Multithreading freezes UI - wpf

Question

I have a mutlithreaded WPF app. But after some background threads are launched, the UI is freezing (the display and the UI inputs).
Here is a simplified (but complete) example :
<Window x:Class="WPF_Multithreading.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Grid>
<Grid.RowDefinitions>
<RowDefinition/>
<RowDefinition Height="auto"/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition/>
<ColumnDefinition/>
</Grid.ColumnDefinitions>
<ListBox ItemsSource="{Binding Path=Scores, UpdateSourceTrigger=PropertyChanged}"/>
<ListBox Grid.Column="1" ItemsSource="{Binding Path=FrameRates, UpdateSourceTrigger=PropertyChanged}"/>
<Button Grid.Row="1"
Content="New Thread" Click="Button_NewThread_Click" />
<Button Grid.Row="1" Grid.Column="1"
Content="UI test" Click="Button_UItest_Click"/>
</Grid>
</Window>
code-behind :
using System.ComponentModel;
using System.Collections.ObjectModel;
using System.Threading;
namespace WPF_Multithreading
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window, INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private void RaisePropertyChanged(string propertyName)
{
if (this.PropertyChanged != null)
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
private List<Thread> threads;
ObservableCollection<int> _scores;
public ObservableCollection<int> Scores
{
get { return _scores; }
set { _scores = value; RaisePropertyChanged("Scores"); }
}
ObservableCollection<int> _framerates;
public ObservableCollection<int> FrameRates
{
get { return _framerates; }
set { _framerates = value; RaisePropertyChanged("FrameRates"); }
}
public MainWindow()
{
InitializeComponent();
DataContext = this;
Scores = new ObservableCollection<int>();
FrameRates = new ObservableCollection<int>();
threads = new List<Thread>();
}
~MainWindow()
{
foreach (Thread t in threads) t.Abort();
}
private void Button_NewThread_Click(object sender, RoutedEventArgs e)
{
Thread t = new Thread(backgroundRun);
Scores.Add(0);
FrameRates.Add(0);
t.Start(this);
threads.Add(t);
}
private void backgroundRun(object o)
{
MainWindow _this = (MainWindow)o;
int index = _this.Scores.Count()-1;
Random r = new Random();
DateTime prevDT = DateTime.Now;
int nbiterations = 0;
while(true)
{
// simulate computation
nbiterations = 0;
while((DateTime.Now-prevDT).Milliseconds < 50)
{
nbiterations++;
}
// Update UI
Dispatcher.Invoke(() =>
{
_this.Scores[index] = nbiterations;// r.Next();
_this.FrameRates[index] = (DateTime.Now - prevDT).Milliseconds;
});
prevDT = DateTime.Now;
}
}
private void Button_UItest_Click(object sender, RoutedEventArgs e)
{
MessageBox.Show("UI input ok");
}
}
}
In this example I have to launch a lot of threads to make UI freezing but in my more complex app (displaying 2D graphics), only 2 or 3 threads would make it happen (with freezing for more than 10 seconds sometimes).
My question is then : is there a way to ensure the Dispatcher to have enough resource to unfreeze UI (especially inputs) ?