Windows Phone 7.1: AutoResetEvent不能与Service Methods一起工作

将Service Reference添加到我的手机应用程序(例如http://www.deeptraining.com/webservices/weather.asmx?op=GetWeather)后，我尝试使用AutoResetEvent进行仿真同步方法调用。但是在调用WaitOne之后，方法Set永远不会被调用。为什么?是bug吗?

public partial class MainPage : PhoneApplicationPage
{
    private readonly AutoResetEvent _autoResetEvent = new AutoResetEvent(false);
    private string _result;
    // Constructor
    public MainPage()
    {
        InitializeComponent();
    }
    private void button1_Click(object sender, RoutedEventArgs e)
    {
        var weatherSoapClient = new WeatherSoapClient();
        weatherSoapClient.GetWeatherCompleted += weatherSoapClient_GetWeatherCompleted;
        weatherSoapClient.GetWeatherAsync("Pekin");
        _autoResetEvent.WaitOne(); // Program stop hire
        textBlock1.Text = _result;
    }
    void weatherSoapClient_GetWeatherCompleted(object sender, GetWeatherCompletedEventArgs e)
    {
        _result = e.Result;
        _autoResetEvent.Set(); // Never invoke! Why???
    }
}