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XAML Namespaces and Namespace Mapping

This topic further explains the presence and purpose of the two XML namespace mappings as found in the root tag of each Extensible Application Markup Language (XAML) file. It also describes how to produce similar mappings for using elements that are defined in your own code, and/or within separate assemblies.

This topic contains the following sections.

Within the namespace declarations in the root tag of many Extensible Application Markup Language (XAML) files, you will see that there are two XML namespace declarations. The first declaration maps the overall Windows Presentation Foundation (WPF) namespace as the default:

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

The second declaration maps a separate Extensible Application Markup Language (XAML) namespace, mapping it (typically) to the x: prefix.

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

The relationship between these declarations is that the x: prefix mapping supports the intrinsics that are part of the XAML language definition, and WPF is one implementation that uses XAML as a language and defines a vocabulary of its objects for XAML. Because the WPF vocabulary's usages will be far more common than the XAML intrinsics usages, the WPF vocabulary is mapped as the default.

The x: prefix convention for mapping the XAML language intrinsics support is followed by project templates, sample code, and the documentation of language features within this SDK. The XAML namespace defines many commonly-used features that are necessary even for basic WPF applications. For instance, in order to join any code-behind to a XAML file through a partial class, you must name that class as the x:Class attribute in the root element of the relevant XAML file. Or, any element as defined in a XAML page that you wish to access as a keyed resource should have the x:Key attribute set on the element in question. For more information on these and other aspects of XAML see XAML Overview or XAML Syntax Terminology.

You can map XML namespaces to assemblies using a series of tokens within an xmlns prefix declaration, similar to how the standard WPF and XAML namespaces are mapped to prefixes.

The syntax takes the following possible named tokens and following values:

clr-namespace: The common language runtime (CLR) namespace declared within the assembly that contains the public types to expose as elements.

assembly= The assembly that contains some or all of the referenced CLR namespace. This value is typically just the name of the assembly, not the path, and does not include the extension (such as .dll or .exe). The path to that assembly must be established as a project reference in the project file that contains the XAML you are trying to map. In order to incorporate versioning and strong-name signing, the assembly value can be a string as defined by AssemblyName, rather than the simple string name.

Note that the character separating the clr-namespace token from its value is a colon (:) whereas the character separating the assembly token from its value is an equals sign (=). The character to use between these two tokens is a semicolon. Also, do not include any whitespace anywhere in the declaration.

A Basic Custom Mapping Example

The following code defines an example custom class:

namespace SDKSample {
    public class ExampleClass : ContentControl {
        public ExampleClass() {
        ...
        }
    }
}

This custom class is then compiled into a library, which per the project settings (not shown) is named SDKSampleLibrary.

In order to reference this custom class, you also need to include it as a reference for your current project, which you would typically do using the Solution Explorer UI in Visual Studio.

Now that you have a library containing a class, and a reference to it in project settings, you can add the following prefix mapping as part of your root element in XAML:

xmlns:custom="clr-namespace:SDKSample;assembly=SDKSampleLibrary"

To put it all together, the following is XAML that includes the custom mapping along with the typical default and x: mappings in the root tag, then uses a prefixed reference to instantiate ExampleClass in that UI:

<Page x:Class="WPFApplication1.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:custom="clr-namespace:SDKSample;assembly=SDKSampleLibrary">
  ...
  <custom:ExampleControl />
...
</Page>

Mapping to Current Assemblies

assembly can be omitted if the clr-namespace referenced is being defined within the same assembly as the application code that is referencing the custom classes. Or, an equivalent syntax for this case is to specify assembly=, with no string token following the equals sign.

Custom classes cannot be used as the root element of a page if defined in the same assembly. Partial classes do not need to be mapped; only classes that are not the partial class of a page in your application need to be mapped if you intend to reference them as elements in XAML.

WPF defines a CLR attribute that is consumed by XAML processors in order to map multiple CLR namespaces to a single XML namespace. This attribute, XmlnsDefinitionAttribute, is placed at the assembly level in the source code that produces the assembly. The WPF assembly source code uses this attribute to map the various common namespaces, such as System.Windows and System.Windows.Controls, to the http://schemas.microsoft.com/winfx/2006/xaml/presentation namespace.

The XmlnsDefinitionAttribute takes two parameters: the XML namespace name, and the CLR namespace name. More than one XmlnsDefinitionAttribute can exist to map multiple CLR namespaces to the same XML namespace. Once mapped, members of those namespaces can also be referenced without full qualification if desired by providing the appropriate using statement in the partial-class code-behind page. For more details, see XmlnsDefinitionAttribute.

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