WPF ProgressBar show value

//xaml
<Window x:Class="WpfApp424.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:WpfApp424"
        mc:Ignorable="d"
        WindowState="Maximized"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <ProgressBar x:Name="progressBar1" 
                     IsIndeterminate="False"
                     Minimum="0"
                     Maximum="100"
                     Height="50"
                     HorizontalAlignment="Stretch"
                     VerticalAlignment="Bottom"/>
        <TextBlock Text="{Binding Path=Value,ElementName=progressBar1,StringFormat={}{0:0}%}"
                   VerticalAlignment="Bottom"
                   HorizontalAlignment="Center"
                   FontSize="20"
                   Foreground="Red"/>
    </Grid>
</Window>


//xaml.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 WpfApp424
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        int idx = 0;
        public MainWindow()
        {
            InitializeComponent();
            System.Timers.Timer tmr = new System.Timers.Timer();
            tmr.Elapsed += Tmr_Elapsed;
            tmr.Interval = 100;
            tmr.Start();
        }

        private void Tmr_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
        {
            Dispatcher.BeginInvoke(new Action(() =>
            {
                progressBar1.Value = ++idx;
                if(idx==100)
                {
                    MessageBox.Show("Completed!");
                }
            }));
        }
    }
}