'App' does not implement inherited abstract member 'MauiWinUIApplication.CreateMauiApp()'

Question

Error CS0534 'App' does not implement inherited abstract member 'MauiWinUIApplication.CreateMauiApp()' MyProject (net6.0-android) C:\Git\MyProject\obj\Debug\net6.0-windows10.0.19041.0\win10-x64\Platforms\Windows\App.g.i.cs 40 Active

AFAIK I have done nothing to change any android config items. I am trying to write a windows desktop app.

I created a new Maui project (it builds/runs fine) and compared the files and they appear to be identical other than project names.
There is no App.xaml in Platforms/Android in my project or the new app I created for comparison.
App.xaml.cs in Platforms/Windows does implement the abstract method:

protected override MauiApp CreateMauiApp() => MauiProgram.CreateMauiApp();  

How can I resolve this?

Visual Studio Info:

Microsoft Visual Studio Community 2022
Version 17.3.0 Preview 1.0
VisualStudio.17.Preview/17.3.0-pre.1.0+32505.426
Microsoft .NET Framework
Version 4.8.04084

Installed Version: Community

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ASP.NET and Web Tools 2019 17.3.122.33185
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Azure App Service Tools v3.0.0 17.3.122.33185
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Azure Data Lake Tools for Visual Studio 2.6.5000.0
Microsoft Azure Data Lake Tools for Visual Studio

Azure Functions and Web Jobs Tools 17.3.122.33185
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Azure Stream Analytics Tools for Visual Studio 2.6.5000.0
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C# Tools 4.3.0-1.22254.1+9919d7e7bd753404a5d2328e5e3fb2de635169f3
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Visual F# Tools 17.1.0-beta.22178.3+6da0245a7ce4bb8483b8d1f2993c8ecaea967ad9
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Xamarin Designer 17.3.0.30 (remotes/origin/main@fb6442dc1)
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Xamarin.Android SDK 12.3.99.58 (main/64b22fc)
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Mono: adf1bc4
Java.Interop: xamarin/java.interop/release/6.0.3xx@2a882d2d
ProGuard: Guardsquare/proguard/v7.0.1@912d149
SQLite: xamarin/sqlite/3.38.2@7b1e016
Xamarin.Android Tools: xamarin/xamarin-android-tools/main@fc3c2ac

Xamarin.iOS and Xamarin.Mac SDK 15.11.0.87 (406f3098e)
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Answer 1

I see the issue now. It appears that you originally created the project as LeaderPivot.XAML.MAUI.Host.WinUI. That means the namespace used to generate the code was the same. However you later started changing the classes to be in the LeaderAnalytics.LeaderPivot... namespace. So the generated code, which is using the original namespace, is not being merged into your partial classes you've defined and hence you're getting the errors. You need to sync up your namespaces of your types to what your XAML files are using.

From what I can tell you should make at least the following changes:

• platforms\windows\app.xaml change the x:class and xmlns:local attributes to use the updated namespace name.
• platforms\windows\app.xaml change 'maui:MauiWinUIApplicationto justApplication`.
• AppShell.xaml make the same changes.

Now the auto-generated files (*.g.cs) should correctly merge with your codebehind files.

Answer 2

The error is coming from the auto-generated file. You didn't happen to check these auto-generated files into your repo did you? Clean the solution and ensure the entire bin\debug directory is wiped. Ensure none of these files were accidentally checked into your repo. Then rebuild.

Answer 3

Create a new project in VS in a temp dir, call it MauiApp1 exactly as shown here.
Ensure the project builds and runs
Open Platforms/Windows/App.xaml

Note the WinUI namespace:

<maui:MauiWinUIApplication
    x:Class="MauiApp1.WinUI.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:maui="using:Microsoft.Maui"
    xmlns:local="using:MauiApp1.WinUI">

</maui:MauiWinUIApplication>

Answer 4

I have done nothing (intentionally) to configure multi-targeting. I only want to run on windows. If it works on Android I wont complain.

Here is Platforms/Windows/App.xaml.cs

public partial class App : MauiWinUIApplication
{
    /// <summary>
    /// Initializes the singleton application object.  This is the first line of authored code
    /// executed, and as such is the logical equivalent of main() or WinMain().
    /// </summary>
    public App()
    {
        this.InitializeComponent();
    }

    protected override MauiApp CreateMauiApp() => MauiProgram.CreateMauiApp();
}

Project:

<Project Sdk="Microsoft.NET.Sdk">

    <PropertyGroup>
        <TargetFrameworks>net6.0-android;net6.0-ios;net6.0-maccatalyst</TargetFrameworks>
        <TargetFrameworks Condition="$([MSBuild]::IsOSPlatform('windows'))">$(TargetFrameworks);net6.0-windows10.0.19041.0</TargetFrameworks>
        <!-- Uncomment to also build the tizen app. You will need to install tizen by following this: https://github.com/Samsung/Tizen.NET -->
        <!-- <TargetFrameworks>$(TargetFrameworks);net6.0-tizen</TargetFrameworks> -->
        <OutputType>Exe</OutputType>
        <RootNamespace>MyProject</RootNamespace>
        <UseMaui>true</UseMaui>
        <SingleProject>true</SingleProject>
        <ImplicitUsings>enable</ImplicitUsings>

        <!-- Display name -->
        <ApplicationTitle>MyProject</ApplicationTitle>

        <!-- App Identifier -->
        <ApplicationId>MyProject</ApplicationId>
        <ApplicationIdGuid>B981403E-C786-4560-B56F-A286A88</ApplicationIdGuid>

        <!-- Versions -->
        <ApplicationDisplayVersion>1.0</ApplicationDisplayVersion>
        <ApplicationVersion>1</ApplicationVersion>

        <SupportedOSPlatformVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'ios'">14.2</SupportedOSPlatformVersion>
        <SupportedOSPlatformVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'maccatalyst'">14.0</SupportedOSPlatformVersion>
        <SupportedOSPlatformVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'android'">21.0</SupportedOSPlatformVersion>
        <SupportedOSPlatformVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'windows'">10.0.17763.0</SupportedOSPlatformVersion>
        <TargetPlatformMinVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'windows'">10.0.17763.0</TargetPlatformMinVersion>
        <SupportedOSPlatformVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'tizen'">6.5</SupportedOSPlatformVersion>
    </PropertyGroup>

    <ItemGroup>
        <!-- App Icon -->
        <MauiIcon Include="Resources\appicon.svg" ForegroundFile="Resources\appiconfg.svg" Color="#512BD4" />

        <!-- Splash Screen -->
        <MauiSplashScreen Include="Resources\appiconfg.svg" Color="#512BD4" BaseSize="128,128" />

        <!-- Images -->
        <MauiImage Include="Resources\Images\*" />
        <MauiImage Update="Resources\Images\dotnet_bot.svg" BaseSize="168,208" />

        <!-- Custom Fonts -->
        <MauiFont Include="Resources\Fonts\*" />

        <!-- Raw Assets (also remove the "Resources\Raw" prefix) -->
        <MauiAsset Include="Resources\Raw\**" LogicalName="%(RecursiveDir)%(Filename)%(Extension)" />
    </ItemGroup>

</Project>

Answer 5

Same problem.

Please see this repo:

https://github.com/leaderanalytics/LeaderPivot.XAML.MAUI.git

I am only trying to build .Host project. The library project will not build and is not referenced from .Host.