MainWindow.xaml.cs

using CommunityToolkit.Mvvm.Input;
using MaterialDesignThemes.Wpf;
using Microsoft.Extensions.DependencyInjection;
using System.Reactive.Linq;
using System.Windows;

namespace DialogHostStartupExample;
public partial class MainWindow
{
    public const string DialogHostIdentifier = "MyDialogHost";

    public MainWindow(IServiceProvider serviceProvider)
    {
        this.serviceProvider = serviceProvider;
        InitializeComponent();

        //Scenario 1: fails due to System.InvalidOperationException: 'No loaded DialogHost instances.'.
        Observable
            .FromEventPattern<RoutedEventArgs>(this, nameof(MainWindow.Loaded))
            .Select(_ => Observable.FromAsync(() => ShowTestViewCommand.ExecuteAsync(null), System.Reactive.Concurrency.TaskPoolScheduler.Default))
            .Concat()
            .Take(1)
            .Subscribe();

        //Scenario 2: works because of small delay. Not ideal.
        //var uiScheduler = new System.Reactive.Concurrency.DispatcherScheduler(Dispatcher, System.Windows.Threading.DispatcherPriority.Background);
        //Observable
        //    .FromEventPattern<RoutedEventArgs>(this, nameof(MainWindow.Loaded))
        //    .Delay(TimeSpan.FromSeconds(1), uiScheduler)
        //    .Select(_ => Observable.FromAsync(() => ShowTestViewCommand.ExecuteAsync(null), System.Reactive.Concurrency.TaskPoolScheduler.Default))
        //    .Concat()
        //    .Take(1)
        //    .Subscribe();

        //Scenario 3: works because we wait for DialogHost.LoadedEvent. I think this introduces a new problem if DialogHost is alraedy loaded.
        //Observable
        //    .FromEventPattern<RoutedEventArgs>(this, nameof(MainWindow.Loaded))
        //    .CombineLatest(Observable.FromEventPattern<RoutedEventArgs>(MyDialogHost, nameof(MyDialogHost.Loaded)), (a, b) => b.Sender)
        //    .Select(_ => Observable.FromAsync(() => ShowTestViewCommand.ExecuteAsync(null), System.Reactive.Concurrency.TaskPoolScheduler.Default))
        //    .Concat()
        //    .Take(1)
        //    .Subscribe();
    }
    private readonly IServiceProvider serviceProvider;


    [RelayCommand]
    private async Task ShowTestViewAsync()
    {
        try
        {
            if (MyDialogHost is { IsLoaded: true } && serviceProvider.GetService<Views.TestView>() is { } view)
            {
                await DialogHost.Show(view, DialogHostIdentifier);
            }
        }
        catch (Exception ex)
        {
            System.Windows.MessageBox.Show(this, ex.Message);
        }
    }
}