无法在ShowMessageAsync上使用await(Can't use await on ShowMessageAsync)

 我正在使用Mahapp并且我正在尝试等待对话框的结果，但是编译器强调了ShowMessageAsync并显示了我： 
 
 
 
 当前上下文中不存在ShowMessageAsync 
 
 
 这是代码： 
 
private async void ShowMessageBox(object sender, RoutedEventArgs e)
{
    var result = await ShowMessageAsync("Hello!", "Welcome to the world of metro!", 
        MahApps.Metro.Controls.MessageDialogStyle.AffirmativeAndNegative);
    if (result == MessageDialogResult.Affirmative)
    {
        this.ShowMessageAsync("Result", "You said: OK");
    }
    else
    {
        this.ShowMessageAsync("Result", "You said: CANCEL");
    }
}

I'm using Mahapp and I'm trying to wait for the result of the dialog, but the compiler underlined ShowMessageAsync and display me: 
 
 
 
ShowMessageAsync doesn't exist in the current context
 
 
this is the code:
 
private async void ShowMessageBox(object sender, RoutedEventArgs e)
{
    var result = await ShowMessageAsync("Hello!", "Welcome to the world of metro!", 
        MahApps.Metro.Controls.MessageDialogStyle.AffirmativeAndNegative);
    if (result == MessageDialogResult.Affirmative)
    {
        this.ShowMessageAsync("Result", "You said: OK");
    }
    else
    {
        this.ShowMessageAsync("Result", "You said: CANCEL");
    }
}

原文：https://stackoverflow.com/questions/37319540