MainWindow.xaml

<Window x:Class="BindingConverter.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="600" Width="800" 
        xmlns:dxb="http://schemas.devexpress.com/winfx/2008/xaml/bars"
        xmlns:dxre="http://schemas.devexpress.com/winfx/2008/xaml/richedit">
    <!--region #1-->
    <Window.Resources>
        <dxre:ContentToPlainTextConverter x:Key="ContentPlainConverter" />
        <dxre:RichEditUICommand x:Key="commands" />
    </Window.Resources>
    
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition />
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <dxb:BarManager Name="barManager1" Grid.Column="0"> 
                        <dxre:RichEditControl x:Name="richEditControl1"  Height="Auto" Width="Auto" 
                           BarManager="{Binding ElementName=barManager1}" ActiveViewType="Simple" />
        </dxb:BarManager>
        <TextBox Text="{Binding ElementName=richEditControl1, Path=Content, Converter={StaticResource ContentPlainConverter}, Mode=OneWay}" 
            x:Name="textBox1" Grid.Column="1" Height="Auto" Width="Auto"/>
    </Grid>
    <!--endregion #1-->
</Window>