docs-desktop/dotnet-desktop-guide/net/wpf/properties/snippets/dependency-property-callbacks-and-validation/csharp/app.xaml

<Application x:Class="CodeSampleCsharp.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:CodeSampleCsharp"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
         
    </Application.Resources>
</Application>