CancellationTokenSource Cancel IsCancellationRequested

//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" Padding="4" Margin="10,0,0,0" IsEnabled="False"
                    x:Name="_cancelBtn" Click="_cancelBtn_Click" />
        </StackPanel>
        <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
    { 
        CancellationTokenSource _cts; 

        public MainWindow()
        {
            InitializeComponent(); 
        }
         
        static int CountPrimes(int from, int to,CancellationToken ct)
        { 
            int total = 0;
            for (int i = from; i <= to; i++)
            {
                if (ct.IsCancellationRequested)
                {
                    return total;
                }
                bool isPrime = true;
                int limit = (int)Math.Sqrt(i);
                for (int j = 2; j <= limit; j++)
                {
                    if (i % j == 0)
                    {
                        isPrime = false;
                        break;
                    } 
                }
                if (isPrime)
                {
                    total++;
                }
            }
            return total;
        }

        private void calc_Click(object sender, RoutedEventArgs e)
        { 
            _result.Text = "Calculating...";
            int first=int.Parse(_from.Text),last=int.Parse(_to.Text);
            calcBtn.IsEnabled = false;
            _cancelBtn.IsEnabled = true;
            _cts=new CancellationTokenSource();
            ThreadPool.QueueUserWorkItem(x =>
            {
                int total = CountPrimes(first, last, _cts.Token);
                Dispatcher.BeginInvoke(new Action(() =>
                {
                    _result.Text = _cts.IsCancellationRequested ? $"Cancelled to :{total}" :
                    $"Finished Total Primes :{total}"; 
                    _cancelBtn.IsEnabled = false;
                    calcBtn.IsEnabled = true;
                }));
            }); 
        }

        private void _cancelBtn_Click(object sender, RoutedEventArgs e)
        {
            if (_cts != null)
            {
                _cts.Cancel(); 
            }
        }
    }
}