WPF

static DependencyPropertyKey BehaviorsKey = DependencyProperty.RegisterAttachedReadOnly(
    "PrivateBehaviors", // REMARK: name is different from property to force GetBehaviors() call
    typeof(BehaviorList),
    typeof(Behavior),
    new UIPropertyMetadata(null));

public static DependencyProperty BehaviorsProperty = BehaviorsKey.DependencyProperty;

public static BehaviorList GetBehaviors(DependencyObject target)
{
    var result = (BehaviorList)target.GetValue(BehaviorsProperty);
    if (result == null)
    {
        result = new BehaviorList(target);
        target.SetValue(BehaviorsKey, result);
    }
    return result;
}

<Slider>
    <gx:Behavior.Behaviors>
        <local:DragLayerGroupBehavior/>
    </gx:Behavior.Behaviors>
</Slider>

public class BehaviorList : IList<Behavior>, System.Collections.IList
{
// .....
}

public class DragLayerGroupBehavior : Behavior
{
// .....
}

Error XLS0503,  A value of type 'DragLayerGroupBehavior' cannot be added to a collection or dictionary of type 'BehaviorList'.
Error XDG0048,  The specified value cannot be assigned to the collection. The following type was expected: "Behavior".

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApplication1"
        mc:Ignorable="d"
        Title="MainWindow" Height="300" Width="300">
    
    <UniformGrid Columns="4">
        <Rectangle Fill="Blue" />
        <Rectangle Fill="Lime" Width="20" />
        <Rectangle Fill="Red" />
        <Rectangle Fill="Yellow" Width="20" />
    </UniformGrid>
    
</Window>

<HierarchicalDataTemplate DataType="{x:Type entities:NavigationGroupEntity}"
                            ItemsSource="{Binding NavigationItems}">

<pre>
<Grid>

    <TextBlock>

        <Hyperlink Style="{StaticResource HyperlinkStyle}"
                    Command="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}, Path=DataContext.NavHeaderLinkClickedCommand}"
                    CommandParameter="{Binding}">>

            <TextBlock Text="{Binding Caption}"/>

            <Hyperlink.ContextMenu>

                <ContextMenu>

                    <MenuItem Header="Open" 
                                Command="{Binding OpenItemCommand}"
                                CommandParameter="{Binding}"/>

                </ContextMenu>

            </Hyperlink.ContextMenu>

        </Hyperlink>
    </TextBlock>

</Grid>

<HierarchicalDataTemplate DataType="{x:Type entities:NavigationGroupEntity}"
                            ItemsSource="{Binding NavigationItems}">

<pre>
<Grid>

    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="*"/>
        <ColumnDefinition Width="Auto"/>
    </Grid.ColumnDefinitions>

    <TextBlock Grid.Column="0"
                HorizontalAlignment="Stretch">
        <Hyperlink Style="{StaticResource HyperlinkStyle}"
                    Command="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}, Path=DataContext.NavHeaderLinkClickedCommand}"
                    CommandParameter="{Binding}">>
            <TextBlock Text="{Binding Caption}"/>
        </Hyperlink>
    </TextBlock>

    <Button Grid.Column="1"
            x:Name="button"
            Visibility="Hidden"
            VerticalAlignment="Center"
            Padding="0"
            Height="24"
            Width="24"
            Style="{StaticResource flatButtonStyle}">

        <TextBlock Text="+"
                    Foreground="SteelBlue"
                    VerticalAlignment="Top"
                    FontSize="18"/>

    </Button>

</Grid>

<HierarchicalDataTemplate.Triggers>
    <Trigger Property="IsMouseOver" Value="True">
        <Setter TargetName="button" Property="Visibility" Value="Visible"/>
    </Trigger>
</HierarchicalDataTemplate.Triggers>

<ScrollViewer VerticalScrollBarVisibility="Auto" Grid.Column="0" Grid.Row="3" Grid.ColumnSpan="2" ScrollChanged="ScrollViewer_ScrollChanged">
            <ItemsControl x:Name="DivisionItems" ItemsSource="{Binding oObsByDiv}">
                <ItemsControl.ItemTemplate>
                    <DataTemplate>
                        <uc:ucObservationsHeader/>
                    </DataTemplate>
                </ItemsControl.ItemTemplate>
            </ItemsControl>
        </ScrollViewer>

<UserControl x:Class="MOATools.Views.ucObservationsHeader"            
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"             
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"             
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:uc="clr-namespace:MOATools.Views"
    mc:Ignorable="d" d:DesignHeight="115" d:DesignWidth="400">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="100"/>
            <ColumnDefinition Width="300*"/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="20"/>
            <RowDefinition Height="100*"/>
        </Grid.RowDefinitions>
        <TextBox x:Name="divNum" Text="{Binding DivNum, Mode=OneWay}" Grid.Column="0" Grid.Row="0" Margin="0,3,0,0"/>
        <TextBox x:Name="divName" Text="{Binding DivName, Mode=OneWay}" Grid.Column="1" Grid.Row="0" Margin="0,3,0,0"/>
            <ItemsControl ItemsSource="{Binding lObs}" Grid.Row="1" Grid.Column="1">
                <ItemsControl.ItemTemplate>
                    <DataTemplate>
                        <uc:ucObservationsView/>
                    </DataTemplate>
                </ItemsControl.ItemTemplate>
            </ItemsControl>
    </Grid>
</UserControl>

<UserControl x:Class="MOATools.Views.ucObservationsView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
             mc:Ignorable="d" d:DesignHeight="75" d:DesignWidth="400">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="25"/>
            <RowDefinition Height="25*"/>
            <RowDefinition Height="25*"/>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width=".2*"/>
            <ColumnDefinition Width=".4*"/>
            <ColumnDefinition Width=".4*"/>
        </Grid.ColumnDefinitions>
        <TextBox x:Name="txtDivObs" HorizontalAlignment="Stretch" TextWrapping="Wrap" Text="{Binding obsFullNum, Mode=OneWay}" Grid.Column="0" Grid.Row="0"/>
        <Label x:Name="lblOpenDate" HorizontalAlignment="Left" Content="Open " Grid.Column="1" Grid.Row="0"/>
        <DatePicker x:Name="dtOpen" HorizontalAlignment="Right" SelectedDate="{Binding Opendate, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Grid.Column="1" Grid.Row="0"/>
        <Label x:Name="lblCLoseDate" HorizontalAlignment="Left" Content="Close " Grid.Column="2" Grid.Row="0"/>
        <DatePicker x:Name="txtCloseDate" HorizontalAlignment="Right" SelectedDate="{Binding Closedate}" Grid.Column="2" Grid.Row="0"/>
        <TextBox x:Name="txtDescription" HorizontalAlignment="Stretch" TextWrapping="Wrap" Text="{Binding Description}" VerticalAlignment="Top" Grid.ColumnSpan="3" Grid.Row="1"/>
        <TextBox x:Name="txtImagePath" HorizontalAlignment="Stretch" TextWrapping="Wrap" Text="{Binding ImagePath}" Grid.Row="2" Grid.ColumnSpan="3">
            <i:Interaction.Triggers>
                <i:EventTrigger EventName="PreviewMouseLeftButtonUp">
                    <i:InvokeCommandAction Command="{Binding DataContext.PreviewImage, RelativeSource={RelativeSource AncestorType={x:Type Window}}}"
                                           CommandParameter="{Binding}"/>
                </i:EventTrigger>
            </i:Interaction.Triggers>
        </TextBox>
    </Grid>
</UserControl>