Extensible Application Markup Language

Extensible Application Markup Language (XAML /ˈzæməl/ ^ⓘ) is a declarative XML-based language developed by Microsoft for initializing structured values and objects. It is available under Microsoft's Open Specification Promise.

XAML is used extensively in Windows Presentation Foundation (WPF), Silverlight, Workflow Foundation (WF), Windows UI Library (WinUI), Universal Windows Platform (UWP), and .NET Multi-platform App UI (.NET MAUI). In WPF and UWP, XAML is a user interface markup language to define UI elements, data binding, and events. In WF, however, XAML defines workflows.

XAML elements map directly to Common Language Runtime (CLR) object instances, while XAML attributes map to CLR properties and events on those objects.

Anything that is created or implemented in XAML can be expressed using a more traditional .NET language, such as C# or Visual Basic .NET. However, a key aspect of the technology is the reduced complexity needed for tools to process XAML, because it is based on XML.

XAML originally stood for Extensible Avalon Markup Language, Avalon being the code-name for Windows Presentation Foundation (WPF). Before the end of .NET Framework 3.0 development, however, Microsoft adopted XAML for Workflow Foundation (WF).

In WPF, XAML describes visual user interfaces. WPF allows for the definition of both 2D and 3D objects, rotations, animations, and a variety of other effects and features. A XAML file can be compiled into a Binary Application Markup Language (BAML) file, which may be inserted as a resource into a .NET Framework assembly. At run-time, the framework engine extracts the BAML file from assembly resources, parses it, and creates a corresponding WPF visual tree or workflow.

In WF contexts, XAML describes potentially long-running declarative logic, such as those created by process modeling tools and rules systems. The serialization format for workflows was previously called XOML, to differentiate it from UI markup use of XAML, but now they are no longer distinguished. However, the file extension for files containing the workflow markup is still ".xoml".

XAML uses a specific way to define look and feel called Templates; differing from Cascading Style Sheet syntax, it is closer to XBL.