Wpf BackgroundWorker WorkerSupportsCancellation CancellationPending

//xaml
<Window x:Class="WpfApp37.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:WpfApp37" WindowState="Maximized"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid TextBlock.FontSize="16">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <StackPanel Grid.Row="0" Orientation="Horizontal" Margin="6">
            <TextBlock Text="From:"/>
            <TextBox Margin="10,2,2,2" Width="120" MaxLength="10" x:Name="_from"/>
            <TextBlock Text="To:" Margin="20,0,0,0"/>
            <TextBox Margin="10,2,2,2" Width="120" MaxLength="10" x:Name="_to"/>
        </StackPanel>
        <StackPanel Grid.Row="1" Orientation="Horizontal" Margin="6">
            <Button Content="Calculate" x:Name="_calcBtn" Padding="4" Click="calc_Click" />
            <Button Content="Cancel" x:Name="_cancelBtn" Padding="4" Click="_cancelBtn_Click"/>
        </StackPanel>
        <ProgressBar x:Name="_progressBar" Grid.Row="2" Height="30" Margin="4"/>
        <TextBlock Grid.Row="3" x:Name="_result" FontSize="20" Margin="6" HorizontalAlignment="Center"/>
         
    </Grid>
</Window>


//cs
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.Linq.Expressions;
using System.Text;
using System.Threading;
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 WpfApp37
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        BackgroundWorker _bgWorker;
        int _cnt = 0;
        public MainWindow()
        {
            InitializeComponent();
        }

        private void calc_Click(object sender, RoutedEventArgs e)
        {
            _bgWorker = new BackgroundWorker();
            _bgWorker.WorkerSupportsCancellation = true;
            _bgWorker.DoWork += _bgWorker_DoWork;
            _bgWorker.RunWorkerCompleted += _bgWorker_RunWorkerCompleted;
            _calcBtn.IsEnabled = false;
            _cancelBtn.IsEnabled = true;
            _result.Text = "Calculating...";
            var data = new PrimeInputData()
            {
                First = int.Parse(_from.Text),
                Last = int.Parse(_to.Text)
            };
            _bgWorker.RunWorkerAsync(data);
        }

        private void _bgWorker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            if (e.Cancelled)
            {
                _result.Text = $"Cancelled Primes:{_cnt}";
            }
            else
            {
                _result.Text = $"Total Primes:{e.Result}";
            }
            _calcBtn.IsEnabled = true;
            _cancelBtn.IsEnabled = false;
        }

        private void _bgWorker_DoWork(object sender, DoWorkEventArgs e)
        {
            var data = e.Argument as PrimeInputData;
            if (data != null)
            {
                for (int i = data.First; i <= data.Last; i++)
                {
                    if (_bgWorker.CancellationPending)
                    {
                        e.Cancel = true;
                        break;
                    }
                    int limit = (int)Math.Sqrt(i);
                    bool isPrime = true;
                    for (int j = 2; j <= limit; j++)
                    {
                        if (i % j == 0)
                        {
                            isPrime = false;
                            break;
                        }
                    }
                    if (isPrime)
                    {
                        _cnt++;
                    }
                }
                e.Result = _cnt;
            }
        }

        private void _cancelBtn_Click(object sender, RoutedEventArgs e)
        {
            _bgWorker.CancelAsync();
            _calcBtn.IsEnabled = true;
            _cancelBtn.IsEnabled = false;
        }
    }

    class PrimeInputData
    {
        public int First { get; set; }
        public int Last { get; set; }
    }
}