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dausa
Asked: 2024-02-19 21:58:33 +0800 CST

如何解决提交按钮后选择选项错误空引用异常

  • 772

我正在做一个项目，涉及到在ASP.Net Core 7.0中使用select选项，我遇到了一些困难。具体来说，我在下拉列表中遇到了与空引用异常相关的错误。但是我观察到，提交数据后，安装到SQL数据库中，提交后返回时，显示空引用异常错误。这些错误使开发过程变得相当具有挑战性，我希望尽快找到解决方案。任何意见或建议将不胜感激。

错误

在此输入图像描述

行动

public IActionResult Apply()
{

    ViewBag.Courselist = new SelectList(GetAllAccountIDList(), "Id", 
    "CourseName");
    return View();
}
[HttpPost]
public IActionResult Apply(PurchasedFormViewModel purchasedFormVM)
{
    if (ModelState.IsValid)
    {
    try
    {
        var transaction = new StudentBiodataViewModels
        {
            CourseId = purchasedFormVM.CourseId,
            SurName = purchasedFormVM.SurName,
            OtherName = purchasedFormVM.OtherName,
            PhoneNumber = purchasedFormVM.PhoneNumber,
            Email = purchasedFormVM.Email,
        };
        _context.StudentBiodata.Add(transaction);
        _context.SaveChangesAsync();
    }
    catch (Exception ex)
    {

        ViewBag.Message = ex.Message + " Error occur while inserting records.";
    }
    }
    return View();
}

楷模

 public class ApplyFormViewModel
 { 
     public int Id { get; set; }
     [Display(Name = "Course Apply")]
     [Required(ErrorMessage = "Course Apply is Required")]
     public int CourseId { get; set; }
     [Display(Name = "SurName")]
     [Required(ErrorMessage = "SurName is Required")]
     public string SurName { get; set; }
     [Display(Name = "Other Name")]
     [Required(ErrorMessage = "Other Name is Required")]
     public string OtherName { get; set; } 
     [Display(Name = "Phone Number")]
     [Required(ErrorMessage = "Phone Number is Required")]
     ErrorMessage = "Entered phone format is not valid.")]
     public string PhoneNumber { get; set; } 
     [Required(ErrorMessage = "Email is Required")]
     public string Email { get; set; }
     
 }

看法

<select id="fundcode" asp-for="CourseId" class="form-control">
    <option value="">--Please select--</option>
    @foreach (var item in ViewBag.Courselist )
    {
        <option value="@item.Value">@item.Text</option>
    }
</select>
c#

2 个回答

  1. 您将从 POST 返回到 GET 处理程序中调用的同一视图，但未设置ViewBag.CourseList。当然，当页面呈现时，该变量为 NULL，并且您会得到异常。

    我认为您需要在返回到 Post 处理程序内的 View 之前添加 GET 中使用的相同代码

    [HttpPost]
public IActionResult Apply(PurchasedFormViewModel purchasedFormVM)
{
    .....
    ViewBag.Courselist = new SelectList(GetAllAccountIDList(), "Id", "CourseName");
    return View();
}
    • 0
  2. Best Answer
    • 确保 Dropdown Population：验证 GetAllAccountIDList() 方法是否返回 Id 和 CourseName 属性具有非空值的有效项目列表。如果任何项目为空或者列表本身为空，则在视图中绑定下拉列表时可能会导致问题。
    • 检查 ModelState 验证：在 POST 操作中访问购买的FormVM 对象的属性之前，请确保 ModelState 有效。如果缺少任何必填字段或存在验证错误，则 ModelState 可能无效，从而导致将空值传递给操作。
    • 同步保存更改：在 POST 操作中，您使用 _context.SaveChangesAsync() 将更改保存到数据库。但是，此方法返回一个任务，因此您应该等待它以确保正确保存更改。否则，该方法可能会在实际保存更改之前返回，从而可能导致问题。
    [HttpPost]
public async Task<IActionResult> Apply(PurchasedFormViewModel purchasedFormVM)
{
    if (ModelState.IsValid)
    {
        try
        {
            var transaction = new StudentBiodataViewModels
            {
                CourseId = purchasedFormVM.CourseId,
                SurName = purchasedFormVM.SurName,
                OtherName = purchasedFormVM.OtherName,
                PhoneNumber = purchasedFormVM.PhoneNumber,
                Email = purchasedFormVM.Email,
            };
            _context.StudentBiodata.Add(transaction);
            await _context.SaveChangesAsync(); // Await SaveChangesAsync
        }
        catch (Exception ex)
        {
            ViewBag.Message = ex.Message + " Error occur while inserting records.";
        }
    }
    // Repopulate ViewBag.Courselist in case of validation errors
    ViewBag.Courselist = new SelectList(GetAllAccountIDList(), "Id", "CourseName");
    return View();
}
    • 0

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