WPF mouse down on canvas and draw shapes which render with random colors

//custom control
//xaml
<UserControl x:Class="WpfApp307.ElpTbk"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:local="clr-namespace:WpfApp307"
             mc:Ignorable="d" 
             d:DesignHeight="450" d:DesignWidth="800">
    <Grid>
        <Ellipse Fill="{Binding ElpBrush,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"/>
        <TextBlock Text="{Binding ElpStr,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
                   HorizontalAlignment="Center" VerticalAlignment="Center"/>
    </Grid>
</UserControl>


//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 WpfApp307
{
    /// <summary>
    /// Interaction logic for ElpTbk.xaml
    /// </summary>
    public partial class ElpTbk : UserControl
    {
        public ElpTbk()
        {
            InitializeComponent();
            this.DataContext = this;
        }

        public SolidColorBrush ElpBrush
        {
            get { return (SolidColorBrush)GetValue(ElpBrushProperty); }
            set { SetValue(ElpBrushProperty, value); }
        }

        // Using a DependencyProperty as the backing store for ElpBrush.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty ElpBrushProperty =
            DependencyProperty.Register("ElpBrush", typeof(SolidColorBrush),
                typeof(ElpTbk), new PropertyMetadata(new SolidColorBrush(Colors.Red)));




        public string ElpStr
        {
            get { return (string)GetValue(ElpStrProperty); }
            set { SetValue(ElpStrProperty, value); }
        }

        // Using a DependencyProperty as the backing store for ElpStr.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty ElpStrProperty =
            DependencyProperty.Register("ElpStr", typeof(string), 
                typeof(ElpTbk), new PropertyMetadata(""));


    }
}

//cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Remoting.Channels;
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 WpfApp307
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        Random rnd { get; set; }
        private int idx = 0;
        Canvas cvs { get; set; }

        public MainWindow()
        {
            InitializeComponent();
            rnd = new Random();
            cvs = new Canvas();
            cvs.HorizontalAlignment = HorizontalAlignment.Stretch;
            cvs.VerticalAlignment = VerticalAlignment.Stretch;
            cvs.Background =new SolidColorBrush(Colors.Transparent);
            cvs.MouseDown += cvs_MouseDown;
            this.Content = cvs;
        }

        private void cvs_MouseDown(object sender, MouseButtonEventArgs e)
        {
            ElpTbk elp = new ElpTbk();
            elp.Width = 100;
            elp.Height = 50;
            Color clr = new Color();
            clr.A = 255;
            clr.R=(byte)rnd.Next(255);
            clr.G=(byte)rnd.Next(255);
            clr.B = (byte)rnd.Next(255);
            elp.ElpBrush = new SolidColorBrush(clr);
            if(!cvs.Children.Contains(elp))
            { 
                Point p = e.GetPosition(this);
                elp.ElpStr = $"{++idx}   ({p.X - 50},{p.Y - 25})";
                Canvas.SetLeft(elp, p.X-50);
                Canvas.SetTop(elp, p.Y-25);
                cvs.Children.Add(elp);
            }
            this.Content = cvs;
        }
    }
}