如何将MainWindow上的自定义属性获取到UserControl中(从UserControl库/DLL)？

public static readonly DependencyProperty MyCustomProperty = DependencyProperty.Register(
    "MyCustom", typeof(string), typeof(MainWindow));

public string MyCustom
{
    get { return (string)GetValue(MyCustomProperty); }
    set { SetValue(MyCustomProperty, value); }
}

<Window x:Class="MyApp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:MyApp"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <local:MyUserControl MyCustom="{Binding MyCustom, RelativeSource={RelativeSource AncestorType=Window}}"/>
    </Grid>
</Window>

public static readonly DependencyProperty MyCustomProperty = DependencyProperty.Register(
    "MyCustom", typeof(string), typeof(MyUserControl));

public string MyCustom
{
    get { return (string)GetValue(MyCustomProperty); }
    set { SetValue(MyCustomProperty, value); }
}

<UserControl x:Class="MyApp.MyUserControl"
             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:local="clr-namespace:MyApp"
             mc:Ignorable="d" 
             d:DesignHeight="450" d:DesignWidth="800">
    <Grid>
        <TextBlock Text="{Binding MyCustom, RelativeSource={RelativeSource AncestorType=UserControl}}"/>
    </Grid>
</UserControl>