<telerik:RadComboBox Margin="5"
                                 ItemsSource="{Binding Path=AvailableServices}"
                                 IsEditable="True" 
                                 Text="{Binding Path=Service, Mode=TwoWay}"/>

            <ComboBox Margin="5"
                                 ItemsSource="{Binding Path=AvailableServices}"
                                 IsEditable="True" 
                                 Text="{Binding Path=Service, Mode=TwoWay}"/>

public class ViewModel
    {
        public ObservableCollection<string> AvailableServices { get; set; } = new ObservableCollection<string> { "UPS Standard", "UPS Express Saver", "UPS Express 12:00", "UPS Express" };

        public string Service { get; set; } = "UPS Express";
    }

var template = this.comboBox.Template;
this.comboBox.Template = null;
this.comboBox.Template = template;

public class CustomRadComboBox : RadComboBox
    {
        protected override bool HandleKeyDown(Key systemKey, int platformKeyCode)
        {
            if(systemKey == Key.Oem5)
            {
                return false;
            }
            return base.HandleKeyDown(systemKey, platformKeyCode);
        }
    }

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;
		}                
	}	
}

<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>

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.

AutomationManager.AutomationMode = AutomationMode.Disabled;

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

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#