对象引用未设置为mvc中对象的实例[Null reference Exception]

Question

我正在创建一个MVC应用程序。我收到一个关于更改密码表单功能的Null引用异常的错误。

我的模型类代码

public partial class tblUser
{
    public int Id { get; set; }
    [Required(ErrorMessage = "Please Enter Your Email Id")]
    [EmailAddress(ErrorMessage = "Invalid Email Address")]
    public string Email { get; set; }
    [Required(ErrorMessage = "Please Enter Your Password")]
    [DataType(DataType.Password)]
    [StringLength(18, ErrorMessage = "The password must be atleast 3 characters long", MinimumLength = 3)]
    public string Password { get; set; }
    [Required(ErrorMessage = "Please Enter Your Password")]
    [DataType(DataType.Password)]
    [StringLength(18, ErrorMessage = "The password must be atleast 3 characters long", MinimumLength = 3)]
    public string NewPassword { get; set; }
}

我的视图（Changepassword.cs.html）

@model MvcWebApplication1.Models.tblUser
<div class="container">
    <div class="row rowspace1">
        <div class="col-sm-12">
            @Html.ActionLink("Log Out", "Login", "Home")
        </div>
    </div>
    <div class="row">
        <div id="CpassTitle">
            <h3>CHANGE PASSWORD</h3>
        </div>
        @using (Html.BeginForm("Changepassword", "Home", FormMethod.Post))
        {
            
                <table class="center">
                    <tr>
                        <td>Old Password</td>
                        <td>
                            @Html.EditorFor(pass => pass.Password)
                        </td>
                        <td>@Html.ValidationMessageFor(pass => pass.Password)</td>
                    </tr>
                    <tr class="rowspace">
                        <td>New Password</td>
                        <td>
                            @Html.EditorFor(pass => pass.NewPassword)
                        </td>
                        <td>@Html.ValidationMessageFor(pass => pass.NewPassword)</td>
                    </tr>
                  
                    <tr class="rowspace">
                        <td colspan="3" id="button">
                            <input type="submit" value="Change Password" /></td>
                    </tr>
                    <tr class="rowspace"><td colspan="3">@ViewBag.Message</td></tr>
                </table>
            
        }
    </div>
</div> 

当我点击更改密码提交按钮时，我在userDetail.Password = login.NewPassword; 线上得到了一个NullReferenceException在操作方法

[HttpPost]
public ActionResult Changepassword(tblUser login)
{
    UserDetailsEntities db = new UserDetailsEntities();
    var detail = db.tblUsers.Where(log => log.Password == login.Password).FirstOrDefault();
    if (detail != null)
    {
        var userDetail = db.tblUsers.FirstOrDefault(c => c.Email == login.Email);
        userDetail.Password = login.NewPassword;
        
        db.SaveChanges();
        ViewBag.Message = "Record Inserted Successfully!";
    }
    else
    {
        ViewBag.Message = "Password not Updated!";
    }
    return View(login);
}

Answer 1

[HttpPost]
public ActionResult Changepassword(tblUser login)
{
    UserDetailsEntities db = new UserDetailsEntities();
    var detail = db.tblUsers.Where(log => log.Password == login.Password).FirstOrDefault();
    // ADDED NULL CHECK HERE
    if (detail != null)
    {
        var userDetail = db.tblUsers.FirstOrDefault(c => c.Email == login.Email);
         if(userDetail !=null)
         {
              userDetail.Password = login.NewPassword;
              db.SaveChanges();
              ViewBag.Message = "Record Inserted Successfully!";
         }
    }
    else
    {
        ViewBag.Message = "Password not Updated!";
    }
    return View(login);
}

Answer 2

检查ur数据库。它要么在你阅读之前就关闭了，要么正在连接到其他表格。