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Keep the UI from becoming non-responsive in single threaded application which performs a long operation

<Window x: Class="WpfApplication1.Window1"     xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"     Title="Prime Numbers" Width="260" Height="75">   <StackPanel Orientation="Horizontal" VerticalAlignment="Center" >     <Button Content="Start" Click="StartOrStop" Name="startStopButton" Margin="5,0,5,0"/>     <TextBlock Margin="10,5,0,0">Number:</TextBlock>     <TextBlock Name="numberTextBlock" Margin="4,5,0,0">3</TextBlock>   </StackPanel> </Window> //File:Window.xaml.vb Imports System Imports System.Windows Imports System.Windows.Controls Imports System.Windows.Threading Imports System.Threading Namespace WpfApplication1   Public Partial Class Window1     Inherits Window     Public Delegate Sub NextPrimeDelegate()     Private num As Long = 1     Private continueCalculating As Boolean = False     Public Sub New()       MyBase.New()       InitializeComponent()     End Sub     Public Sub StartOrStop(sender As Object, e As EventArgs)       If continueCalculating Then         continueCalculating = False         startStopButton.Content = "Resume"       Else         continueCalculating = True         startStopButton.Content = "Stop"         startStopButton.Dispatcher.BeginInvoke(DispatcherPriority.Normal, New NextPrimeDelegate(AddressOf CheckNextNumber))       End If     End Sub     Public Sub CheckNextNumber()       numberTextBlock.Text = num.ToString()       num += 2       If continueCalculating Then         startStopButton.Dispatcher.BeginInvoke(System.Windows.Threading.DispatcherPriority.SystemIdle, New NextPrimeDelegate(AddressOf Me.CheckNextNumber))       End If     End Sub   End Class End Namespace