如何在表單發佈後保留模型數據

Question

當表單最初顯示時，我會顯示問題及其答案。我顯示多個單選按鈕來回答。所以用戶只能選擇一個答案。當我提交我的表單和操作方法調用時會發生問題。當我看到表格發佈後答案模型變爲空時。因此，請指導我如何編寫代碼，以便表單發佈時答案模型不應該爲null。

這是完整的代碼。請通過代碼，如果需要更改結果答案模型將不會爲空當表單將被髮布到操作方法。

我的ViewModels代碼

namespace ViewModels 
{ 
    public class Question 
    { 
     public int ID { set; get; } 
     public string QuestionText { set; get; } 

     public List<Answer> Answers { set; get; } 
     [Required] 
     public string SelectedAnswer { set; get; } 

     public Question() 
     { 
      Answers = new List<Answer>(); 
     } 
    } 

    public class Answer 
    { 
     public int ID { set; get; } 
     public string AnswerText { set; get; } 
    } 

    public class Evaluation 
    { 
     public List<Question> Questions { set; get; } 
     public Evaluation() 
     { 
      Questions = new List<Question>(); 
     } 
    } 
} 

controller code 

public ActionResult Index() 
     { 
      var evalVM = new Evaluation(); 

      //the below is hardcoded for DEMO. you may get the data from some 
      //other place and set the questions and answers 

      var q1 = new Question { ID = 1, QuestionText = "What is your favourite language" }; 
      q1.Answers.Add(new Answer { ID = 12, AnswerText = "PHP" }); 
      q1.Answers.Add(new Answer { ID = 13, AnswerText = "ASP.NET" }); 
      q1.Answers.Add(new Answer { ID = 14, AnswerText = "Java" }); 
      evalVM.Questions.Add(q1); 

      var q2 = new Question { ID = 2, QuestionText = "What is your favourite DB" }; 
      q2.Answers.Add(new Answer { ID = 16, AnswerText = "SQL Server" }); 
      q2.Answers.Add(new Answer { ID = 17, AnswerText = "MySQL" }); 
      q2.Answers.Add(new Answer { ID = 18, AnswerText = "Oracle" }); 
      evalVM.Questions.Add(q2); 

      return View(evalVM);   
     } 

[HttpPost] 
     public ActionResult Index(Evaluation model) 
     { 
      if (ModelState.IsValid) 
      { 
       foreach (var q in model.Questions) 
       { 
        if(q.Answers==null) 
        { 
         // Answers is null 
        } 
        var qId = q.ID; 
        var selectedAnswer = q.SelectedAnswer; 
        // Save the data 

       } 
       return RedirectToAction("ThankYou"); //PRG Pattern 
      } 
      //reload questions 
      return View(model); 
     } 

index.cshtml code 

@model ViewModels.Evaluation 
<h2>Quiz 24</h2> 
@using (Html.BeginForm()) 
{ 
    @Html.EditorFor(x=>x.Questions) 
    <input type="submit" /> 
} 

and view code which is stored in EditorTemplates folder 

@model ViewModels.Question 
<div> 
    @Html.HiddenFor(x=>x.ID) 
    <h3> @Model.QuestionText </h3> 

    @foreach (var a in Model.Answers) 
    { 
     <p> 
      @Html.RadioButtonFor(b=>b.SelectedAnswer,a.ID) @a.AnswerText 
     </p> 

    } 
</div> 

的問題是在這裏

[HttpPost] 
     public ActionResult Index(Evaluation model) 
     { 
      if (ModelState.IsValid) 
      { 
       foreach (var q in model.Questions) 
       { 
        if(q.Answers==null) 
        { 
         // Answers is null 
        } 
        var qId = q.ID; 
        var selectedAnswer = q.SelectedAnswer; 
        // Save the data 

       } 
       return RedirectToAction("ThankYou"); //PRG Pattern 
      } 
      //reload questions 
      return View(model); 
     } 

q.Answers == NULL越來越空，當我張貼的形式。我想知道如何以這種方式編寫代碼時，表單將被髮布到操作然後答案應該爲空。 很多人告訴我，我需要手動重建模型，因爲Answers將始終爲空。 MVC中是否存在任何機制來堅持所有的數據並正確地反序列化它以在表單發佈時進行建模。

Answer 1

AnswerText通常我會建議使用Answer模型的另一個嵌套Editor Template，但在這種情況下，它會導致問題正確地構造單選按鈕組，並向後檢索SelectedAnswer的值。舉一個簡單的解決方案，我會適應Editor Template爲Question模型通過以下方式：

<div> 
    @Html.HiddenFor(x=>x.ID) 
    <h3> @Model.QuestionText </h3> 
    <div> 
     @for (int i = 0; i < Model.Answers.Count; i++) 
     { 
      <p> 
       @Html.RadioButtonFor(model => model.SelectedAnswer, Model.Answers[i].AnswerText @Model.Answers[i].AnswerText 
       @Html.HiddenFor(model => model.Answers[i].ID) 
       @Html.HiddenFor(model => model.Answers[i].AnswerText) 
      </p> 
     } 
    </div> 
</div> 

這將創建下列HTML（跳過不相關的屬性）：

<input type="radio" name="Questions[0].SelectedAnswer" ...> 
<input type="hidden" name="Questions[0].Answers[0].ID" ...> 
<input type="hidden" name="Questions[0].Answers[0].AnswerText" ...> 

輸出將使MVC到妥善重建Answer集合。

更新

在你原來的代碼，你要找的Answer.AnswerText，所以我已經更新了代碼，包括它代替Answer.ID的。

希望這會有所幫助。