Ursa.Avalonia for Cross-Platform Avalonia Applications

What Ursa Adds to an Avalonia Application

Ursa addresses the part of application development that sits above individual Avalonia controls: a reusable collection of controls and supporting services for applications that need a consistent interface across desktop platforms. The project describes itself as an enterprise level UI library for cross-platform applications built with Avalonia UI. That target audience is relevant to developers who are already using Avalonia and do not want to implement every interaction pattern, dialog service, or themed control separately in each application.

The repository contains the core `Irihi.Ursa` package, theme packages, and optional extensions for Prism.Avalonia and ReactiveUI.Avalonia. The README demonstrates `TagInput`, which indicates that Ursa is intended to add application-level controls rather than merely alter the default Avalonia appearance. Its scope is therefore narrower than a general-purpose application framework. It does not replace Avalonia's windowing, layout, styling, or platform integration. It builds on those APIs and supplies additional UI components and services.

The repository also includes demos for desktop, browser, Android, and iOS, plus `Ursa.PrismDialogDemo`. Based on the repository layout, the library is designed for shared UI work across those Avalonia targets. The documentation does not promise identical behavior on every platform, so platform-specific behavior still needs checking in the application where the control is used.

Package, Theme, and Control Data Flow

Ursa's documented setup has three layers. First, the application references `Irihi.Ursa`, which provides the core library. Second, it references a theme package. For the Semi theme, the README names `Semi.Avalonia` and `Irihi.Ursa.Themes.Semi`. Third, the application registers the theme resources in its `Application.Styles` collection. Only after those resources are registered does the README say that Ursa controls can be used in an Avalonia `Window`.

The example uses the `https://irihi.tech/semi` namespace and adds both `semi:SemiTheme` and `semi:UrsaSemiTheme`, each configured with `Locale="zh-CN"`. The control example then uses the `https://irihi.tech/ursa` namespace and places `u:TagInput` inside a `StackPanel`. This is a resource registration model, not a separate runtime service shown in the example. The theme package supplies styles that the controls consume through Avalonia's resource and styling system.

The optional integration packages extend that model. `Irihi.Ursa.PrismExtension` provides dialog-related services for using Ursa Dialogs with the Prism flavor. `Irihi.Ursa.ReactiveUIExtension` provides ReactiveUI versions of `UrsaWindow` and `UrsaView`. The README directs ReactiveUI users to the project wiki for details, so the complete initialization steps for that extension are not established by the README itself.

Installing Ursa and Adding the First Control

The README starts with the .NET CLI package installation. Run the core package command inside the Avalonia application project. This adds the `Irihi.Ursa` dependency:

dotnet add package Irihi.Ursa

For the documented Semi theme setup, add both packages named by the README:

dotnet add package Semi.Avalonia
dotnet add package Irihi.Ursa.Themes.Semi

Then register the theme styles in the Avalonia application resources. The following block is the exact abbreviated structure shown in the README. Replace the existing `Application` attributes with those in the application project, and preserve the documented namespaces:

<Application...
    xmlns:semi="https://irihi.tech/semi"
    ....>

    <Application.Styles>
        <semi:SemiTheme Locale="zh-CN" />
        <semi:UrsaSemiTheme Locale="zh-CN"/>
    </Application.Styles>

After the styles are registered, a window can reference Ursa and place a documented control in its visual tree:

<Window
    ...
    xmlns:u="https://irihi.tech/ursa"
    ...>
    <StackPanel Margin="20">
        <u:TagInput />
    </StackPanel>
</Window>

The result is an Avalonia window containing `TagInput`, with the Semi and Ursa Semi themes available to the application. The README does not document a command-line demo launch, a sample project name, or a complete project template, so the first use here is limited to the published setup example.

Framework Extensions and Compatibility Boundaries

Ursa supports two documented integration paths, but they solve different problems. The Prism extension is for applications using Prism.Avalonia and focuses on dialog-related services for Ursa Dialogs. The ReactiveUI extension is for applications using ReactiveUI.Avalonia and supplies ReactiveUI versions of `UrsaWindow` and `UrsaView`. These packages are optional. An application that uses neither framework should follow the core package and theme setup rather than installing an extension it does not need.

The clearest stated boundary is Avalonia compatibility. The README says that Ursa is compatible with Avalonia `11.1.x` and `11.2.x`, while specifically excluding `11.2.0`. That makes the dependency choice more important than a generic statement that the library supports Avalonia 11. The project should be checked against the exact Avalonia version in the application, especially when the application is already pinned to a specific patch release.

The documentation also says that a theme package designed for Ursa is required for Ursa controls to appear. This is a practical failure point for an initial adoption: adding `Irihi.Ursa` alone does not make the README's example complete. Missing `Semi.Avalonia`, `Irihi.Ursa.Themes.Semi`, or the `UrsaSemiTheme` resource can leave the control without the expected theme resources. Ursa is therefore a poor choice for a project that requires theme-independent controls or that refuses to add a theme package.

Maintenance, Packaging, and Adoption Limits

The repository is not archived, and its last push was on 2026-09-30. Recent release records include `v2.2.0` on 2026-07-31, `v2.1.0` on 2026-07-01, and `v2.0.1` on 2026-06-12. These facts indicate that the repository has recent repository activity and published package releases, but they do not establish the support duration or compatibility policy for every future Avalonia version.

The project uses the MIT license. That permits use and modification under the license's terms, while the repository also contains `THIRD-PARTY-NOTICES.md`, `SECURITY.md`, and contribution files. An organization should review those files for its own distribution and compliance process. The README identifies the .NET Foundation as a supporter, but it does not describe a warranty, migration service, or long-term support commitment.

The most important adoption cost is integration work. Developers must choose a theme, register the correct resource namespaces, match the Avalonia version constraint, and add a framework extension when Prism or ReactiveUI is present. The limited free community support mentioned in the README also means that the documentation does not promise a particular response time for implementation questions.

Editorial conclusion

Adopt Ursa when an Avalonia application needs a reusable enterprise UI layer and you can accept its Semi theme dependency. Do not adopt it as a drop-in replacement for every Avalonia control library, because compatibility is limited to Avalonia 11.1.x and 11.2.x, with 11.2.0 explicitly unsupported. Verify the target Avalonia version, the required theme setup, and the availability of Prism or ReactiveUI integration before adding the packages.

Frequently asked questions

Official sources

1. irihitech/Ursa.Avalonia on GitHub
2. License: MIT
3. Project website
4. README
5. Releases