哪里是对IObservable的引用

| VS2010，SL5和Reactive Extensions版本1.1.10605.1（2011-06-05）上未编译以下内容。 \'System.Collections.Generic.IEnumerable \'不包含\'ToObservable \'的定义，也没有扩展方法\'ToObservable \'接受类型为''System.Collections.Generic.IEnumerable \'的第一个参数找到（您是否缺少using指令或程序集引用？）

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;

public partial class MainPage : UserControl
{
    public MainPage()
    {
        InitializeComponent();
 }


 private void ButtonNormal_Click(object sender, RoutedEventArgs e)
    {
        //MyList.ItemsSource = GetModel();
        var items = new System.Collections.ObjectModel.ObservableCollection<string>();
        MyList.ItemsSource = items;
        foreach (string item in GetModel())
            items.Add(item);
    }

    private void ButtonRx_Click(object sender, RoutedEventArgs e)
    {
        var items = new System.Collections.ObjectModel.ObservableCollection<string>();
        MyList.ItemsSource = items;

        IObservable<string> observable = GetModel()
            .ToObservable(System.Concurrency.Scheduler.NewThread);
        observable.ObserveOnDispatcher().Subscribe(item => items.Add(item));
    }
}