If you select an item in RadMultiColumnComboBox and then replace the ItemsSource of the GridViewItemsSourceProvider, the selection in the autocompletebox part of the the control is not cleared. There are items that are still selected and are not presented in the ItemsSource.

To work this around, you can create a custom GridViewItemsSourceProvider and override its OnPropertyChanged method. When the ItemsSource is changed, you can reset the selection.

public class CustomGridViewItemsSourceProvider : GridViewItemsSourceProvider
{
	public RadMultiColumnComboBox Owner
	{
		get { return (RadMultiColumnComboBox)GetValue(OwnerProperty); }
		set { SetValue(OwnerProperty, value); }
	}
	
	public static readonly DependencyProperty OwnerProperty =
		DependencyProperty.Register(
			"Owner", 
			typeof(RadMultiColumnComboBox), 
			typeof(CustomGridViewItemsSourceProvider), 
			new PropertyMetadata(null));

	protected override void OnPropertyChanged(DependencyPropertyChangedEventArgs e)
	{
		base.OnPropertyChanged(e);
		if (e.Property.Name == "ItemsSource" && this.Owner != null)
		{
			this.Owner.SelectionBridge.ClearOwnerSelection();
		}
	}
}

 <telerik:RadMultiColumnComboBox.ItemsSourceProvider>
                <local:CustomGridViewItemsSourceProvider Owner="{Binding ElementName=mccb}" />
            </telerik:RadMultiColumnComboBox.ItemsSourceProvider>

The Validation.ErrorTemplate attached property is not set. To workaround this, you can use the following resource.

<ControlTemplate x:Key="ValidationTooltipTemplate">
    <Grid SnapsToDevicePixels="True" VerticalAlignment="Top">
        <Border Background="Transparent" HorizontalAlignment="Right" VerticalAlignment="Top" Width="3" Height="3"/>
        <AdornedElementPlaceholder x:Name="Holder"/>
        <Border BorderBrush="{StaticResource ValidationTooltipOuterBorder}" BorderThickness="1" CornerRadius="{StaticResource ValidationTooltip_CornerRadius}"/>
        <Path Data="M2,1 L6,1 6,5 Z" Fill="{StaticResource ValidationInnerTick}" Width="7" Height="7" HorizontalAlignment="Right" VerticalAlignment="Top"/>
        <Path Data="M0,0 L2,0 7,5 7,7 Z" Fill="{StaticResource ValidationOuterTick}" Width="7" Height="7" HorizontalAlignment="Right" VerticalAlignment="Top"/>
        <ToolTipService.ToolTip>
            <ToolTip x:Name="PART_ToolTip"
                DataContext="{Binding RelativeSource={RelativeSource Mode=Self}, Path=PlacementTarget.DataContext}"
                Template="{StaticResource ErrorTooltipTemplate}"
                Placement="Right"/>
        </ToolTipService.ToolTip>
    </Grid>
</ControlTemplate>

<Style TargetType="telerik:RadMultiColumnComboBox" BasedOn="{StaticResource RadMultiColumnComboBoxStyle}">
    <Setter Property="Validation.ErrorTemplate" Value="{StaticResource ValidationTooltipTemplate}" />
</Style>

The following error is thrown when the dropdown of the control is opened: Cannot find governing FrameworkElement or FrameworkContentElement for target element. ... ; target element is 'GridViewItemsSourceProvider; target property is 'ItemsSource' (type 'Object')

If the combobox is placed inside a RadGridView, the error is thrown each time the dropdown is opened (after a commit action).

If set the AutoCompleteMode property in a Style Setter, and exception is thrown.

To work this around set the property directly on the control.
<telerik:RadMultiColumnComboBox AutoCompleteMode="Search" />

Hello Telerik,

Exception is thrown after typing in the autocompletebox and selecting an item right after that. 

Thanks in advance.

The event should fire when the DropDownContentManager gets initialized.

To work this around use the InitializeSelectionBridge event. The DropDownContentManager is initialized when this event fires.

Hello,

when selecting more items and dropdown positioning is down and input filed has fixed width, the dropdown covers the input field.

It should move down to the bottom edge of input field.