C# 控制入院审批和未审批状态

大家好，我正在MVC中制作一个大学网站项目，它有一张录取表，所以每当用户填写并提交录取表时，它就会进行审批，用户会收到他的录取代码来检查他/她的帐户审批状态，所以我在这里遇到了一个问题

守则：

型号：

  public partial class admission_tbl
{
    public int adm_id { get; set; } //- by this user checks his status//
    [Required]
    [StringLength(50)]
    public string std_name { get; set; }
    [Required]
    [StringLength(50)]
    public string std_father { get; set; }
    [Required]
    [StringLength(50)]
    public string std_mother { get; set; }
    [AgeRangeValidation(ErrorMessage = "Age must be between 18 - 28", MinAge = 18, MaxAge = 28)]
    [Required]
    [DataType(DataType.Date), DisplayFormat(DataFormatString = "{0:dd/MM/yyyy}", ApplyFormatInEditMode = true)]

    public System.DateTime DOB { get; set; }
    [Required]
    public string std_gender { get; set; }
    [Required]
    public string R_address { get; set; }
    [Required]
    public string P_address { get; set; }
    [Required]
    public string adm_for { get; set; }
    [Required]
    public string university { get; set; }
    [Required]
    public string E_no { get; set; }
    [Required]
    public string Center { get; set; }
    [Required]
    public string City { get; set; }
    [Required]
    public string Field { get; set; }
    [Required]
    public string Marks_secured { get; set; }
    [Required]
    public string out_of { get; set; }
    [Required]
    public string Class_obtained { get; set; }

    public string Sports_details { get; set; }
    public string adm_status { get; set; } //--This is the method which i am using to control account 
                                                approval or non approval status.
           }}

 public ActionResult Status_check()
    {
        return View();
    }

    [HttpPost]
    public ActionResult Status_check(admission_tbl svm)
    {
        admission_tbl u = db.admission_tbl.Where(a => a.adm_id == svm.adm_id).SingleOrDefault();


        if (u.adm_status == "Approved")
        {
            ViewBag.Message1 = "Your account is Approved";
        }
        else if (u.adm_status == "Unapproved")
        {
            ViewBag.Message = "Your account is not Approved ";

        }
        else
        {
            ViewBag.Message = "Your account is in pending please wait";
        }

        return View();
    }

控制器：

  public partial class admission_tbl
{
    public int adm_id { get; set; } //- by this user checks his status//
    [Required]
    [StringLength(50)]
    public string std_name { get; set; }
    [Required]
    [StringLength(50)]
    public string std_father { get; set; }
    [Required]
    [StringLength(50)]
    public string std_mother { get; set; }
    [AgeRangeValidation(ErrorMessage = "Age must be between 18 - 28", MinAge = 18, MaxAge = 28)]
    [Required]
    [DataType(DataType.Date), DisplayFormat(DataFormatString = "{0:dd/MM/yyyy}", ApplyFormatInEditMode = true)]

    public System.DateTime DOB { get; set; }
    [Required]
    public string std_gender { get; set; }
    [Required]
    public string R_address { get; set; }
    [Required]
    public string P_address { get; set; }
    [Required]
    public string adm_for { get; set; }
    [Required]
    public string university { get; set; }
    [Required]
    public string E_no { get; set; }
    [Required]
    public string Center { get; set; }
    [Required]
    public string City { get; set; }
    [Required]
    public string Field { get; set; }
    [Required]
    public string Marks_secured { get; set; }
    [Required]
    public string out_of { get; set; }
    [Required]
    public string Class_obtained { get; set; }

    public string Sports_details { get; set; }
    public string adm_status { get; set; } //--This is the method which i am using to control account 
                                                approval or non approval status.
           }}

 public ActionResult Status_check()
    {
        return View();
    }

    [HttpPost]
    public ActionResult Status_check(admission_tbl svm)
    {
        admission_tbl u = db.admission_tbl.Where(a => a.adm_id == svm.adm_id).SingleOrDefault();


        if (u.adm_status == "Approved")
        {
            ViewBag.Message1 = "Your account is Approved";
        }
        else if (u.adm_status == "Unapproved")
        {
            ViewBag.Message = "Your account is not Approved ";

        }
        else
        {
            ViewBag.Message = "Your account is in pending please wait";
        }

        return View();
    }

---

因此，在“状态检查”中，用户输入其准入代码以检查帐户是否已批准，所有操作是否正常，但问题是，当我尝试输入一个随机数输入时，它会抛出空异常错误，而我想抛出一个消息错误，例如“无效准入代码不存在”我抓不到它我缺少什么？

对于

svm.adm\u id

在

入院记录中不存在的情况

admission_tbl u = db.admission_tbl.SingleOrDefault(a => a.adm_id == svm.adm_id);

此

.SingleOrDefault（）

将返回NULL。因此，当您尝试从

u

访问

adm\u status

时（该值为空），您将获得

NullReferenceException

要解决此问题，需要添加验证以检查

u

对象在继续访问其内部属性之前不能为null

[HttpPost]
public ActionResult Status_check(admission_tbl svm)
{
    admission_tbl u = db.admission_tbl.Where(a => a.adm_id == svm.adm_id).SingleOrDefault();

    // Validate u cannot be null, if null do error handling
    if (u == null)
    {
        ViewBag.Message = "Invalid Admission Code no such exists";
        return View();
    }

    if (u.adm_status == "Approved")
    {
        ViewBag.Message1 = "Your account is Approved";
    }
    else if (u.adm_status == "Unapproved")
    {
        ViewBag.Message = "Your account is not Approved ";

    }
    else
    {
        ViewBag.Message = "Your account is in pending please wait";
    }

    return View();
}

注: 此声明

db.admission_tbl.Where(a => a.adm_id == svm.adm_id).SingleOrDefault();

可替换为：

db.admission_tbl.SingleOrDefault(a => a.adm_id == svm.adm_id);

两者都将返回相同的结果，如果找不到记录，则返回null

第二编辑版本： 您可以在ViewBag中绑定错误消息，或添加场景的任何处理并立即退出

ViewBag.Message = "Invalid Admission Code no such exists";
return View();

---

这回答了你的问题吗？是的，我对此进行了检查，但仍然没有找到任何解决方案。感谢@yong的详细解释，但是有可能在同一页面上只抛出一条错误消息而不是抛出异常吗？嗨@Mac，是的，当然，只需将

抛出新异常（）

替换为

ViewBag.message=“无效的准入代码不存在”

。我更新了代码。你可以参考一下。谢谢。是的，这是我的答案！再次感谢@yong