Asp.net 其他型号的mvc5下拉列表

第一次使用MVC开发应用程序时，遇到了一个问题，即在student create视图的下拉列表中填充教师列表

我有两种型号

public class Student
{

    [Required]
    [Display(Name = "ID")]
    public int ID { get; set; }
    [Required]
    [StringLength(100)]
    [DataType(DataType.Text)]
    [Display(Name = "First Name")]
    public string FirstName { get; set; }
    [Required]
    [StringLength(100)]
    [DataType(DataType.Text)]
    [Display(Name = "Last Name")]
    public string LastName { get; set; }
    [Required]
    [DataType(DataType.Date)]
    [Display(Name = "Date of Birth")]
    public DateTime? DateOfBirth { get; set; }
    [Required]
    [DataType(DataType.EmailAddress)]
    [Display(Name = "Email Address")]
    public string EmailAddress { get; set; }
    [Required]
    [DataType(DataType.PhoneNumber)]
    [Display(Name = "Contact Number")]
    public string ContactNumber { get; set; }
    [Required]
    [StringLength(100)]
    [DataType(DataType.Text)]
    [Display(Name = "Parent/Guardian Name")]
    public string ParentGuardianName { get; set; }

    public Teacher Teacher { get; set; }


    public MembershipOption MembershipOption { get; set; }

    public ICollection<Progress> Progress { get; set; }
    public ICollection<Report> Report { get; set; }
    [DataType(DataType.Date)]
    [Display(Name = "Signed Up Date")]
    public DateTime? SignedUpDate { get; set; }
    [Display(Name = "Active")]
    public bool Active { get; set; }



}

 public class Teacher
{
    [Required]
    [Display(Name = "ID")]
    public int ID { get; set; }
    [Required]
    [StringLength(100)]
    [DataType(DataType.Text)]
    [Display(Name = "First Name")]
    public string FirstName { get; set; }
    [StringLength(100)]
    [DataType(DataType.Text)]
    [Display(Name = "Last Name")]
    public string LastName { get; set; }
    [Display(Name = "Active")]
    public bool Active { get; set; }
    [Required]
    [EmailAddress]
    [Display(Name = "Email")]
    public string Email { get; set; }

}

}

我有学生模型的创建视图，如下所示

    <div class="form-group">
    @Html.LabelFor(model => model.ParentGuardianName, htmlAttributes: new { @class = "control-label col-md-2" })
    <div class="col-md-10">
        @Html.DropDownListFor(m => m.Teacher, // 1. Store selected value in Model.State;
                                         // when page is rendered after postback,
                                         // take selected value from Model.State.

                                       // 2. Take list of values from Model.States
                                       Model.Teacher,

                                       // 3. Text for the first 'default' option
                                       "- Please select a Teacher -",

                                       //4. A class name to assign to <select> tag
                                       new { @class = "form-control" })
        @Html.ValidationMessageFor(model => model.Teacher, "", new { @class = "text-danger" })
    </div>
</div>

教师的dropdownlist由于教师为null而引发异常

如何将教师下拉列表添加到此学生创建cshtml

---

我假设我需要在调用学生模型时找到教师，但不确定它需要放在studentscontroller中的什么位置？

第二个参数应该是SelectListItem的列表，以便帮助器方法可以使用该列表生成选择选项。但是您正在尝试通过教师导航属性而不是该属性

您应该为视图创建一个视图模型，该视图具有接受表单输入值的属性。由于要向视图添加下拉列表，请向视图模型添加两个属性。一个用于选定值，一个用于显示下拉列表所需的项目列表

public class CreateStudent
{
    [Required]
    [StringLength(100)]
    [Display(Name = "First Name")]
    public string FirstName { get; set; }

    [Required]
    [StringLength(100)]
    [Display(Name = "Last Name")]
    public string LastName { get; set; } 

    // to do : Add other properties AS NEEDED BY THE VIEW
    // Do not just copy all the properties from Entity class

    public List<SelectListItem> Teachers { set; get; }

    [Required]
    [DisplayName("Select Teacher")]
    public int SelectedTeacherId { set; get; }
}

现在在您的视图中，它是视图模型的强类型，我们可以使用dropdownlistforhelper方法来渲染SELECT元素

@model CreateStudent
@using(Html.BeginForm())
{
    @Html.LabelFor(a=>a.FirstName)
    @Html.TextBoxFor(a=>a.FirstName)

    @Html.LabelFor(a=>a.LastName)
    @Html.TextBoxFor(a=>a.LastName)

    @Html.LabelFor(a=>a.SelectedTeacherId)
    @Html.DropDownListFor(a=>a.SelectedTeacherId , Model.Teachers, "select one")

    <input type="submit" />
}

---

您不能将a绑定到复杂对象，而教师就是这样的对象-a绑定到a后发回单个简单值。建议您查看中的代码以了解如何生成dropdownlist感谢您的帮助。要添加的是，如何在此下拉列表中设置所选索引？在“获取”操作中，将SelectedTeacherId设置为其中一个TeacherId值，助手将选择该选项。

@model CreateStudent
@using(Html.BeginForm())
{
    @Html.LabelFor(a=>a.FirstName)
    @Html.TextBoxFor(a=>a.FirstName)

    @Html.LabelFor(a=>a.LastName)
    @Html.TextBoxFor(a=>a.LastName)

    @Html.LabelFor(a=>a.SelectedTeacherId)
    @Html.DropDownListFor(a=>a.SelectedTeacherId , Model.Teachers, "select one")

    <input type="submit" />
}

[HttpPost]
public ActionResult Create(CreateStudent model)
{
  if(ModelState.IsValid)
  { 
     var s = new Student { FirstName=model.FirstName };
     s.LastName = model.LastName;
     // to do : Map other property values as well from view model
     db.Students.Add(s);
     db.SaveChanges();
     return RedirectToAction("Index");
  }
  //Make sure to RELOAD the Teachers list for dropdown
  model.Teachers = GetTeachers();
  return View(model);
}