WPF ItemsControl IsItemsHost true

//customzie control
//xaml
<UserControl x:Class="WpfApp306.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:WpfApp306"
             mc:Ignorable="d" 
             d:DesignHeight="450" d:DesignWidth="800">
    <Grid>
        <Ellipse Fill="{Binding ElpBrush,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"/>
        <TextBlock HorizontalAlignment="Stretch" 
                   VerticalAlignment="Center" 
                   Text="{Binding ElpStr,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"/>
    </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 WpfApp306
{
    /// <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.Brown)));



        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(""));

    }
}

ItemsHost

//xaml
<Window x:Class="WpfApp306.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:WpfApp306"
        mc:Ignorable="d" WindowState="Maximized"
        Title="MainWindow" Height="450" Width="800">

    <Grid MouseDown="Grid_MouseDown">
        <ListBox BorderBrush="Blue" BorderThickness="3" x:Name="lbx">
            <Style TargetType="{x:Type ListBoxItem}">
                <Setter Property="Height" Value="500"/>
                <Setter Property="ContentTemplate">
                    <Setter.Value>
                        <DataTemplate>
                            <Canvas IsItemsHost="True" 
                                    Width="{Binding Path=ActualWidth,RelativeSource={RelativeSource AncestorType=ListBoxItem}}" 
                                    Height="500"/>
                        </DataTemplate>
                    </Setter.Value>
                </Setter>
            </Style>
        </ListBox>
    </Grid>
</Window>



//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 WpfApp306
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        private int idx = 0;
        private Random rnd { get; set; }

        public MainWindow()
        {
            rnd= new Random();
            InitializeComponent();
        }
        
        private void Grid_MouseDown(object sender, MouseButtonEventArgs e)
        {
            ElpTbk elpTbk = new ElpTbk();
            Color cl = new Color();
            cl.A = 255;
            cl.R = (byte)rnd.Next(0, 255);
            cl.G = (byte)rnd.Next(0,255);
            cl.B = (byte)rnd.Next(0, 255);
            elpTbk.ElpBrush = new SolidColorBrush(cl);
            elpTbk.Width = 200;
            elpTbk.Height = 100;
            elpTbk.ElpStr = $"{++idx}___({e.GetPosition(this).X},{e.GetPosition(this).Y})";
            Canvas.SetLeft(elpTbk, e.GetPosition(this).X);
            Canvas.SetTop(elpTbk, e.GetPosition(this).Y);
            lbx.Items.Add(elpTbk);
        }
    }
}