Form1.cs

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        _repo = new ObservableHelper<Customer>();

        ControlScheduler cs = new ControlScheduler(this);//WE NEED THIS.
        Observable.FromEventPattern(h => textBox1.TextChanged += h,
                                    h => textBox1.TextChanged -= h)//tell Rx about our event
            .Throttle(TimeSpan.FromMilliseconds(500), cs)///throttle
            .Do(a => SearchList(textBox1.Text))//do this method 
            .Subscribe();//this is where we tell it to begin all the magic
    }
    IObservableHelper<Customer, Database1Entities> _repo;
    void SearchList(string query)
    {
        listBox1.Items.Clear();
        listBox1.BeginUpdate();
        var getfn = _repo.GetAllAsObservables
            (d => d.Customers.Where(c => c.FirstName.Contains(query) || c.LastName.Contains(query)));
        getfn.ObserveOn(this).Subscribe(custList =>
            {
                foreach (var item in custList)
                {
                    listBox1.Items.Add(string.Format("{0} {1},{2}", item.FirstName, item.LastName, item.Age));
                }
                listBox1.EndUpdate();
            });
        //we could also do ObserveOn(WindowsFormsSynchronizationContext.Current)
    }
}