WPF AutoComplete Folder TextBox

<Window x:Class="AutoCompleteTextBox.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:t="clr-namespace:QuickZip.Tools"
    xmlns:local="clr-namespace:QuickZip.Controls" x:Name="root"
    Title="Window1" Height="300" Width="300">
    <DockPanel>
        <Border BorderBrush="Silver" BorderThickness="1" Padding="2" DockPanel.Dock="Bottom">
            <DockPanel >
                <local:SelectFolderTextBox  DockPanel.Dock="Bottom" Margin="4,0,0,0" />
                <local:SelectFolderTextBox  x:Name="stb" DockPanel.Dock="Bottom" Margin="4,0,0,0">
                    <local:SelectFolderTextBox.Text>
                        <Binding Path="Text" UpdateSourceTrigger="Explicit" >
                            <Binding.ValidationRules>
                                <t:DirectoryExistsRule />
                            </Binding.ValidationRules>
                        </Binding>
                    </local:SelectFolderTextBox.Text>
                </local:SelectFolderTextBox>
                <Label Content="_Enter Path" Target="{Binding ElementName=stb}"  />
            </DockPanel>
        </Border>
        <TextBlock Text="{Binding Text}" />
    </DockPanel>
</Window>