This sample app demonstrates a plugin solution for WinUI3 applications. The host application is a WinUI3 desktop application project. A plugin is a WinUI3 class project that uses a custom build script to pack and install the content as an optional package targeting the host application.

The plugin assembly code is loaded via reflection and an AssemblyLoadContext which isolates plugin dependencies from the host application. This is based on https://docs.microsoft.com/en-us/dotnet/core/tutorials/creating-app-with-plugin-support.

The plugin UI Xaml is packaged in the plugin optional package. The default InitializeComponent() method in UserControl code behind files incorrectly searches the host application package for Xaml data. Each plugin UserControl must replace this with a workaround which instead locates the data using the full path to the Xaml file in the plugin package. See SimpleUI.xaml.cs for an example.

• Plugins cannot introduce their own framework. Framework assemblies must be provided by the host application. https://docs.microsoft.com/en-us/dotnet/core/tutorials/creating-app-with-plugin-support#plugin-framework-references
• Plugins cannot use the resource system e.g. strings/localization/images*
• Plugins cannot use 3rd party controls*

*URI location of data in the plugin package's resources.pri does not work. The plugin's resources.pri file is loaded into the host application's package graph at runtime (inspect ResourceManager.Current to confirm for yourself) however the data is never located correctly by URI. This means any feature dependent on the plugin's resources.pri will not work correctly. This includes resources defined in a .resw file and 3rd party controls which embed their Xaml in the resources.pri file.

Please contact me if you have a solution for the above issue! :)

1. Before the first build you will need to create and install a certificate for msix package signing. See instructions in section below. Then modify HostApp.SimplePlugin.csproj to use your certificate path and password.
2. Build and Deploy HostApp.csproj
3. Build HostApp.SimplePlugin.csproj (the custom post build script will deploy it as an optional package to your local machine)

https://docs.microsoft.com/en-us/windows/msix/package/create-certificate-package-signing

In PowerShell run the following to generate a new certificate. -Subject must match the Publisher property of AppxManifest.xml in the plugin project.

New-SelfSignedCertificate -Type Custom -Subject "CN=WilliamF" -KeyUsage DigitalSignature -FriendlyName "my certificate" -CertStoreLocation "Cert:\CurrentUser\My" -TextExtension @("2.5.29.37={text}1.3.6.1.5.5.7.3.3", "2.5.29.19={text}")

In PowerShell run the following to export the new certificate.

1. Set-Location Cert:\CurrentUser\My

2. Get-ChildItem | Format-Table Subject, FriendlyName, Thumbprint

3. Note the Thumbprint value of your certificate e.g. BDDC57EBFEECECC2B17448FB4AB94F1F307DF5F4 and replace in below commands.

4. Choose a password and replace with your chosen password in below commands.

5. Choose a path to export your certificate and replace with your chosen path in the below commands.

6. $password = ConvertTo-SecureString -String <My Password> -Force -AsPlainText

7. Export-PfxCertificate -cert "Cert:\CurrentUser\My\<Certificate Thumbprint>" -FilePath <FilePath>.pfx -Password $password

Modify the HostApp.SimplePlugin.csproj PostBuild script arguments to include the path to the exported certificate and the password you chose.

<Target Name="PostBuild" AfterTargets="PostBuildEvent">
	<Exec Command="powershell -file $(MSBuildProjectDirectory)\build\deploy.ps1 $(TargetDir) my_certificate_path my_password" />
</Target>