[Bug]: README example Throws for ReactiveUI.Avalonia.Microsoft.Extensions.DependencyInjection

[MidgeOnGithub]

Describe the bug 🐞

In the README's example for the MS DI extension package, the order of the methods shown is:

• .UseReactiveUIWithMicrosoftDependencyResolver(...)
• .RegisterReactiveUIViewsFromEntryAssembly()

This results in a runtime System.InvalidOperationException. I believe this is related to the fact that .UseReactiveUIWithMicrosoftDependencyResolver(...) methods call serviceCollection.BuildServiceProvider() and use Splat MS DI extension methods to resolve with the built MS DI container.

ReactiveUI.Avalonia/src/ReactiveUI.Avalonia.Microsoft.Extensions.DependencyInjection/AvaloniaMixins.cs

Line 52 in 78f5dc4

var serviceProvider = serviceCollection.BuildServiceProvider();

Running .RegisterReactiveUIViewsFromEntryAssembly() afterwards tries to add the View services to the finalized resolver.

ReactiveUI.Avalonia/src/ReactiveUI.Avalonia/AppBuilderExtensions.cs

Line 234 in 78f5dc4

resolver.Register(

Reversing the order of the methods results in the application not throwing (though for a new Avalonia MVVM template project as shown in the steps to recreate, .RegisterReactiveUIViewsFromEntryAssembly() isn't necessary and can be removed).

I believe we get the same behaviors from any of the view registration methods, so you can also run the example with .RegisterReactiveUIViews(Assembly.GetExecutingAssembly()) after adding using System.Reflection and see the same runtime Exception.

Truncated stack trace:

Unhandled exception. System.InvalidOperationException: This container has already been built and cannot be modified.
   at Splat.Microsoft.Extensions.DependencyInjection.MicrosoftDependencyResolver.Register(Func`1 factory, Type serviceType, String contract) in c:\temp\releaser\splat\src\Splat.Microsoft.Extensions.DependencyInjection\MicrosoftDependencyResolver.cs:line 161
   at ReactiveUI.Avalonia.AppBuilderExtensions.RegisterViewsInternal(IMutableDependencyResolver resolver, Assembly[] assemblies) in c:\temp\releaser\ReactiveUI.Avalonia\src\ReactiveUI.Avalonia\AppBuilderExtensions.cs:line 234
   at ReactiveUI.Avalonia.AppBuilderExtensions.<>c__DisplayClass2_0.<RegisterReactiveUIViews>b__0(AppBuilder _) in c:\temp\releaser\ReactiveUI.Avalonia\src\ReactiveUI.Avalonia\AppBuilderExtensions.cs:line 124

Step to reproduce

1. Create a new Avalonia MVVM project with ReactiveUI; one can run dotnet new avalonia.mvvm -o TestApp -m ReactiveUI after installing Avalonia dotnet templates (dotnet new install Avalonia.Templates)
2. (Optionally) Update the project's target framework to .NET 10 and language version to C# 14; the error happens just as well if you stick with the template's .NET 9 and C# 13 output
3. Add the Nuget package ReactiveUI.Avalonia.Microsoft.Extensions.DependencyInjection
4. Chain onto BuildAvaloniaApp the methods shown in the documentation example for MS DI, like so:

using Avalonia;
using System;
using ReactiveUI.Avalonia.Splat;

namespace TestApp;

sealed class Program
{
    [STAThread]
    public static void Main(string[] args) => BuildAvaloniaApp().StartWithClassicDesktopLifetime(args);

    public static AppBuilder BuildAvaloniaApp()
        => AppBuilder.Configure<App>()
            .UsePlatformDetect()
            .WithInterFont()
            .LogToTrace()
            .UseReactiveUIWithMicrosoftDependencyResolver(_ => { }, null)
            .RegisterReactiveUIViewsFromEntryAssembly();
}

5. Run the project

Reproduction repository

https://github.com/reactiveui/ReactiveUI

Expected behavior

Using README examples would not throw.

Screenshots 🖼️

No response

IDE

Rider Windows

Operating system

Windows

Version

No response

Device

No response

ReactiveUI.Avalonia Version

11.3.8

Additional information ℹ️

No response

[fuguiKz]

Opened a PR to update the README MS DI example so it doesn’t throw (and to add a short note about registering services/views before the ServiceProvider is built):

#50