presence/Presence.Desktop/Views/MainWindow.axaml.cs

using Avalonia.Controls;
using Avalonia.Markup.Xaml;
using Avalonia.Platform.Storage;
using Avalonia.ReactiveUI;
using Presence.Desktop.ViewModels;
using ReactiveUI;
using System;
using System.Linq;
using System.Reactive;
using System.Threading.Tasks;

namespace Presence.Desktop.Views
{
    public partial class MainWindow : ReactiveWindow<MainWindowViewModel>
    {
        public MainWindow()
        {
            InitializeComponent();

            this.WhenActivated(action =>
            {
                action(ViewModel.ShowOpenFileDialog.RegisterHandler(ShowOpenFileDialogAsync));
            });
        }

        public static FilePickerFileType FileCsv { get; } = new("Comma-Separated Values")
        {
            Patterns = ["*.csv"],
            AppleUniformTypeIdentifiers = ["public.comma-separated-values-text"],
            MimeTypes = ["application/vnd.ms-excel", "text/csv"]
        };

        private async Task ShowOpenFileDialogAsync(InteractionContext<Unit, string?> interaction)
        {
            var topLevel = GetTopLevel(this);
            FilePickerOpenOptions fpOptions = new FilePickerOpenOptions
            {
                AllowMultiple = false,
                Title = "students csv",
                FileTypeFilter = [FileCsv]
            };
            var storageFile = await topLevel.StorageProvider.OpenFilePickerAsync(fpOptions);

            interaction.SetOutput(storageFile.First().Path.ToString());
        }

    }
}