Referencing a MAUI library gives "Type 'App' already defines a member called 'InitializeComponent' with the same parameter types"

[BlueRaja]

Description

I'm not sure if this is a WPF bug, a MAUI bug, Visual Studio, Roslyn, etc.

With the latest VS/Maui/WPF/.Net, if you have a WPF app and reference a MAUI Class Library, the WPF application suddenly stops compiling with weird errors, even in a brand new WPF project with no changes from the defaults.

Specifically, the following files are generated by Visual Studio when creating a new WPF project:

App.xaml

<Application x:Class="Deletemetest.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:Deletemetest"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
         
    </Application.Resources>
</Application>

App.xaml.cs

using System.Configuration;
using System.Data;
using System.Windows;

namespace Deletemetest
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : Application
    {
    }

}

And results in the following errors:

App.xaml.sg.cs: 20: Type 'App' already defines a member called 'InitializeComponent' with the same parameter types
App.xaml.cs:10: 'Application' is an ambiguous reference between 'Microsoft.Maui.Controls.Application' and 'System.Windows.Application'

The second error (which is confusing because the code is already specifying using System.Windows) can be fixed by changing App : Application to App : System.Windows.Application, but the first error seems to be unfixable on our end.

Reproduction Steps

1. Create a new Maui Class Library
2. Create a new WPF App which references the Maui Class Library
3. Attempt to compile

Expected behavior

App compiles successfully

Actual behavior

Compiler gives non-sensical, unfixable compilation errors

Regression?

No, this is new behavior. It was working prior to upgrading to .Net 9

Known Workarounds

none

Impact

MAJOR. App is now completely uncompilable.

Configuration

No response

Other information

No response

[miloush]

Since the executable is WPF and MAUI is only a library, I am not sure why MAUI should be generating an app class. Perhaps this would be more useful in dotnet/maui than here.

[BlueRaja]

@miloush The issue seems to be deeper than simply Maui defining a class called "App". If I create a new WPF XAML file with any name, the compiler complains that IntializeComponent() is defined twice. If I add any component to that the page's Content and give it a variable name in the XAML, the compiler complains that the variable is defined twice.

In other words, referencing a Maui library completely prevents any WPF XAML component from compiling, with ridiculous error messages.

[miloush]

Sounds like MAUI library is bringing in source generators to the project that run on XAML files.

[BlueRaja]

I'm not sure what that means. Do you know of any way to stop or workaround that?

[miloush]

You can disable source generators, for example in the project file:

<Project>
    <Target Name="DisableAnalyzers" BeforeTargets="CoreCompile">
        <ItemGroup>
            <Analyzer Remove="@(Analyzer)" />
        </ItemGroup>
    </Target>
</Project>

This disables them all. You can disable specific ones, but you need a path. I am not set up to run MAUI projects, so you have to find out yourself in the solution explorer:

ref: dotnet/roslyn#55518 (comment)

[himgoyalmicro]

@BlueRaja Does the comment from @miloush resolve your issue?

[StephaneDelcroix]

<EnableDefaultMauiItems>false</EnableDefaultMauiItems> should disable handling xaml files as maui xaml files

[BlueRaja]

The latest comment resolved the issue.

For posterity, you need to open the .csproj for the WPF project in a text editor, and add the above line inside of the <PropertyGroup>