MainWindow.xaml

<Window x:Class="FrontEndTask.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:telerik="clr-namespace:Telerik.Windows.Controls;assembly=Telerik.Windows.Controls"
        Title="Front-end task by Ivan Yankov Ivanov"
        MinWidth="200"
        MinHeight="220"
        Width="525"
        Height="400"
        DataContext="{Binding RelativeSource={RelativeSource Self}}">
    <Grid x:Name="LayoutRoot">
        <Grid.RowDefinitions>
            <RowDefinition Height="50" />
            <RowDefinition />
        </Grid.RowDefinitions>
        <CheckBox x:Name="testCheckBox" IsThreeState="True">Test checkbox</CheckBox>
        <telerik:RadListBox Grid.Row="1"
                            Style="{StaticResource CheckedRadListBoxStyle}"
                            ItemsSource="{Binding CheckedRadListBoxItemsSource}"
                            SelectionMode="Multiple" />
    </Grid>
</Window>