WPF select uielement via drawing rectangle

//xaml
<Window x:Class="WpfApp68.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp68"
        WindowState="Maximized"
        WindowStyle="None"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Canvas x:Name="cvs"
                Background="Transparent"
                MouseDown="cvs_MouseDown"
                MouseMove="cvs_MouseMove"
                MouseUp="cvs_MouseUp"
                MouseRightButtonUp="cvs_MouseRightButtonUp">
            <Rectangle x:Name="rect"
                       Visibility="Collapsed"
                       Fill="LightBlue"
                       Stroke="Blue"
                       StrokeThickness="5"/>
        </Canvas>
    </Grid>
</Window>



//cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApp68
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        private Point startPt, endPt;
        private int width, height, idx = 0;
        private Random rnd;
        public MainWindow()
        {
            InitializeComponent();
            this.Loaded += MainWindow_Loaded;
        }

        private void MainWindow_Loaded(object sender, RoutedEventArgs e)
        {
            rnd = new Random();
            rect.Visibility = Visibility.Collapsed;
            width = (int)this.ActualWidth - 120;
            height = (int)this.ActualHeight - 120;
            InitEllipses();
        }

        private void InitEllipses()
        {
            for (int i = 0; i < 100; i++)
            {
                Ellipse elp = new Ellipse();
                elp.Width = 100;
                elp.Height = 100;
                elp.Stroke = new SolidColorBrush(Colors.Blue);
                elp.Fill = new SolidColorBrush(Colors.Red);
                elp.ToolTip = $"{++idx},{Guid.NewGuid().ToString("N")}";
                Canvas.SetLeft(elp, rnd.Next(10, width));
                Canvas.SetTop(elp, rnd.Next(10, height));
                if (!cvs.Children.Contains(elp))
                {
                    cvs.Children.Add(elp);
                }
            }
        }

        private void cvs_MouseRightButtonUp(object sender, MouseButtonEventArgs e)
        {
            rect.Visibility = Visibility.Collapsed;
        }

        private void cvs_MouseDown(object sender, MouseButtonEventArgs e)
        {
            startPt = e.GetPosition(cvs);
            ResetEllipses();
        }

        private void ResetEllipses()
        {
            int childrenCount = cvs.Children.Count;
            for (int i = 0; i < childrenCount; i++)
            {
                var elp = cvs.Children[i] as Ellipse;
                if (elp != null)
                {
                    elp.Stroke = new SolidColorBrush(Colors.Blue);
                    elp.StrokeThickness = 5;
                }
            }
        }

        private void cvs_MouseMove(object sender, MouseEventArgs e)
        {
            if (Mouse.LeftButton == MouseButtonState.Pressed)
            {
                endPt = e.GetPosition(cvs);
                rect.Visibility = Visibility.Visible;
                rect.Width = Math.Abs(startPt.X - endPt.X);
                rect.Height = Math.Abs(startPt.Y - endPt.Y);
                Canvas.SetLeft(rect, Math.Min(startPt.X, endPt.X));
                Canvas.SetTop(rect, Math.Min(startPt.Y, endPt.Y));
                SelectEllipsesInRect();
            }
        }

        private void SelectEllipsesInRect()
        {
            int childrenCount = cvs.Children.Count;
            for (int i = 0; i < childrenCount; i++)
            {
                var elp = cvs.Children[i] as Ellipse;
                if (elp != null)
                {
                    var elpX = Canvas.GetLeft(elp);
                    var elpY = Canvas.GetTop(elp);
                    if (elpX > Math.Min(startPt.X, endPt.X)
                        && elpY > Math.Min(startPt.Y, endPt.Y)
                        && elpX + 100 < Math.Max(startPt.X, endPt.X)
                        && elpY + 100 < Math.Max(startPt.Y, endPt.Y))
                    {
                        elp.Stroke = new SolidColorBrush(Colors.Orange);
                        elp.StrokeThickness = 10;
                    }
                    else
                    {
                        elp.Stroke = new SolidColorBrush(Colors.Blue);
                        elp.StrokeThickness = 5;
                    }
                }
            }
        }

        private void cvs_MouseUp(object sender, MouseButtonEventArgs e)
        {

        }
    }
}