MainWindow.xaml.cs

using Microsoft.UI.Xaml;
using Microsoft.UI.Xaml.Controls;
using Microsoft.UI.Xaml.Controls.Primitives;
using Microsoft.UI.Xaml.Data;
using Microsoft.UI.Xaml.Input;
using Microsoft.UI.Xaml.Media;
using Microsoft.UI.Xaml.Navigation;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices.WindowsRuntime;
using Windows.Foundation;
using Windows.Foundation.Collections;

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

namespace BlankProximityWinUI3Desktop
{
    /// <summary>
    /// An empty window that can be used on its own or navigated to within a Frame.
    /// </summary>
    public sealed partial class MainWindow : Window
    {
        public MainWindow()
        {
            this.InitializeComponent();
        }

        private async void myButton_Click(object sender, RoutedEventArgs e)
        {
            var proximityDevice = Windows.Networking.Proximity.ProximityDevice.GetDefault();
            if (proximityDevice != null )
            {
                var messageSubscriptionId = proximityDevice.SubscribeForMessage("NDEF", async (device, message) =>
                {
                    Console.WriteLine(message.Data.ToArray());

                    ContentDialog dialog = new()
                    {
                        XamlRoot = Content.XamlRoot,
                        Title = "NDEF Message",
                        Content = message.Data.ToArray(),
                        CloseButtonText = "Ok"
                    };

                    await dialog.ShowAsync();
                });
            }
            else
            {
                ContentDialog dialog = new()
                {
                    XamlRoot = Content.XamlRoot,
                    Title = "No ProximityDevice",
                    Content = "No ProximityDevice found",
                    CloseButtonText = "Ok"
                };

                await dialog.ShowAsync();
            }
            
        }
    }
}