MainWindow.cs

using System;
using Avalonia.Controls;
using Avalonia.Media.Imaging;
using Avalonia.Threading;
using Avalonia.VisualTree;
using NekoDot.Core;

namespace NekoDot.Views;

public partial class MainWindow : Window {
    private Image? petImage;
    
    private const int GWL_EXSTYLE = -20;
    private const uint WS_EX_TOOLWINDOW = 0x00000080;

    public MainWindow() {
        Loaded += SearchImage;

        InitializeComponent();

        ShowInTaskbar = false;
        WindowStartupLocation = WindowStartupLocation.Manual;
        var visualRoot = this.GetVisualRoot() as TopLevel;
        if (visualRoot != null && visualRoot.TryGetPlatformHandle() is { } platformHandle) {
            var hwnd = platformHandle.Handle;
            NativeMethods.SetWindowLong(hwnd, GWL_EXSTYLE,
                (uint)NativeMethods.GetWindowLong(hwnd, GWL_EXSTYLE) | WS_EX_TOOLWINDOW);
        }
    }

    private void SearchImage(Object? o, EventArgs eventArgs) {
        petImage = this.FindControl<Image>("pet");
    }

    private void SetSprite(Bitmap bitmap) {
        petImage.Source = bitmap;
    }
    
    public void SetPetSprite(Bitmap bitmap) {
        Dispatcher.UIThread.Invoke(() => SetSprite(bitmap));
    }
}