Web服务功能等待，直到执行另一个Web服务功能

Question

我是Async的新手。我有一个Web服务返回我（在Windows Phone 8应用程序中）一些问题和DB的一些答案（2个功能）当调用GetQuestions函数时，我把问题放在一个ListBox和我想为每个问题返回一些ansewrs（GetAnswers方法），但我希望在listBox中像这样： 问题1 ANSWER1 ANSWER2 ... 问题2 ANSWER1 ANSWER2

下面的鳕鱼把我放在ListBox中 问题1 问题2 问题3 ANSWER1 ANSWER2 ...

如何停止GetQuestions方法并在Listbox答案中添加每个问题？

      private  void PhoneApplicationPage_Loaded(object sender, RoutedEventArgs e)
    {
           ServiceReference1.Service1SoapClient client = new  ServiceReference1.Service1SoapClient();
           client.GetQuestionsCompleted += client_GetQuestionsCompleted;
           client.GetQuestionsAsync(ID);
    }



    void client_GetQuestionsCompleted(object sender, ServiceReference1.GetQuestionsCompletedEventArgs e)
    {

        questions = e.Result.Cast<string>().ToList();

            for(int i=0;i<questions.Count;i+=2)
            {

     Listbox1.Items.Add(questions[i]);

             ServiceReference1.Service1SoapClient client2 = new ServiceReference1.Service1SoapClient();
             client2.GetAnswersCompleted += client2_GetAnswersCompleted;
             client2.GetAnswersAsync(questions[i]);
            }
    }

        void client2_GetAnswersCompleted(object sender, ServiceReference1.GetAnswersCompletedEventArgs e)
        {
          answers = e.Result.Cast<string>().ToList();

              for (int i = 1; i < answers.Count; i += 2)
                     Listbox1.Items.Add(answers[i]);

     }