElement.RaiseEventImpl(System.Windows.DependencyObject sender, System.Windows.RoutedEventArgs args) Unknown
  Telerik.Windows.Controls.Input.dll!Telerik.Windows.Controls.RadComboBoxItem.OnSelected(System.Windows.RoutedEventArgs e) Line 59 C#
  PresentationFramework.dll!System.Windows.Controls.ListBoxItem.OnIsSelectedChanged(System.Windows.DependencyObject d, System.Windows.DependencyPropertyChangedEventArgs e) Unknown
  PresentationFramework.dll!System.Windows.FrameworkElement.OnPropertyChanged(System.Windows.DependencyPropertyChangedEventArgs e) Unknown
  WindowsBase.dll!System.Windows.DependencyObject.NotifyPropertyChange(System.Windows.DependencyPropertyChangedEventArgs args) Unknown
  WindowsBase.dll!System.Windows.DependencyObject.UpdateEffectiveValue(System.Windows.EntryIndex entryIndex, System.Windows.DependencyProperty dp, System.Windows.PropertyMetadata metadata, System.Windows.EffectiveValueEntry oldEntry, ref System.Windows.EffectiveValueEntry newEntry, bool coerceWithDeferredReference, bool coerceWithCurrentValue, System.Windows.OperationType operationType) Unknown
  PresentationFramework.dll!System.Windows.StyleHelper.ApplyStyleOrTemplateValue(MS.Internal.FrameworkObject fo, System.Windows.DependencyProperty dp) Unknown
  PresentationFramework.dll!System.Windows.StyleHelper.InvalidateContainerDependents(System.Windows.DependencyObject container, ref MS.Utility.FrugalStructList<System.Windows.ContainerDependent> exclusionContainerDependents, ref MS.Utility.FrugalStructList<System.Windows.ContainerDependent> oldContainerDependents, ref MS.Utility.FrugalStructList<System.Windows.ContainerDependent> newContainerDependents) Unknown
  PresentationFramework.dll!System.Windows.StyleHelper.DoStyleInvalidations(System.Windows.FrameworkElement fe, System.Windows.FrameworkContentElement fce, System.Windows.Style oldStyle, System.Windows.Style newStyle) Unknown
  PresentationFramework.dll!System.Windows.StyleHelper.UpdateStyleCache(System.Windows.FrameworkElement fe, System.Windows.FrameworkContentElement fce, System.Windows.Style oldStyle, System.Windows.Style newStyle, ref System.Windows.Style styleCache) Unknown
  PresentationFramework.dll!System.Windows.FrameworkElement.OnStyleChanged(System.Windows.DependencyObject d, System.Windows.DependencyPropertyChangedEventArgs e) Unknown
  PresentationFramework.dll!System.Windows.FrameworkElement.OnPropertyChanged(System.Windows.DependencyPropertyChangedEventArgs e) Unknown
  WindowsBase.dll!System.Windows.DependencyObject.NotifyPropertyChange(System.Windows.DependencyPropertyChangedEventArgs args) Unknown
  WindowsBase.dll!System.Windows.DependencyObject.UpdateEffectiveValue(System.Windows.EntryIndex entryIndex, System.Windows.DependencyProperty dp, System.Windows.PropertyMetadata metadata, System.Windows.EffectiveValueEntry oldEntry, ref System.Windows.EffectiveValueEntry newEntry, bool coerceWithDeferredReference, bool coerceWithCurrentValue, System.Windows.OperationType operationType) Unknown
  WindowsBase.dll!System.Windows.DependencyObject.InvalidateProperty(System.Windows.DependencyProperty dp, bool preserveCurrentValue) Unknown
  PresentationFramework.dll!System.Windows.FrameworkElement.UpdateStyleProperty() Unknown
  PresentationFramework.dll!System.Windows.FrameworkElement.OnInitialized(System.EventArgs e) Unknown
  Telerik.Windows.Controls.Input.dll!Telerik.Windows.Controls.RadComboBoxItem.OnInitialized(System.EventArgs e) Line 106 C#
  PresentationFramework.dll!System.Windows.FrameworkElement.OnVisualParentChanged(System.Windows.DependencyObject oldParent) Unknown
  PresentationFramework.dll!System.Windows.Controls.ListBoxItem.OnVisualParentChanged(System.Windows.DependencyObject oldParent) Unknown
  PresentationCore.dll!System.Windows.Media.Visual.FireOnVisualParentChanged(System.Windows.DependencyObject oldParent) Unknown
  PresentationCore.dll!System.Windows.Media.Visual.AddVisualChild(System.Windows.Media.Visual child) Unknown
  PresentationCore.dll!System.Windows.Media.VisualCollection.Add(System.Windows.Media.Visual visual) Unknown
  PresentationFramework.dll!System.Windows.Controls.UIElementCollection.AddInternal(System.Windows.UIElement element) Unknown
  PresentationFramework.dll!System.Windows.Controls.Panel.GenerateChildren() Unknown
  PresentationFramework.dll!System.Windows.Controls.Panel.OnItemsChangedInternal(object sender, System.Windows.Controls.Primitives.ItemsChangedEventArgs args) Unknown
  PresentationFramework.dll!System.Windows.Controls.Panel.OnItemsChanged(object sender, System.Windows.Controls.Primitives.ItemsChangedEventArgs args) Unknown
  PresentationFramework.dll!System.Windows.Controls.ItemContainerGenerator.OnRefresh() Unknown
  PresentationFramework.dll!System.Windows.Controls.ItemContainerGenerator.OnCollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs args) Unknown
  WindowsBase.dll!System.Windows.WeakEventManager.ListenerList<System.Collections.Specialized.NotifyCollectionChangedEventArgs>.DeliverEvent(object sender, System.EventArgs e, System.Type managerType) Unknown
  WindowsBase.dll!System.Windows.WeakEventManager.DeliverEvent(object sender, System.EventArgs args) Unknown
  PresentationFramework.dll!System.Windows.Data.CollectionView.OnCollectionChanged(System.Collections.Specialized.NotifyCollectionChangedEventArgs args) Unknown
  WindowsBase.dll!System.Windows.WeakEventManager.ListenerList<System.Collections.Specialized.NotifyCollectionChangedEventArgs>.DeliverEvent(object sender, System.EventArgs e, System.Type managerType) Unknown
  WindowsBase.dll!System.Windows.WeakEventManager.DeliverEvent(object sender, System.EventArgs args) Unknown
  PresentationFramework.dll!System.Windows.Data.CollectionView.OnCollectionChanged(System.Collections.Specialized.NotifyCollectionChangedEventArgs args) Unknown
  PresentationFramework.dll!System.Windows.Data.ListCollectionView.RefreshOverride() Unknown
  PresentationFramework.dll!System.Windows.Data.CollectionView.RefreshInternal() Unknown
  PresentationFramework.dll!System.Windows.Data.CollectionView.DeferHelper.Dispose() Unknown
  PresentationFramework.dll!System.Windows.Controls.ItemCollection.SetCollectionView(System.Windows.Data.CollectionView view) Unknown
  PresentationFramework.dll!System.Windows.Controls.ItemsControl.OnItemsSourceChanged(System.Windows.DependencyObject d, System.Windows.DependencyPropertyChangedEventArgs e) Unknown
  PresentationFramework.dll!System.Windows.FrameworkElement.OnPropertyChanged(System.Windows.DependencyPropertyChangedEventArgs e) Unknown
  WindowsBase.dll!System.Windows.DependencyObject.NotifyPropertyChange(System.Windows.DependencyPropertyChangedEventArgs args) Unknown
  WindowsBase.dll!System.Windows.DependencyObject.UpdateEffectiveValue(System.Windows.EntryIndex entryIndex, System.Windows.DependencyProperty dp, System.Windows.PropertyMetadata metadata, System.Windows.EffectiveValueEntry oldEntry, ref System.Windows.EffectiveValueEntry newEntry, bool coerceWithDeferredReference, bool coerceWithCurrentValue, System.Windows.OperationType operationType) Unknown
  WindowsBase.dll!System.Windows.DependencyObject.InvalidateProperty(System.Windows.DependencyProperty dp, bool preserveCurrentValue) Unknown
  PresentationFramework.dll!System.Windows.Data.BindingExpression.TransferValue(object newValue, bool isASubPropertyChange) Unknown
  PresentationFramework.dll!MS.Internal.Data.PropertyPathWorker.UpdateSourceValueState(int k, System.ComponentModel.ICollectionView collectionView, object newValue, bool isASubPropertyChange) Unknown
  PresentationFramework.dll!MS.Internal.Data.ClrBindingWorker.OnSourcePropertyChanged(object o, string propName) Unknown
  WindowsBase.dll!System.Windows.WeakEventManager.ListenerList<System.ComponentModel.PropertyChangedEventArgs>.DeliverEvent(object sender, System.EventArgs e, System.Type managerType) Unknown
  WindowsBase.dll!System.ComponentModel.PropertyChangedEventManager.OnPropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs args) Unknown
  Telerik.Windows.Controls.dll!Telerik.Windows.Controls.ViewModelBase.OnPropertyChanged(string propertyName) Unknown
> WpfApp180.dll!WpfApp180.MainViewModel.ResetData() Line 44 C#

Unplanned

Last Updated: 06 Mar 2025 10:02 by Martin Ivanov

ComboBox: Add a filter search box in the drop down element

Created by: Martin Ivanov

Comments: 0

Category: ComboBox

Type: Feature Request

ComboBox: Add a filter search box in the drop down element

Currently RadComboBox supports filtering of the data when IsEditable is True.

Add a mode where a search box is displayed (when IsEditable is False) in the drop down of the control. The searchbox should filter the items in the dropdown.

Unplanned

Last Updated: 24 Dec 2024 09:38 by Stenly

ComboBox: Add Extended selection option

Created by: Stenly

Comments: 0

Category: ComboBox

Type: Feature Request

ComboBox: Add Extended selection option

Currently, the RadComboBox provides support for single or multiple selections. We can add an Extended selection mode similar to the one present in the RadListBox control.

Unplanned

Last Updated: 28 Nov 2024 15:01 by Scott Waye

ADMIN

ComboBox: Make possible to virtualization with filtering

Created by: Kalin

Comments: 4

Category: ComboBox

Type: Feature Request

ComboBox: Make possible to virtualization with filtering

Currently filtering with virtualized RadComboBox is not supported as the filtering feature works with the containers (changes their Visibility).

Won't Fix

Last Updated: 20 May 2024 09:33 by ADMIN

ComboBox: When in the RadGridView details template on a touch device subsequent clicks are not registered

Created by: Mauro

Comments: 1

Category: ComboBox

Type: Bug Report

ComboBox: When in the RadGridView details template on a touch device subsequent clicks are not registered

When in the RadGridView details template on a touch device subsequent clicks are not registered.

Unplanned

Last Updated: 08 Apr 2024 10:23 by Stenly

ComboBox: Add NoResultsContent and NoResultsContentTemplate properties

Created by: Stenly

Comments: 0

Category: ComboBox

Type: Feature Request

ComboBox: Add NoResultsContent and NoResultsContentTemplate properties

Add NoResultsContent and NoResultsContentTemplate properties to the RadComboBox control.

Completed

Last Updated: 11 Oct 2023 11:05 by ADMIN

Release R3 2023

ADMIN

ComboBox: Highlight the searched symbols in the RadComboBox items

Created by: Telerik Admin

Comments: 1

Category: ComboBox

Type: Feature Request

ComboBox: Highlight the searched symbols in the RadComboBox items

Unplanned

Last Updated: 19 Sep 2023 15:20 by Martin Ivanov

ComboBox: Add the option to include additional content in the drop down area

Created by: Martin Ivanov

Comments: 0

Category: ComboBox

Type: Feature Request

ComboBox: Add the option to include additional content in the drop down area

Currently, the drop down of RadComboBox contains only the RadComboBoxItems and the Clear Selection button. Add new content presenter that allows you to include extra visuals, like buttons or text.

The feature is similar to the AdditionalContent of RadTabControl.

Completed

Last Updated: 12 Jul 2023 08:52 by ADMIN

Release R2 2023 SP1

ComboBox: ArgumentException is thrown when mixed typed values are presented in the ItemsSource

Created by: Dinko

Comments: 0

Category: ComboBox

Type: Feature Request

ComboBox: ArgumentException is thrown when mixed typed values are presented in the ItemsSource

ArgumentException is thrown when mixed typed values are presented in the ItemsSource. For example, if you use an enum type and present an additional numeric value that is not presented in the corresponding enum definition.

The exception message is the following: System.ArgumentException: 'The value '102' is not a valid value for the enum 'FilterOperator'.'

To work this around, avoid using mixed typed values in the ItemsSource.

Completed

Last Updated: 20 Jun 2023 06:04 by ADMIN

Release LIB 2023.2.619 (19 Jun 2023)

ComboBox: The drop-down popup is sometimes not closed when hosted in an ElementHost in a WinForms application

Created by: Stenly

Comments: 0

Category: ComboBox

Type: Bug Report

ComboBox: The drop-down popup is sometimes not closed when hosted in an ElementHost in a WinForms application

The drop-down popup is sometimes not closed when hosted in an ElementHost in a WinForms application.

Completed

Last Updated: 22 May 2023 08:27 by ADMIN

Release LIB 2023.1.522 (22 May 2023)

ComboBox: Items in the drop-down menu flicker when Windows 11 theme is applied

Created by: Stenly

Comments: 0

Category: ComboBox

Type: Bug Report

ComboBox: Items in the drop-down menu flicker when Windows 11 theme is applied

The items of the RadComboBox control flicker when scrolling and the Windows 11 theme is applied.

To work this around, a new Style with TargetType="RadComboBoxItem" could be created for the ItemContainerStyle property of RadComboBox. Then, set the BorderBrush and ThemeHelper.MouseOverBrush properties to "#F9F9F9".

xmlns:helpers="clr-namespace:Telerik.Windows.Controls.Theming.Helpers;assembly=Telerik.Windows.Controls"

<telerik:RadComboBox.ItemContainerStyle>
    <Style TargetType="telerik:RadComboBoxItem">
        <Setter Property="helpers:ThemeHelper.MouseOverBrush" Value="#F9F9F9" />
        <Setter Property="BorderBrush" Value="#F9F9F9"/>
    </Style>
</telerik:RadComboBox.ItemContainerStyle>

Completed

Last Updated: 19 Jan 2023 14:50 by ADMIN

ComboBox: Provide public API that allows to determine if partial matches can be selected

Created by: Martin Ivanov

Comments: 0

Category: ComboBox

Type: Feature Request

ComboBox: Provide public API that allows to determine if partial matches can be selected

Currently, there is an internal API that determines whether partial matches can be selected or not. Provide public options to change this behavior. By default partial matches are selected.

In the meantime, the default behavior can be changed using reflection to set the CanAutocompleteSelectPartialMatches property of RadComboBox to False.

Completed

Last Updated: 17 Nov 2022 07:11 by ADMIN

Release LIB 2022.3.1121 (21 Nov 2022)

ComboBox: In a multiple selection scenario, having a type converter does not correctly visualize the selected items

Created by: Stenly

Comments: 0

Category: ComboBox

Type: Bug Report

ComboBox: In a multiple selection scenario, having a type converter does not correctly visualize the selected items

The selected items of the RadComboBox will not be displayed correctly if the ItemsSource is bound to a collection of enums that are converted via a custom EnumConverter.

Currently, you could work around this behavior by implementing a custom DataTemplate for the MultipleSelectionBoxTemplate property of RadComboBox:

<telerik:RadComboBox x:Name="comboBox"
                     AllowMultipleSelection="True">
    <telerik:RadComboBox.MultipleSelectionBoxTemplate>
        <DataTemplate>
            <ItemsControl ItemsSource="{Binding ElementName=comboBox, Path=SelectedItems}"
                          AlternationCount="{Binding RelativeSource={RelativeSource Self}, Path=Items.Count}">
                <ItemsControl.ItemsPanel>
                    <ItemsPanelTemplate>
                        <StackPanel Orientation="Horizontal"/>
                    </ItemsPanelTemplate>
                </ItemsControl.ItemsPanel>
                <ItemsControl.ItemTemplate>
                    <DataTemplate>
                        <StackPanel Orientation="Horizontal">
                            <TextBlock x:Name="commaTextBlock" Text=", "/>
                            <TextBlock Text="{Binding}" HorizontalAlignment="Left"/>
                        </StackPanel>
                        <DataTemplate.Triggers>
                            <Trigger Property="ItemsControl.AlternationIndex" Value="0">
                                <Setter Property="Visibility" TargetName="commaTextBlock" Value="Collapsed" />
                            </Trigger>
                        </DataTemplate.Triggers>
                    </DataTemplate>
                </ItemsControl.ItemTemplate>
            </ItemsControl>
        </DataTemplate>
    </telerik:RadComboBox.MultipleSelectionBoxTemplate>
</telerik:RadComboBox>

Completed

Last Updated: 13 Dec 2021 08:00 by ADMIN

Release LIB 2021.3.1206 (6 Dec 2021)

ComboBox: The Background property setting doesn't change the background color of the control

Created by: Martin Ivanov

Comments: 0

Category: ComboBox

Type: Feature Request

ComboBox: The Background property setting doesn't change the background color of the control

Setting the Background property of RadComboBox no longer changes the background color of the control.

To work this around, you can add a global event handler for the Loaded event of RadComboBox and set the corresponding backgrounds in code.

static MainWindow()
{
	EventManager.RegisterClassHandler(typeof(RadComboBox), RadComboBox.LoadedEvent, new RoutedEventHandler(OnComboBoxLoaded));
}

private static void OnComboBoxLoaded(object sender, RoutedEventArgs e)
{
	var comboBox = (RadComboBox)sender;
	if (!comboBox.IsEditable)
	{
		if (comboBox.IsEnabled)
		{
			var buttonChrome = comboBox.FindChildByType<ButtonChrome>();
			buttonChrome.Background = Brushes.Purple;
		}
		else
		{
			comboBox.Background = Brushes.Purple;
		}                
	}	
}

Completed

Last Updated: 05 Nov 2021 09:04 by ADMIN

Release R3 2021 SP

ComboBox: Visual Studio becomes unresponsive when editing the code of the control with both designer and code view opened

Created by: Martin

Comments: 0

Category: ComboBox

Type: Bug Report

ComboBox: Visual Studio becomes unresponsive when editing the code of the control with both designer and code view opened

While editing the XAML document, I'm experiencing weird behavior in VS2019.

All examples apply when the Design editor is active together with XAML editor.

Example 1:

Selecting XAML with mouse stops when I start selecting the telerik:RadComboBox.

Example 2:

Copy Pasting bugs out, either the selection is empty or incorrect.

Example 3:

'Forcus' in Visual Studio is sometimes overtaken by the RadComboBox.

Completed

Last Updated: 15 Oct 2021 10:47 by ADMIN

Release LIB 2021.3.1018 (18 Oct 2021)

ComboBox: Expose a property to change the drop-down icon

Created by: Stenly

Comments: 0

Category: ComboBox

Type: Feature Request

ComboBox: Expose a property to change the drop-down icon

Create a property that allows to easily change the drop-down icon of the RadComboBox control. Now, the default control template needs to be extracted and modified for the theme that is used.

Completed

Last Updated: 15 Sep 2021 15:20 by Arif

Release LIB 2021.2.816 (16 Aug 2021)

Combobox erases entered text if searching for string with spaces when user types space

Created by: Arif

Comments: 11

Category: ComboBox

Type: Bug Report

Combobox erases entered text if searching for string with spaces when user types space

Steps to Create Problem using EditableTextBoxTemplate_WPF sample project provided by Telerik:(modified project is zipped and attached just in case)

- Edit Example.xaml to add "AllowMultipleSelection="True":

<telerik:RadComboBox IsEditable="True" ItemsSource="{Binding Companies}" SelectAllTextEvent="None" FontFamily="Calibri" FontSize="14"
DisplayMemberPath="Name" EditableTemplate="{StaticResource EditableComboBox}"
Width="220" Height="50" DropDownWidth="*" x:Name="comboBox" Margin="5 10" AllowMultipleSelection="True"/>
</StackPanel>

1) Run project

2) Click drop down button to see list in the drop down

3) Type "Global" (at this point it should autocomplete "Global Corporation")

4) Without clicking on anything or Press space bar to continue typing (type space character)

5) Typed text will disappear leaving only space character behind - At this point, nothing is selected.

This does NOT happen if typed text before space does NOT match with any item from the list.

This does NOT happen if AllowMultipleSelection set to false.

This does NOT happen if dropdown is NOT open while typing.

Completed

Last Updated: 13 Aug 2021 12:44 by ADMIN

Release LIB 2021.2.816 (16 Aug 2021)

ComboBox: NullReferenceException when the Text becomes null in multi-selection scenario

Created by: Martin Ivanov

Comments: 0

Category: ComboBox

Type: Bug Report

ComboBox: NullReferenceException when the Text becomes null in multi-selection scenario

NullReferenceException appears when the multiple selection is enabled, items are selected and the Text property of the RadComboBox control becomes null.

To resolve this, you can override the OnTextChanged method of RadComboBox and replace the null value with an empty string.

public class CustomRadComboBox : RadComboBox
{
	protected override void OnTextChanged(string oldValue, string newValue)
	{
		if (string.IsNullOrEmpty(newValue))
		{
			newValue = string.Empty;
			Dispatcher.BeginInvoke(new Action(() =>
			{
					SelectedItems.Clear();
			}));
		}
		base.OnTextChanged(oldValue, newValue);
	}
}

Unplanned

Last Updated: 04 Aug 2021 09:29 by ADMIN

ComboBox: Add SelectAll button or template

Created by: Dinko

Comments: 0

Category: ComboBox

Type: Feature Request

ComboBox: Add SelectAll button or template

The RadComboBox control could provide SelectAll button or template functionality. This way the user can choose whether to use the default button or create a custom select all behavior.