Developer · XAML
What Is XAML? From Avalon to WinUI 3 and .NET MAUI
XAML is an XML-based markup language for describing a user interface declaratively, instead of building it in code. It began life inside Microsoft’s "Avalon" project — the codename for what shipped as Windows Presentation Foundation — and the name originally stood for eXtensible Avalon Markup Language before becoming eXtensible Application Markup Language. In 2026 you will meet it in WPF, WinUI 3, .NET MAUI and the non-Microsoft Avalonia.
If you have written a Windows desktop app in the last twenty years, you have probably written XAML. If you are coming back to Windows development after a long gap, the landscape has changed more than the language has.
What XAML is
XAML is an XML-based markup language for declaring a user interface: what the controls are, how they nest, how they are laid out and how they bind to data. The alternative is constructing the same tree in code. Declarative markup is easier to read, easier to tool, and separates the shape of the interface from the logic behind it.
Where the name came from
This is the part with a genuinely interesting paper trail. XAML was created for the project codenamed Avalon, which shipped as Windows Presentation Foundation. The acronym originally stood for eXtensible Avalon Markup Language.
Rob Relyea, who worked on the team, explained the change in a post dated 31 January 2004:
XAML originally meant "eXtensible Avalon Markup Language". Before one of our first design previews we didn't think the "Avalon" code name would be public at all, so it morphed into "eXtensible Application Markup Language".
He added that the team later learned the technology "wouldn't only work with Avalon, but would be more generally useful as well, confirming that name change was a good one" — which turned out to be an understatement, given where XAML ended up.
You will sometimes read that the rename happened because Microsoft adopted XAML for Workflow Foundation. Relyea’s own account, written at the time and quoted above, gives a different reason: the Avalon codename was not expected to be public. We have gone with the first-hand account, and flag the discrepancy rather than quietly picking a side.
Where you meet XAML in 2026
The dialects differ in detail, but the concept transfers:
- WPF — the original. Windows-only, mature, still supported and still widely used.
- WinUI 3, shipped in the Windows App SDK — the current Windows desktop direction.
- .NET MAUI — one codebase across Windows, Android, iOS and macOS.
- Avalonia — independent and open source. Version 12, released in April 2026, targets Windows, macOS, Linux, iOS, Android and WebAssembly from a single codebase, and is the widest reach of any of them.
On the .NET side, the runtime underneath is on a predictable footing: .NET 10 is the current LTS release, supported until November 2028. That is the stable part of the picture. The UI frameworks on top of it are where the churn is.
On support timelines, the thing worth internalising is that .NET MAUI has its own support policy that does not follow the .NET release it ships with. A major MAUI version is supported for a minimum of six months after its successor ships, which is a much shorter window than the underlying .NET LTS release. If you are planning a maintenance budget, check the live policy page rather than assuming the .NET dates apply.
Choosing one
The honest answer is that this is the least stable part of the picture, and anyone who gives you a confident permanent recommendation is overreaching. Microsoft’s guidance has shifted more than once, and the Windows App SDK version numbering is genuinely confusing.
What can be said usefully:
- Windows-only, and the team already knows WPF? Staying on WPF is defensible. It is supported and enormously well documented.
- New Windows desktop app, want the current platform direction? WinUI 3 with the Windows App SDK. Expect to hand-write markup — see the designer question below.
- Need Windows and mobile from one codebase? .NET MAUI, with the support-window caveat above.
- Need Linux or the browser too? Avalonia is the only one of these that covers that ground.
Whichever you choose, check Microsoft’s current Windows developer documentation before you commit. The XAML you write will transfer between them far more readily than the surrounding platform decisions will.
If you are returning from the Avalon era
The markup will look familiar — elements, attributes, attached properties, data binding, styles and templates all survive. What has changed is everything around it: the packaging, the deployment story, the tooling, and the fact that a XAML dialect no longer implies a Microsoft framework at all.
And the naming question that started this page has a pleasing answer. The "A" that used to stand for a codename now stands for Application — which, given XAML has ended up describing interfaces on Linux and in web browsers, was the right call.
Frequently asked
What does XAML stand for?
XAML stands for Extensible Application Markup Language. It originally stood for eXtensible Avalon Markup Language, after the Avalon codename for what became Windows Presentation Foundation. Rob Relyea, who worked on the team, wrote in January 2004 that the name changed because Microsoft did not expect the Avalon codename to be public, and that the technology later turning out to be more broadly useful confirmed the change was a good one.
Which XAML framework should I use for a new Windows app?
It depends on where the app needs to run, and Microsoft's guidance shifts, so check the current Windows developer documentation before committing. Broadly: WinUI 3 with the Windows App SDK is the current Windows desktop direction, WPF remains widely used and supported for Windows-only desktop apps, .NET MAUI targets Windows alongside mobile, and Avalonia is a non-Microsoft option that also covers Linux and WebAssembly.
Is WPF still supported?
Yes. WPF is still a supported Windows desktop framework on modern .NET, and it remains in wide use. What has changed is that it is no longer the only XAML option for Windows desktop work, and new platform investment has been directed at WinUI 3 and the Windows App SDK.
Is XAML only a Microsoft thing?
No. Avalonia is an independent open-source XAML UI framework for .NET, developed outside Microsoft, and it targets Windows, macOS, Linux, iOS, Android and WebAssembly from one codebase. So XAML as a way of describing interfaces has outlived the specific Microsoft frameworks it was created for.
Does WinUI 3 have a visual designer?
Microsoft's own documentation is inconsistent on this point. The WinUI quickstart page mentions visual designer support in Visual Studio, while the Windows developer FAQ and the WPF-to-WinUI migration guidance both state there is no XAML design surface for WinUI 3. Treat the FAQ and migration pages as authoritative and expect to write markup by hand.
The naming history above is not folklore. Rob Relyea, who worked on the team, wrote it down in a post dated 31 January 2004, and that post — restored here as Rob Relyea: January 2004 — is the single source Wikipedia cites for the claim that XAML originally stood for Extensible Avalon Markup Language.