docs-desktop/dotnet-desktop-guide/samples/snippets/xaml/wpf/introduction-to-wpf/introduction-to-wpf_37.xaml

<Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    x:Class="SDKSample.UserControlWindow"
    xmlns:local="clr-namespace:SDKSample" 
    Title="User Control Window">

  <!-- Numeric Up/Down user control -->
  <local:NumericUpDown />

</Window>