Asp.net mvc下拉列表给出空引用错误
本文关键字：引用 错误 net mvc 下拉列表 Asp | 更新日期: 2023-09-27 18:13:20
我正试图填充下拉列表，然后将其发布到数据库，但由于某种原因，它给出了错误，请建议。
控制器 public class StudentController : BaseController
{
private List<SelectListItem> _gendersList;
[HttpGet]
public ActionResult Create()
{
var model = new CreateStudent();
_gendersList = new List<SelectListItem>()
{
new SelectListItem { Text = Constants.Gender.Boy, Value = Constants.Gender.Boy},
new SelectListItem { Text = Constants.Gender.Girl, Value = Constants.Gender.Girl},
};
model.Genders = _gendersList;
return View(model);
}
[HttpPost]
public ActionResult Create(CreateStudent student)
{
var result = true;
_gendersList = new List<SelectListItem>()
{
new SelectListItem { Text = Constants.Gender.Boy, Value = Constants.Gender.Boy},
new SelectListItem { Text = Constants.Gender.Girl, Value = Constants.Gender.Girl},
};
if (ModelState.IsValid)
{
result = _student.Insert(mappedStudent);
if (result)
{
return RedirectToAction("Index");
}
else
{
TempData["Message"] = "Failed to create new student";
return View();
}
}
return View("Create");
}
}
@model CreateStudent
@{
ViewBag.Title = "Create";
Layout = "~/Views/Shared/_Layout.cshtml";
}
<script src="~/Scripts/jquery-3.1.1.js"></script>
<script src="~/Scripts/jquery.validate.min.js"></script>
<script src="~/Scripts/jquery.validate.unobtrusive.js"></script>
<h2>Create</h2>
@{
if (TempData["Message"] != null)
{
<h3>
@TempData["Message"].ToString()
</h3>
}
}
@{
}
@using (Html.BeginForm("Create","Student",FormMethod.Post))
{
@Html.AntiForgeryToken()
<div class="form-horizontal">
<h4>Student</h4>
<hr />
@Html.ValidationSummary(true, "", new { @class = "text-danger" })
<div class="form-group">
@Html.LabelFor(model => model.FirstName, htmlAttributes: new { @class = "control-label col-md-2" })
<div class="col-md-4">
@Html.EditorFor(model => model.FirstName, new { htmlAttributes = new { @class = "form-control" } })
@Html.ValidationMessageFor(model => model.FirstName, "", new { @class = "text-danger" })
</div>
</div>
<div class="form-group">
@Html.LabelFor(model => model.MiddleName, htmlAttributes: new { @class = "control-label col-md-2" })
<div class="col-md-4">
@Html.EditorFor(model => model.MiddleName, new { htmlAttributes = new { @class = "form-control" } })
@Html.ValidationMessageFor(model => model.MiddleName, "", new { @class = "text-danger" })
</div>
</div>
<div class="form-group">
@Html.LabelFor(model => model.LastName, htmlAttributes: new { @class = "control-label col-md-2" })
<div class="col-md-4">
@Html.EditorFor(model => model.LastName, new { htmlAttributes = new { @class = "form-control" } })
@Html.ValidationMessageFor(model => model.LastName, "", new { @class = "text-danger" })
</div>
</div>
<div class="form-group">
@Html.LabelFor(model => model.Gender, htmlAttributes: new { @class = "control-label col-md-2" })
<div class="col-md-10">
@*@Html.DropDownListFor(o=>o.Gender,Model.StudentGender, "", new { @class = "form-control" })*@
@Html.DropDownListFor(o=>o.Gender,(List<SelectListItem>)Model.Genders,"1", new { @class = "form-control" })
@Html.ValidationMessageFor(model => model.Gender, "", new { @class = "text-danger" })
</div>
</div>
<div class="form-group">
@Html.LabelFor(model => model.IdNo, htmlAttributes: new { @class = "control-label col-md-2" })
<div class="col-md-10">
@Html.EditorFor(model => model.IdNo, new { htmlAttributes = new { @class = "form-control" } })
@Html.ValidationMessageFor(model => model.IdNo, "", new { @class = "text-danger" })
</div>
</div>
<div class="form-group">
@Html.LabelFor(model => model.SchoolIdNo, htmlAttributes: new { @class = "control-label col-md-2" })
<div class="col-md-10">
@Html.EditorFor(model => model.SchoolIdNo, new { htmlAttributes = new { @class = "form-control" } })
@Html.ValidationMessageFor(model => model.SchoolIdNo, "", new { @class = "text-danger" })
</div>
</div>
<div class="form-group">
@Html.LabelFor(model => model.ServiceType, htmlAttributes: new { @class = "control-label col-md-2" })
<div class="col-md-10">
@Html.EditorFor(model => model.ServiceType, new { htmlAttributes = new { @class = "form-control" } })
@Html.ValidationMessageFor(model => model.ServiceType, "", new { @class = "text-danger" })
</div>
</div>
<div class="form-group">
@Html.LabelFor(model => model.Comments, htmlAttributes: new { @class = "control-label col-md-2" })
<div class="col-md-10">
@Html.EditorFor(model => model.Comments, new { htmlAttributes = new { @class = "form-control" } })
@Html.ValidationMessageFor(model => model.Comments, "", new { @class = "text-danger" })
</div>
</div>
<div class="form-group">
@Html.LabelFor(model => model.IsEnabled, htmlAttributes: new { @class = "control-label col-md-2" })
<div class="col-md-10">
<div class="checkbox">
@Html.EditorFor(model => model.IsEnabled)
@Html.ValidationMessageFor(model => model.IsEnabled, "", new { @class = "text-danger" })
</div>
</div>
</div>
<div class="form-group">
@Html.LabelFor(model => model.FirstNameAr, htmlAttributes: new { @class = "control-label col-md-2" })
<div class="col-md-10">
@Html.EditorFor(model => model.FirstNameAr, new { htmlAttributes = new { @class = "form-control" } })
@Html.ValidationMessageFor(model => model.FirstNameAr, "", new { @class = "text-danger" })
</div>
</div>
<div class="form-group">
@Html.LabelFor(model => model.MiddleNameAr, htmlAttributes: new { @class = "control-label col-md-2" })
<div class="col-md-10">
@Html.EditorFor(model => model.MiddleNameAr, new { htmlAttributes = new { @class = "form-control" } })
@Html.ValidationMessageFor(model => model.MiddleNameAr, "", new { @class = "text-danger" })
</div>
</div>
<div class="form-group">
@Html.LabelFor(model => model.LastNameAr, htmlAttributes: new { @class = "control-label col-md-2" })
<div class="col-md-10">
@Html.EditorFor(model => model.LastNameAr, new { htmlAttributes = new { @class = "form-control" } })
@Html.ValidationMessageFor(model => model.LastNameAr, "", new { @class = "text-danger" })
</div>
</div>
<div class="form-group">
<div class="col-md-offset-2 col-md-10">
<input type="submit" value="Create" class="btn btn-primary" />
</div>
</div>
</div>
}
<div>
@Html.ActionLink("Back to List", "Index")
</div>
@section Scripts {
@Scripts.Render("~/bundles/jqueryval")
}
你的视图是强类型的CreateStudent类和视图代码使用Model.Genders集合属性时，你使用DropDownListFor助手方法。但是在您的http post操作中，您在没有传递有效的CreateStudent对象的情况下调用return View()方法，并且在另一种情况下没有填充Genders属性。因此，当razor执行视图代码时，模型值是null(因为您没有向视图传递任何内容)
在将发布的视图模型返回给视图之前，您需要再次设置Genders属性。
[HttpPost]
public ActionResult Create(CreateStudent student)
{
var genderList = new List<SelectListItem>()
{
new SelectListItem { Text = Constants.Gender.Boy, Value = Constants.Gender.Boy},
new SelectListItem { Text = Constants.Gender.Girl, Value = Constants.Gender.Girl},
};
if (ModelState.IsValid)
{
var result = _student.Insert(mappedStudent);
if (result)
{
return RedirectToAction("Index");
}
else
{
student.Genders = genderList;
TempData["Message"] = "Failed to create new student";
return View(student); // Passing the object here to view
}
}
//Model validation fails. Return the same view
student.Genders = genderList;
return View(student);
}
}
也不需要额外的强制转换。Model.Genders的类型为List<SelectListItem>
@Html.DropDownListFor(o=>o.Gender,Model.Genders, new { @class = "form-control" })