ASP.NET Core MVC：列表索引的发布列表超出范围错误

Question

我需要在表单中发布对象列表，它包含另一个列表，但我收到了这个错误：

索引超出范围。必须是非负数且小于集合的大小

我的模型类：

public class SurveyQuestionDto
{
    public SurveyQuestionDto()
    {
        SurveyChoices = new List<SurveyChoiceDto>();
    }

    public int SurveyQuesId { get; set; }
    public DateTime? DateStart { get; set; }
    public DateTime? DateEnd { get; set; }

    [Required(ErrorMessage = "Required")]
    public short ChoicesType { get; set; }

    [StringLength(500, ErrorMessage = "MaxLength")]
    public string TextAnswer { get; set; }

    public virtual List<SurveyChoiceDto> SurveyChoices { get; set; }
}

public class SurveyChoiceDto
{
    [Required(ErrorMessage = "Required")]
    public int? LanguageId { get; set; }

    [Required(ErrorMessage = "Required"),
    StringLength(200, ErrorMessage = "MaxLength")]
    public string Title { get; set; }

    public long ChoiceId { get; set; }
    public int SurveyQuesId { get; set; }
    public long Count { get; set; }
    public bool MarkedToDelete { get; set; }
    public bool IsSelected { get; set; }
}

我的看法：

@model List<SurveyQuestionDto>

@if (Model != null && Model.Count() > 0)
{
    <form action="@Url.Action("Answer", "Survey", new { typeId })" method="post" class="form form-horizontal" enctype="multipart/form-data">
        @Html.AntiForgeryToken()
        @Html.ValidationSummary(true)

        @foreach (var question in Model)
        {
            int i = 0;
            @Html.HiddenFor(m => m[i].SurveyQuesId, new { Value = question.SurveyQuesId })
            <div style="background:#cccccc; padding:20px; margin:20px;">
                <h2>@question.Title</h2>
                @if (question.ChoicesType == (int)ChoicesTypeEnum.Text)
                {
                    @Html.TextAreaFor(m => m[i].TextAnswer, new Dictionary<string, object> { { "data-val", "true" }, { "data-val-required", _loc["Required"] } })
                }
                else
                {
                    <ul style="list-style-type:none;">
                        @foreach (var choice in question.SurveyChoices)
                        {
                            int a = 0;
                            <li>
                                @Html.HiddenFor(m => m[i].SurveyChoices[a].ChoiceId, new { Value = choice.ChoiceId })
                                @if (question.ChoicesType == (int)ChoicesTypeEnum.SingleChoice)
                                {
                                    @Html.RadioButtonFor(m => m[i].SurveyChoices[a].IsSelected, null, new Dictionary<string, object> { { "data-val", "true" }, { "data-val-required", _loc["Required"] } })
                                }
                                else
                                {
                                    @Html.CheckBoxFor(m => m[i].SurveyChoices[a].IsSelected, new Dictionary<string, object> { { "data-val", "true" }, { "data-val-required", _loc["Required"] } })
                                }
                                <label>@choice.Title</label>
                            </li>
                            a++;
                        }
                    </ul>
                }
            </div>
            i++;
        }
        <button type="submit">Submit now</button>
    </form>
}

模型对象已经有了它的值：

public async Task<IActionResult> Index(int id)
{
    return View(await _rep.GetSurveyQuestionsWithChoices(id));
}

问题从这行代码开始

 m[i].SurveyChoices[a].ChoiceId

有什么想法吗？

Answer 1

您的代码中有错误

@foreach (var question in Model)
{
            int i = 0;
 @Html.HiddenFor(m => m[i].SurveyQuesId, new { Value = question.SurveyQuesId })

对于每个项目，您将始终拥有 i=0，因为它会重复分配零。

用for循环会好很多

@for(var i=0; i < Model.Count; i++)
{
 @Html.HiddenFor(m => m[i].SurveyQuesId, new { Value = @Model[i].SurveyQuesId })
....and so on

代码中的第二个 foreach 循环也是如此。应该是

@for (var a=0;  a < @Model[i].SurveyChoices.Count; a++)

Answer 2

你的代码有一些错误，变量i，a永远不会改变。 您应该在 foreach 循环中手动处理此问题，如下所示：

@foreach (var question in Model)
{
   int i = 0;
   // rest of your code
   i++;
}

或者您可以使用for 循环而不是foreach：

for(int index=0;index<Model.Count;index++){...}

除了这些，你可以在你的代码中省略这个 if 语句，因为当没有项目时，for 循环不会运行：

@if (Model != null && Model.Count() > 0)

或者您可以使用Any()，如下所示：

@if (Model.Any()){...}

请记住，如果您的存储中没有数据，您应该返回空列表而不是null