<myctrls:CustomExpander ExpandDirection="Down" Header="Фильтры">
                    <myctrls:CustomExpander.ControlContent>
                        <StackPanel Margin="2" VerticalAlignment="Center" Orientation="Vertical">
                                
 				...

                                <toolkit:Separator></toolkit:Separator>

                                <myctrls:OsMenuItem Margin="0,2,0,0" Header="Test">
                                    <toolkit:MenuItem.Icon>
                                        <CheckBox IsChecked="{Binding Path=FilterData.IsBuy, ElementName=_CmpView, Mode=TwoWay}"></CheckBox>
                                    </toolkit:MenuItem.Icon>
                                </myctrls:OsMenuItem>

                            </toolkit:ContextMenu>
                        </StackPanel>
                    </myctrls:CustomExpander.ControlContent>
                </myctrls:CustomExpander>

<Grid x:Name="LayoutRoot" Background="White">
        <toolkit:Expander x:Name="expander" Style="{StaticResource ExpanderScaleStyle}" ExpandDirection="{Binding ExpandDirection}"
                          Header="{Binding Header}">
            <Popup x:Name="popup">
                <ContentPresenter Content="{Binding ControlContent}">

                </ContentPresenter>
            </Popup>
        </toolkit:Expander>
    </Grid>

public partial class CustomExpander : UserControl
    {
        public object ExpandDirection
        {
            get { return (object)GetValue(ExpandDirectionProperty); }
            set { SetValue(ExpandDirectionProperty, value); }
        }

        // Using a DependencyProperty as the backing store for ExpandDirection.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty ExpandDirectionProperty =
            DependencyProperty.Register("ExpandDirection", typeof(object), typeof(CustomExpander), new PropertyMetadata(null));


        public object Header
        {
            get { return (object)GetValue(HeaderProperty); }
            set { SetValue(HeaderProperty, value); }
        }

        // Using a DependencyProperty as the backing store for Header.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty HeaderProperty =
            DependencyProperty.Register("Header", typeof(object), typeof(CustomExpander), new PropertyMetadata(null));

        public object ControlContent
        {
            get { return (object)GetValue(ControlContentProperty); }
            set { SetValue(ControlContentProperty, value); }
        }

        // Using a DependencyProperty as the backing store for ControlContent.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty ControlContentProperty =
            DependencyProperty.Register("ControlContent", typeof(object), typeof(CustomExpander), new PropertyMetadata(null));



        public CustomExpander()
        {
            InitializeComponent();
            this.Loaded += new RoutedEventHandler(CustomExpander_Loaded);
        }


        void CustomExpander_Loaded(object sender, RoutedEventArgs e)
        {
           this.DataContext = this;
        }

    }

public class Filter : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        private bool _isBuy;
        public bool IsBuy
        {
            get { return _isBuy; }
            set
            {
                if (value != this._isBuy)
                {
                    this._isBuy = value;
                    this.NotifyPropertyChanged("IsBuy");
                }
            }
        }
        private void NotifyPropertyChanged(String info)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(info));
            }
        }

       
    }