How can I implement an AppService in WinUi3?

I'm attempting to upgrade my UWP app to WinUI3 and I cannot get the AppService working in WinUI3.

So far I've tried the following:

• Upgrade tool on working UWP minimal example. - AppUnavailable
• Use Universal Windows project. - Error versions not compatible.
• .Net 7 project - AppUnavailable
• OutputType: winmdobj - Error versions not compatible.
• Experimental Windows Runtime Component C# - Error versions not compatible.

The Microsoft documentation is lacking around AppServices and only the UWP documentation is available.

Is it possible to get an AppService working with WinUI3? And if so, what are the requirements of the projects?

Here's my current setup, not much is different from UWP version.

RuntimeComponent1.csproj

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net7.0-windows10.0.22621</TargetFramework>
    <OutputType>Library</OutputType>
    <DefaultLanguage>en-US</DefaultLanguage>
    <TargetPlatformMinVersion>10.0.17763.0</TargetPlatformMinVersion>
    <AllowCrossPlatformRetargeting>false</AllowCrossPlatformRetargeting>
    <CsWinRTComponent>true</CsWinRTComponent>
    <UseWinUI>true</UseWinUI>
    <ApplicationManifest>app.manifest</ApplicationManifest>
    <EnablePreviewMsixTooling>true</EnablePreviewMsixTooling>
    <Platforms>x86;x64;arm64</Platforms>
    <PublishProfile>win10-$(Platform).pubxml</PublishProfile>
    <RuntimeIdentifiers>win10-x86;win10-x64;win10-arm64</RuntimeIdentifiers>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
    <DefineConstants>DEBUG;TRACE;NETFX_CORE;WINDOWS_UWP</DefineConstants>
    <WarningLevel>4</WarningLevel>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
    <DefineConstants>TRACE;NETFX_CORE;WINDOWS_UWP</DefineConstants>
    <WarningLevel>4</WarningLevel>
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Debug|x86'">
    <OutputPath>bin\x86\Debug\</OutputPath>
    <DefineConstants>DEBUG;TRACE;NETFX_CORE;WINDOWS_UWP</DefineConstants>
    <NoWarn>;2008</NoWarn>
    <UseVSHostingProcess>false</UseVSHostingProcess>
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Release|x86'">
    <OutputPath>bin\x86\Release\</OutputPath>
    <DefineConstants>TRACE;NETFX_CORE;WINDOWS_UWP</DefineConstants>
    <NoWarn>;2008</NoWarn>
    <UseVSHostingProcess>false</UseVSHostingProcess>
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Debug|ARM'">
    <OutputPath>bin\ARM\Debug\</OutputPath>
    <DefineConstants>DEBUG;TRACE;NETFX_CORE;WINDOWS_UWP</DefineConstants>
    <NoWarn>;2008</NoWarn>
    <UseVSHostingProcess>false</UseVSHostingProcess>
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Release|ARM'">
    <OutputPath>bin\ARM\Release\</OutputPath>
    <DefineConstants>TRACE;NETFX_CORE;WINDOWS_UWP</DefineConstants>
    <NoWarn>;2008</NoWarn>
    <UseVSHostingProcess>false</UseVSHostingProcess>
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Debug|ARM64'">
    <OutputPath>bin\ARM64\Debug\</OutputPath>
    <DefineConstants>DEBUG;TRACE;NETFX_CORE;WINDOWS_UWP</DefineConstants>
    <NoWarn>;2008</NoWarn>
    <UseVSHostingProcess>false</UseVSHostingProcess>
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Release|ARM64'">
    <OutputPath>bin\ARM64\Release\</OutputPath>
    <DefineConstants>TRACE;NETFX_CORE;WINDOWS_UWP</DefineConstants>
    <NoWarn>;2008</NoWarn>
    <UseVSHostingProcess>false</UseVSHostingProcess>
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Debug|x64'">
    <OutputPath>bin\x64\Debug\</OutputPath>
    <DefineConstants>DEBUG;TRACE;NETFX_CORE;WINDOWS_UWP</DefineConstants>
    <NoWarn>;2008</NoWarn>
    <UseVSHostingProcess>false</UseVSHostingProcess>
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Release|x64'">
    <OutputPath>bin\x64\Release\</OutputPath>
    <DefineConstants>TRACE;NETFX_CORE;WINDOWS_UWP</DefineConstants>
    <NoWarn>;2008</NoWarn>
    <UseVSHostingProcess>false</UseVSHostingProcess>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.WindowsAppSDK" Version="1.1.0" />
	  <PackageReference Include="Microsoft.Windows.CsWinRT" Version="2.0.2" />
  </ItemGroup>
</Project>

MyAppService.cs

using System;
using Windows.ApplicationModel.Background;

namespace RuntimeComponent1
{
    public sealed class MyAppService : IBackgroundTask
    {
        public void Run(IBackgroundTaskInstance taskInstance)
        {
            //breakpoint
            Console.WriteLine("Hello World");
        }
    }
}

Currently, there is no way to use the mechanism. in a WinUI3 application. I'd suggest that you could go with a named pipe or other IPC mechanism.

I've discovered a workaround.

I can use a Windows Application Packaging Project (.wapproj) to combine a UWP app with a WinUI3 app.

The steps are:

• Create UWP Runtime Component project for the AppService as per usual.
• Reference the UWP Runtime Component project from the UWP Application project.
• Remove everything from UWP project except the manifest and create a dummy main method.
• Create WinUI3 project with WAP project. Or if using existing, ensure WinUI3 project is default application of WAP project
• Reference UWP project from WAP project.
• In WAP manifest ensure entry point is WinUI3 project.
• In WAP manifest declarations set up AppService by pointing to AppService class in Runtime Component.