WPF C#

    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="WPF" Width="220" Height="104" >
    
        Start
        
            Biggest Prime Found:
            
        
    

//File:Window.xaml.cs
using System;
using System.Windows;
using System.Windows.Threading;
namespace WpfApplication1
{
    public partial class Window1 : Window
    {
        private bool continueCalculating = false;
        public Window1() : base()
        {
            InitializeComponent();
        }
        private void StartStop_Click(object sender, RoutedEventArgs e)
        {
            if(continueCalculating)
            {
                continueCalculating = false;
                btnStartStop.Content = "Start";
            }
            else
            {
                continueCalculating = true;
                btnStartStop.Content = "Stop";
                this.Dispatcher.BeginInvoke(DispatcherPriority.Normal,new Action(UpdateNumber), 3);
            }
        }
        public void UpdateNumber(int current)
        {
            txtNumber.Text = current.ToString();
            if(continueCalculating)
            {
                this.Dispatcher.BeginInvoke(DispatcherPriority.SystemIdle,new Action(UpdateNumber), current + 2);
            }
        }
    }
}