'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

.NET Core Debugging with WSL 1.0
.NET Core Debugging with WSL

ADL Tools Service Provider 1.0
This package contains services used by Data Lake tools

ASA Service Provider 1.0

ASP.NET and Web Tools 2019 17.3.122.33185
ASP.NET and Web Tools 2019

Azure App Service Tools v3.0.0 17.3.122.33185
Azure App Service Tools v3.0.0

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
Azure Functions and Web Jobs Tools

Azure Stream Analytics Tools for Visual Studio 2.6.5000.0
Microsoft Azure Stream Analytics Tools for Visual Studio

C# Tools 4.3.0-1.22254.1+9919d7e7bd753404a5d2328e5e3fb2de635169f3
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Common Azure Tools 1.10
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Cookiecutter 17.0.22115.6
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Extensibility Message Bus 1.2.6 (master@34d6af2)
Provides common messaging-based MEF services for loosely coupled Visual Studio extension components communication and integration.

Microsoft Azure Hive Query Language Service 2.6.5000.0
Language service for Hive query

Microsoft Azure Stream Analytics Language Service 2.6.5000.0
Language service for Azure Stream Analytics

Microsoft Azure Tools for Visual Studio 2.9
Support for Azure Cloud Services projects

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Mono Debugging for Visual Studio 17.3.7 (c1ffb2a)
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NuGet Package Manager 6.3.0
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Python - Django support 17.0.22115.6
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Python with Pylance 17.0.22115.6
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Razor (ASP.NET Core) 17.0.0.2222701+751db1ebea5e6a9ecc7fa57fe447180422afa610
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SQL Server Data Tools 17.0.62204.01010
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ToolWindowHostedEditor 1.0
Hosting json editor into a tool window

TypeScript Tools 17.0.10420.2001
TypeScript Tools for Microsoft Visual Studio

Visual Basic Tools 4.3.0-1.22254.1+9919d7e7bd753404a5d2328e5e3fb2de635169f3
Visual Basic components used in the IDE. Depending on your project type and settings, a different version of the compiler may be used.

Visual F# Tools 17.1.0-beta.22178.3+6da0245a7ce4bb8483b8d1f2993c8ecaea967ad9
Microsoft Visual F# Tools

Visual Studio IntelliCode 2.2
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VisualStudio.DeviceLog 1.0
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VisualStudio.Mac 1.0
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VSPackage Extension 1.0
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Xamarin 17.3.0.91 (main@7802af5)
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Xamarin Designer 17.3.0.30 (remotes/origin/main@fb6442dc1)
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Xamarin Templates 17.2.15 (2e3b60e)
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Xamarin.Android SDK 12.3.99.58 (main/64b22fc)
Xamarin.Android Reference Assemblies and MSBuild support.
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)
Xamarin.iOS and Xamarin.Mac Reference Assemblies and MSBuild support.

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

Are you sure you are using the latest templates?? Please post your App.xaml and App.xaml.cs.

https://devblogs.microsoft.com/dotnet/dotnet-maui-rc-3/

According to the above this is latest, generating AppShell, etc. I updated VS this weekend. I don't see versions for Maui Templates but the new objects being generated correspond to the blog post above.

Answer 3

Sorry but unless I am using outdated templates, your answer about namespaces is incorrect.

It appears that you originally created the project as LeaderPivot.XAML.MAUI.Host.WinUI.

Nope.

Here are classes from project that builds and runs. Again, please note the WinUI namespace which is added by the template:

Platforms/Windows/App.xaml

<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>

Platforms/Windows/App.xaml.cs

using Microsoft.UI.Xaml;

// To learn more about WinUI, the WinUI project structure,
// and more about our project templates, see: http://aka.ms/winui-project-info.

namespace MauiApp1.WinUI;

/// <summary>
/// Provides application-specific behavior to supplement the default Application class.
/// </summary>
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();
}

App.xaml

<?xml version = "1.0" encoding = "UTF-8" ?>
<Application xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:MauiApp1"
             x:Class="MauiApp1.App">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Resources/Colors.xaml" />
                <ResourceDictionary Source="Resources/Styles.xaml" />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>

Appl.xaml.cs

namespace MauiApp1;

public partial class App : Application
{
    public App()
    {
        InitializeComponent();

        MainPage = new AppShell();
    }
}

Answer 4

LeaderPivot... as the namespace. Your codebehind files are using LeaderAnalytics.LeaderPivot...

THIS is the problem! I thought you were saying I was adding .WinUI... that is added by the template.

Thank you, one millllyon stars for you today.