简单通用应用程序MVVM的例子不工作的按钮点击

文本绑定良好。这很简单。如果你发现少了什么，请告诉我。现在，当我点击按钮SHOWMESSAGE没有被调用。我遵循了这个例子，唯一不同的是它是WPF，并在XAML而不是代码中设置虚拟机。看到遗漏了什么吗?

http://www.codeproject.com/Articles/126249/MVVM-Pattern-in-WPF-A-Simple-Tutorial-for-Absolute

XAML

    <StackPanel>
            <TextBlock Text="Get Quote Questions" Style="{StaticResource TitleTextBlockStyle}" />
            <TextBox Text="{x:Bind ViewModel.City}" PlaceholderText="City " />
            <TextBox Text="{x:Bind ViewModel.State}" PlaceholderText="State Code " />
            <TextBox Text="{x:Bind ViewModel.ZipCode}" PlaceholderText="Zip Code " />
            <Button Content="Get Quote Questions" Command="{Binding ViewModel.GetQuoteQuestionsCommand, Mode=OneWay}" />
        </StackPanel>

背后的代码:

      public GetQuoteQuestionsVm ViewModel { get; private set; }
    public GetQuoteQuestions()
    {
        this.InitializeComponent();
        var vm = new GetQuoteQuestionsVm();
        this.ViewModel = vm;
    }

ViewModel:

  public class GetQuoteQuestionsVm : NotifyPropertyChanged
{
    private string city;
    public string City
    {
        get { return city; }
        set { Set(ref city, value); }
    }
    private ICommand getQuoteQuestionsCommand;
    public ICommand GetQuoteQuestionsCommand
    {
        get
        {
            return getQuoteQuestionsCommand;
        }
        set
        {
            getQuoteQuestionsCommand = value;
        }
    }
    public GetQuoteQuestionsVm()
    {
        City = "Woodbridge";
        GetQuoteQuestionsCommand = new RelayCommand(new Action<object>(ShowMessage));
    }
    public async void ShowMessage(object obj)
    {
        var dialog = new MessageDialog("Your message here");
        await dialog.ShowAsync();
    }
}

RelayCommand:

    public class RelayCommand : ICommand
{
    private Action<object> _action;
    public RelayCommand(Action<object> action)
    {
        _action = action;
    }
    public bool CanExecute(object parameter)
    {
        return true;
    }
    public event EventHandler CanExecuteChanged;
    public void Execute(object parameter)
    {
        if (parameter != null)
        {
            _action(parameter);
        }
        else
        {
            _action("Hello World");
        }
    }
}