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Create app extensions for Windows App SDK apps

App extensions let your app host add-in content and functionality from other packages. This is the Windows equivalent of a plug-in model. An extension provider declares content or services, and your host app discovers and loads them at runtime.

Important

App extensions require MSIX package identity for both the host and extension apps. The extensions are discovered through the package catalog.

How app extensions work

The app extension model has two roles:

  • Extension host — Your app declares which extension types it accepts and discovers installed extensions.
  • Extension provider — Another app (or the same app) declares that it provides an extension of a particular type.

Extensions communicate through the package manifest declarations, not through code. The host app uses the AppExtensionCatalog API to find and manage installed extensions.

Declare an extension host

Add an AppExtensionHost declaration to your app's Package.appxmanifest:

<Package xmlns:uap3="http://schemas.microsoft.com/appx/manifest/uap/windows10/3" ...>
  <Applications>
    <Application ...>
      <Extensions>
        <uap3:Extension Category="windows.appExtensionHost">
          <uap3:AppExtensionHost>
            <uap3:Name>com.example.myapp.extensions</uap3:Name>
          </uap3:AppExtensionHost>
        </uap3:Extension>
      </Extensions>
    </Application>
  </Applications>
</Package>

Declare an extension provider

In the extension app's Package.appxmanifest:

<Package xmlns:uap3="http://schemas.microsoft.com/appx/manifest/uap/windows10/3" ...>
  <Applications>
    <Application ...>
      <Extensions>
        <uap3:Extension Category="windows.appExtension">
          <uap3:AppExtension Name="com.example.myapp.extensions"
                             Id="MathExtension"
                             DisplayName="Math Functions"
                             Description="Provides math operations"
                             PublicFolder="Public">
          </uap3:AppExtension>
        </uap3:Extension>
      </Extensions>
    </Application>
  </Applications>
</Package>

The PublicFolder points to a folder in the extension package that the host app can read.

Discover and load extensions

In your host app, use AppExtensionCatalog to discover installed extensions:

private AppExtensionCatalog? _catalog;

private async Task InitializeExtensionsAsync()
{
    _catalog = AppExtensionCatalog.Open("com.example.myapp.extensions");

    _catalog.PackageInstalled += OnExtensionInstalled;
    _catalog.PackageUpdated += OnExtensionUpdated;
    _catalog.PackageUninstalling += OnExtensionRemoved;

    var extensions = await _catalog.FindAllAsync();
    foreach (var extension in extensions)
    {
        await LoadExtensionAsync(extension);
    }
}

private async Task LoadExtensionAsync(AppExtension extension)
{
    string extensionId = extension.Id;
    string displayName = extension.DisplayName;

    // Get the extension's public folder
    var folder = await extension.GetPublicFolderAsync();
    if (folder != null)
    {
        // Read configuration, content, or scripts from the folder
        var configFile = await folder.TryGetItemAsync("config.json");
        if (configFile != null)
        {
            // Parse and use the extension configuration
        }
    }
}

// See "Handle extension lifecycle events" below for the
// OnExtensionInstalled, OnExtensionUpdated, and OnExtensionRemoved handlers.
private async void OnExtensionInstalled(AppExtensionCatalog sender,
    AppExtensionPackageInstalledEventArgs args) { }
private async void OnExtensionUpdated(AppExtensionCatalog sender,
    AppExtensionPackageUpdatedEventArgs args) { }
private void OnExtensionRemoved(AppExtensionCatalog sender,
    AppExtensionPackageUninstallingEventArgs args) { }

Handle extension lifecycle events

private async Task LoadExtensionAsync(AppExtension extension) => await Task.CompletedTask;
// (See "Discover and load extensions" above for the real implementation.)

private async void OnExtensionInstalled(AppExtensionCatalog sender,
    AppExtensionPackageInstalledEventArgs args)
{
    // Re-query the catalog to get the AppExtension instances added by this package
    var extensions = await sender.FindAllAsync();
    foreach (var extension in extensions.Where(e => e.Package.Id.FamilyName == args.Package.Id.FamilyName))
    {
        await LoadExtensionAsync(extension);
    }
}

private void OnExtensionRemoved(AppExtensionCatalog sender,
    AppExtensionPackageUninstallingEventArgs args)
{
    // args.Package identifies the package that is uninstalling
    string familyName = args.Package.Id.FamilyName;
    // Remove any extensions from that package from your app
}

private async void OnExtensionUpdated(AppExtensionCatalog sender,
    AppExtensionPackageUpdatedEventArgs args)
{
    // Re-query the catalog to get the refreshed AppExtension instances
    var extensions = await sender.FindAllAsync();
    foreach (var extension in extensions.Where(e => e.Package.Id.FamilyName == args.Package.Id.FamilyName))
    {
        // Reload with updated content
        await LoadExtensionAsync(extension);
    }
}

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