c# - 无法将所选项目从 DropDownList 设置为控制器

Question

在我看来，DropDownList 显示了正确的字段，但是当我在“编辑或创建”中选择一个时，该字段将被保存/修改为 NULL。调试时我可以看到没有发送新值。我认为 ID 和 SurveyID 之间存在不匹配...

看法：

@model Project_ASP_2012.Models.QuestionGroup

@{
ViewBag.Title = "Edit";
}

<h2>Edit</h2>

@using (Html.BeginForm()) {
@Html.ValidationSummary(true)

<fieldset>
    <legend>QuestionGroup</legend>

    @Html.HiddenFor(model => model.ID)


    <div class="editor-label">
        @Html.LabelFor(model => model.Description)
    </div>
    <div class="editor-field">
        @Html.EditorFor(model => model.Description)
        @Html.ValidationMessageFor(model => model.Description)
    </div>

    <div class="editor-label">
        @Html.LabelFor(model => model.SurveyID, "Survey")
    </div>
    <div class="editor-field">
        @Html.DropDownList("Id", String.Empty)
        @Html.ValidationMessageFor(model => model.SurveyID)
    </div>

    <p>
        <input type="submit" value="Save" />
    </p>
</fieldset>
}

<div>
@Html.ActionLink("Back to List", "Index")
</div>

 @section Scripts {
@Scripts.Render("~/bundles/jqueryval")
}

模型：

public class Survey : IEntity
{


    [Key]
    [Display(Name = "SurveyID")]
    public int ID { get; set; }

    [Required(ErrorMessage = "Survey title is required.")]
    [Display(Name = "Survey Title")]
    [MaxLength(20, ErrorMessage = "Title cannot be longer than 20 characters.")]
    public string Title { get; set; }

    [MaxLength(50, ErrorMessage = "Description cannot be longer than 50 characters.")]
    public string Description { get; set; }

    public virtual ICollection<QuestionGroup> QuestionGroups { get; set; }

}

public class QuestionGroup : IEntity
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int ID { get; set; }

    [MaxLength(50, ErrorMessage = "Description cannot be longer than 50 characters.")]
    public string Description { get; set; }

    [Display(Name = "SurveyID")]
    public int? SurveyID { get; set; }

    public virtual Survey Survey { get; set; }

}

控制器：

    public ActionResult Edit(int id)
    {
        QuestionGroup questiongroup = unitOfWork.QuestionGroupRepository.GetById(id);
        if (questiongroup == null)
        {
            return HttpNotFound();
        }

        PopulateSurveysDropDownList(questiongroup.SurveyID);
        return View(questiongroup);
    }

    //
    // POST: /QuestionGroup/Edit/5

    [HttpPost]
    public ActionResult Edit(QuestionGroup questiongroup)
    {
        try
        {
           if (ModelState.IsValid)
            {
                unitOfWork.UoWContext.Entry(questiongroup).State = EntityState.Modified;
                unitOfWork.SaveChanges();
                return RedirectToAction("Index");
            }
        }
        catch (DataException)
        {
            ModelState.AddModelError("", "Unable to save changes. Try again, and if the problem persists see your system administrator.");
        }
        PopulateSurveysDropDownList(questiongroup.SurveyID);
        return View(questiongroup);
    }


    private void PopulateSurveysDropDownList(object selectedSurvey = null)
    {
        var surveyQuery = unitOfWork.SurveyRepository.Get(
            orderBy: q => q.OrderBy(d => d.Title));
        ViewBag.Id = new SelectList(surveyQuery, "Id", "Title", selectedSurvey);
    }

Answer 1

尝试改变：

private void PopulateSurveysDropDownList(object selectedSurvey = null)
{
    var surveyQuery = unitOfWork.SurveyRepository.Get(
        orderBy: q => q.OrderBy(d => d.Title));
    ViewBag.Id = new SelectList(surveyQuery, "Id", "Title", selectedSurvey);
}

....

<div class="editor-field">
    @Html.DropDownList("Id", String.Empty)
    @Html.ValidationMessageFor(model => model.SurveyID)
</div>

至：

private void PopulateSurveysDropDownList(object selectedSurvey = null)
{
    var surveyQuery = unitOfWork.SurveyRepository.Get(
        orderBy: q => q.OrderBy(d => d.Title));
    //don't provide the select value here.  you will bind to it in your view
    ViewBag.SurveySelectList = new SelectList(surveyQuery, "Id", "Title");
}

....

<div class="editor-field">
    //selected value will be whatever SurveyID is on your model.
    @Html.DropDownListFor(model => model.SurveyID, ViewBag.SurveySelectList, String.Empty)
    @Html.ValidationMessageFor(model => model.SurveyID)
</div>

希望有帮助。

Answer 2

在您看来，您有 Id 而不是 SurveyId 的下拉菜单。另外，您有两次 ID - 在下拉列表和隐藏字段中。

<div class="editor-field">
    @Html.DropDownList("Id", String.Empty)
    @Html.ValidationMessageFor(model => model.SurveyID)
</div>