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MyApp.csproj

<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">

  <!--
    Insert the following code into your project file:
    
    <Import Project="PreprocessXaml.targets" />
  -->

  <PropertyGroup>
    <MarkupCompilePass1DependsOn>
      $(MarkupCompilePass1DependsOn)
      PreprocessXaml;
    </MarkupCompilePass1DependsOn>
  </PropertyGroup>

  <Target Name="PreprocessXaml">

    <!-- Run the preprocessor on every page that is added to the project as link. The processed file will be saved at the link position. -->
    <Exec Condition="('%(Page.Link)' != '')" Command="XamlPreprocessor.exe &quot;%(Page.FullPath)&quot; &quot;%(Page.Link)&quot;"/>

    <ItemGroup>

      <ProcessedPages Include="@(Page->'%(Link)')" />

      <!-- Remove the original (linked) pages so that they are not compiled. -->
      <Page Remove="@(Page)" Condition="('%(Page.Link)' != '')" />

      <!-- Include the processed pages instead. -->
      <Page Include="@(ProcessedPages)" />

    </ItemGroup>
  </Target>

</Project>

MyTemplatedControl.cs

#if WPF
using System.Windows;
using System.Windows.Controls;
#endif

#if NETFX_CORE
using Windows.UI.Xaml.Controls;
#endif

namespace MyApp.Controls
{
    public sealed class MyTemplatedControl : Control
    {
        public MyTemplatedControl()
        {
#if NETFX_CORE
            this.DefaultStyleKey = typeof(MyTemplatedControl);
#endif
        }

#if WPF
        static MyTemplatedControl()
        {
            DefaultStyleKeyProperty.OverrideMetadata(typeof(MyTemplatedControl), new FrameworkPropertyMetadata(typeof(MyTemplatedControl)));
        }
#endif
    }
}

Resources.xaml

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:MyApp.Controls">

    <Style TargetType="local:MyTemplatedControl">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="local:MyTemplatedControl">
                    <Border
                        Background="{TemplateBinding Background}"
                        BorderBrush="{TemplateBinding BorderBrush}"
                        BorderThickness="{TemplateBinding BorderThickness}">
                    </Border>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>

XamlPreprocessor.cs

namespace XamlPreprocessor
{
    public class XamlPreprocessor
    {
        const string regex = @"""using:(?<namespace>[\w\.\-_]*)""";

        public static void Main(string[] args)
        {
            string unprocessedXaml = File.ReadAllText(args[0]);

            //Create destination directory if it doesn't exist
            (new FileInfo(args[1])).Directory.Create();
            
            var preprocessor = new XamlPreprocessor();
            var processedXaml = preprocessor.Process(unprocessedXaml);

            File.WriteAllText(args[1], processedXaml);
        }

        public string Process(string text)
        {
            return Regex.Replace(text, regex, RegExMatchEvaluator);
        }

        private string RegExMatchEvaluator(Match match)
        {
            return match.Result("\"clr-namespace:" + match.Groups["namespace"] + "\"");
        }
    }
}