Implicit XAML Namespaces

Uno Platform supports implicit XAML namespaces, allowing you to write XAML files without repetitive xmlns boilerplate declarations. This feature is inspired by .NET MAUI's global usings for XAML and is enabled by default in Uno.Sdk projects.

Overview

Traditionally, every XAML file requires at minimum:

<Page
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    x:Class="MyApp.MainPage">
    <StackPanel>
        <Button Content="Hello" />
        <TextBlock Text="World" />
    </StackPanel>
</Page>

With implicit XAML namespaces enabled, the same page becomes:

<Page x:Class="MyApp.MainPage">
    <StackPanel>
        <Button Content="Hello" />
        <TextBlock Text="World" />
    </StackPanel>
</Page>

The default WinUI presentation namespace (xmlns) and the XAML namespace (xmlns:x) are implicitly available without explicit declaration.

How It Works

When UnoEnableImplicitXamlNamespaces is set to true (the default), the Uno Platform source generator automatically injects the missing xmlns declarations into the root element of each XAML file before parsing. This happens transparently at compile time and does not modify your source files.

The following namespaces are implicitly available:

Registering Custom Namespaces

You can register your own CLR namespaces to be available globally (unprefixed) in XAML by using the XmlnsDefinition attribute targeting the global namespace URI:

[assembly: System.Windows.Markup.XmlnsDefinition(
    "http://schemas.microsoft.com/winfx/2006/xaml/presentation/global",
    "MyApp.Controls")]

Types from registered namespaces resolve unprefixed in XAML:

<Page x:Class="MyApp.MainPage">
    <StackPanel>
        <!-- MyCustomControl is from MyApp.Controls, resolved via global namespace -->
        <MyCustomControl Label="Hello" />
        <Button Content="Standard WinUI Button" />
    </StackPanel>
</Page>

[!NOTE] Standard WinUI types always take precedence over custom types with the same name. If your global namespace contains a type named Button, the WinUI Button will be resolved instead.

Registering Prefixed Namespaces

You can make a CLR namespace implicitly available with a specific prefix using XmlnsPrefix:

using System.Windows.Markup;

[assembly: XmlnsPrefix("MyApp.Controls.Advanced", "adv")]

The adv: prefix is then available in XAML without declaring xmlns:adv:

<Page x:Class="MyApp.MainPage">
    <StackPanel>
        <adv:AdvancedControl Value="42" />
    </StackPanel>
</Page>

Alternatively, if you already have a custom XML namespace URI defined via XmlnsDefinition, you can associate a prefix with that URI:

using System.Windows.Markup;

[assembly: XmlnsDefinition("http://myapp.com/controls", "MyApp.Controls.Advanced")]
[assembly: XmlnsPrefix("http://myapp.com/controls", "adv")]

Cross-Assembly Support

The XmlnsDefinition attribute works across assembly boundaries. If a referenced library declares:

[assembly: XmlnsDefinition(
    "http://schemas.microsoft.com/winfx/2006/xaml/presentation/global",
    "MyLibrary.Controls")]

Types from MyLibrary.Controls will automatically be available unprefixed in any consuming project that has implicit XAML namespaces enabled.

Backward Compatibility

Implicit XAML namespaces are fully backward compatible:

• XAML files with explicit xmlns declarations continue to work identically
• Explicit per-file xmlns declarations take precedence over implicit ones
• The mc:Ignorable, d:DesignHeight, and other design-time attributes work alongside implicit namespaces (just declare xmlns:mc and xmlns:d as usual)

Disambiguation

If two globally registered namespaces contain types with the same name, use an explicit xmlns prefix to resolve the ambiguity:

<Page x:Class="MyApp.MainPage"
      xmlns:libA="using:LibraryA.Controls"
      xmlns:libB="using:LibraryB.Controls">
    <StackPanel>
        <libA:SharedTypeName />
        <libB:SharedTypeName />
    </StackPanel>
</Page>

Configuration

Disabling Implicit Namespaces

To opt out of implicit XAML namespaces, set the property in your project file or Directory.Build.props:

<PropertyGroup>
    <UnoEnableImplicitXamlNamespaces>false</UnoEnableImplicitXamlNamespaces>
</PropertyGroup>

When disabled, all XAML files must include explicit xmlns declarations as before.

Custom Global Namespace URI

The default global namespace URI is http://schemas.microsoft.com/winfx/2006/xaml/presentation/global. You can customize this if needed:

<PropertyGroup>
    <UnoGlobalXamlNamespaceUri>http://myapp.com/global</UnoGlobalXamlNamespaceUri>
</PropertyGroup>

MSBuild Properties

Limitations

• Implicit namespaces are a compile-time feature. IDE XAML designers may not fully support the reduced syntax until they are updated.
• Hot Reload correctly handles XAML without explicit xmlns declarations.
• The feature requires Uno.Sdk. Projects not using Uno.Sdk need to manually set UnoEnableImplicitXamlNamespaces to true in their project file.