native-android-csharp/DemoAndroid/MainActivity.cs

using Android.Content;
using Android.Graphics;
using QRCoder; // QRCoder-ImageSharp else it crashes

namespace DemoAndroid;

[Activity(Label = "@string/app_name", MainLauncher = false)]
public class MainActivity : Activity
{
    private string _text = "";
    private ImageView? _ivQr;
    
    protected override void OnCreate(Bundle? savedInstanceState)
    {
        base.OnCreate(savedInstanceState);

        // Set our view from the "main" layout resource
        SetContentView(Resource.Layout.activity_main);

        var etInput = FindViewById<EditText>(Resource.Id.etInput) ?? throw new Exception("EditText #etInput not found");
        _ivQr = FindViewById<ImageView>(Resource.Id.ivQR) ?? throw new Exception("ImageView #ivQR not found");

        etInput.TextChanged += (sender, args) =>
        {
            _text = args.Text?.ToString() ?? string.Empty;
        
            if (!string.IsNullOrEmpty(_text))
            {
                CreateQr();
            }
        };

        var btnWol = FindViewById<Button>(Resource.Id.btnWolPage) ?? throw new Exception("Button #btnWol not found");
        btnWol.Click += (_, _) =>
        {
            StartActivity(new Intent(this, typeof(WakeOnLanActivity)));
        };
    }

    private void CreateQr()
    {
        using var qrGenerator = new QRCodeGenerator();
        using var qrCodeData = qrGenerator.CreateQrCode(_text, QRCodeGenerator.ECCLevel.Q);
        using var qrCode = new PngByteQRCode(qrCodeData);
        
        var qrCodeImage = qrCode.GetGraphic(20);
        var javaBitmap = BitmapFactory.DecodeByteArray(qrCodeImage, 0, qrCodeImage.Length);
        _ivQr?.SetImageBitmap(javaBitmap);
    }
}