Binding WPF command using WPFToolkit.Extended

Binding WPF command

//xmlns:interactivity="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
//xmlns:toolkit="clr-namespace:Xceed.Wpf.Toolkit;assembly=WPFToolkit.Extended"

<toolkit:WatermarkTextBox Padding="6,4"
TextOptions.TextRenderingMode="Aliased" Watermark="Введите что нибудь для поиска" >
                        <toolkit:WatermarkTextBox.BorderBrush>
                            <SolidColorBrush Color="#DDD"/>
                        </toolkit:WatermarkTextBox.BorderBrush>
                        <interactivity:Interaction.Triggers>
                            <interactivity:EventTrigger EventName="TextChanged">
                                <wpfTools:EventToCommand PassEventArgsToCommand="True" Command="{Binding AllSearch}"></wpfTools:EventToCommand>
                            </interactivity:EventTrigger>
                        </interactivity:Interaction.Triggers>
                    </toolkit:WatermarkTextBox>

//in code

        private RelayCommand<EventArgs> _allSearchCommand;
        public ICommand AllSearch
        {
            get
            {
                return _allSearchCommand ?? (_allSearchCommand = new RelayCommand<EventArgs>(
                    e =>
                    {
                        var search = ((WatermarkTextBox)((TextChangedEventArgs)e).Source).Text;
                    }));
            }
        }