docs-desktop/dotnet-desktop-guide/net/wpf/properties/snippets/how-to-register-an-attached-property/csharp/MainWindow.xaml

<Window x:Class="CodeSampleCsharp.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"/>