ViewModel 的 List<model> 在 POST 方法中为空

Question

我正在做以下工作。当我运行代码时，我可以通过GET 方法从数据库中以正确的形式获取数据，并且可以在View 中正确显示它。但是，在编辑/更改某些数据时，比如Type，并在按下Save Data 时，会调用POST 方法，但作为参数的studentsViewModel 值是null。不知道为什么。

我在网站和网上其他地方也遇到过类似的问题。但他们没有我正在寻找的东西。另外，他们中的大多数人都在使用HTMLHelpers。

这里是sn-ps的代码：

控制器

public IActionResult StudentsDetails(int Id)
{
    try
    {
        var typeItems = _context.Type.Where(t => t.IsActive == true).Select(us => new SelectListItem { Value = us.TypeId.ToString(), Text = us.TypeName.ToString() }).ToList();
        ViewBag.TypeItems = typeItems;

        var studentsData = _context.Students
            .Where(r => r.StudentId == Id).ToList();

        var studentsViewModelData = new tudentsViewModel();

        studentsViewModelData.Students = studentsData;
        studentsViewModelData.StudentId = Id;

        return View(studentsViewModelData);
    }
    catch (Exception ex)
    {
        ....      
    }

    ....
}

[HttpPost]
public IActionResult StudentsDetails(int Id, StudentsViewModel studentsViewModel)
{
    try
    { 
        if(ModelState.IsValid)
        {
            ...
            studentsViewModel.StudentId = Id;
            ....     
        }
        catch(Exception ex)
        {
            ....
        }

        ....
    }
}

Details.cshtml

<div class="card-body card-padding">
    <form method="post" asp-action="Details" asp-controller="Students" id="studentsForm">
        ....
        <table id="data-table-basic" class="table table-striped table-vmiddle">                    
            @if(Model.Students.Count() != 0)
            {
                <thead>
                    <tr>
                        <th data-column-id="StudentNumber">Student Number</th>
                        <th data-column-id="Type">Student Type</th>
                    </tr>
                </thead>
            }
            <tbody>
                @for(int count = 0;count < Model.Students.Count; count++)
                {
                    <tr>
                        <td>                                           
                            @Model.Students[count].StudentNumber
                        </td>
                        <td>
                            <select asp-for="@Model.Students[count].TypeId" asp-items="@ViewBag.TypeItems" class="chosen" data-placeholder="Select student type" name="selectType" id="selectType">
                                <option value=""></option>
                            </select>        
                        </td>
                    </tr>                                          
                }
            </tbody>                       
        </table>
        <div>
            <button type="submit" value="Save">Save Data</button>
        </div>
    </form>
</div>

型号

public class Students
{
    [Key]
    public int StudentId { get; set; }

    public string StudentNumber { get; set; }

    [ForeignKey("Type")]
    public int TypeId { get; set; }
    public virtual Type Type { get; set; }

}

StudentsViewModel.cs

public class StudentsViewModel
{
    public int StudentId { get; set; }

    public List<Students> Students { get; set; }  
}

编辑：添加类型类

Type.cs

public class Type
{
    [Key]
    public int TypeId { get; set; }

    public string TypeName { get; set; }
    public bool IsActive { get; set; }
}

Answer 1

您不应将 id 和 name 明确设置为 select。删除name="selectType" id="selectType"。

<select asp-for="@Model.Students[count].TypeId" 
        asp-items="@ViewBag.TypeItems"
        class="chosen" 
        data-placeholder="Select student type">
    <option value=""></option>
</select>