XAML Namespaces

Every XAML file begins by declaring one or more XML namespaces on its root element using xmlns attributes, which tell the parser which CLR namespaces and assemblies to search when resolving an unprefixed or prefixed tag name to an actual .NET type. Without these declarations the parser would have no way to know that a tag named Button should resolve to System.Windows.Controls.Button rather than some other class with the same name, since XAML files never hardcode a fully-qualified type name directly in an element tag.

Two XAML namespaces appear at the top of nearly every file: the default namespace, http://schemas.microsoft.com/winfx/2006/xaml/presentation, usually declared without a prefix so it covers ordinary WPF controls like Button and Grid without needing a prefix on every tag; and the XAML language namespace, http://schemas.microsoft.com/winfx/2006/xaml, conventionally mapped to the x: prefix, which supplies core XAML language features such as x:Class to link a file to its code-behind partial class, x:Key for resource dictionary entries, x:Name to give an element a field-accessible name, and x:Type for type references.

To use your own classes in XAML — a custom UserControl, a converter, or a domain model — you declare an additional xmlns mapped with the clr-namespace and assembly keywords, such as xmlns:local="clr-namespace:MyApp.Controls;assembly=MyApp", after which any public type in MyApp.Controls can be used as <local:MyCustomButton .../>. The conventional prefix local is typically used for types defined in the same assembly as the XAML file, while a distinct, descriptive prefix is chosen for types imported from other assemblies or third-party libraries to keep their origin clear at a glance.

Namespace declarations are what let a single XAML file safely mix framework controls, first-party custom controls, and third-party control libraries like MahApps.Metro or Telerik's UI suite without any class-name collisions, because each prefix scopes its tags to an unambiguous CLR namespace and assembly. This plays the same structural role that using directives play in C# — resolving short names to fully-qualified types — except namespaces in XAML are declared per-file (or inherited from a parent element) rather than once per project, and they're expressed as URIs or clr-namespace strings rather than simple dotted identifiers.