EF 外键为 NULL

Question

我有以下型号class：

[key]
public int CustomerId { get; set; }
public string CustomerName { get; set; }
[ForeignKey("CustomerType")]
public int? CustomerTypeId { get; set; }
public virtual CustomerType CustomerType { get; set; }

[Key]
public int CustomerTypeId { get; set; }
public string CustomerTypeCode { get; set; }

然后我想用这个控制器代码将它加载到视图中：

public async Task<IActionResult> Index()
{
     var obj = _db.Customer.Include(i => i.CustomerType);
     return View(await obj.ToListAsync());
}

在视图中：

@using IEnumerable<myProject.Models.Customer>
<table>
<thead>
   <tr>
     <th>Customer
     <th>CustomerType
   </tr>
</thead>
<tbody>
   @foreach (var item in Model)
   {
   <tr>
     <td>@item.Customer</td>
     <td>@item.CustomerType.Code</td>
   </tr>
   }
</tbody>
</table>

当我尝试运行它时，我收到 @item.CustomerType.Code 的错误，因为数据库中的数据为空：

System.NullReferenceException: 'Object reference not set to an instance of an object.'

有谁知道为什么？即使我尝试将 ? 放在模型上，它也不起作用。

非常感谢。

Answer 1

保留所有现有代码，我为您找到了这个解决方案。只需要改变一下你的观点。控制器没有变化。

型号：

public class Customer
    {
        [key]
        public int CustomerId { get; set; }
        public string CustomerName { get; set; }
        [ForeignKey("CustomerType")]
        public int? CustomerTypeId { get; set; }
        public virtual CustomerType CustomerType { get; set; }
    }

public class CustomerType
    {
        [Key]
        public int CustomerTypeId { get; set; }
        public string CustomerTypeCode { get; set; }
      
    }

控制器：

  public async Task<IActionResult> CustomerIndex()
        {
            var obj = _context.Customers.Include(i => i.CustomerType);
            return View(await obj.ToListAsync());
        }

查看：

@model IEnumerable<MVCApps.Models.Customer>
<table class="table table table-bordered">
    <thead>
        <tr>
            <th>Customer
            <th>Customer Type
        </tr>
    </thead>
    <tbody>
        @foreach (var item in Model)
        {
            <tr>
                <td>@item.CustomerName</td>
                <td>@(item.CustomerType == null ? "" : item.CustomerType.CustomerTypeCode)</td>
            </tr>
        }
    </tbody>
</table>

Note: There are few other way to handle this scenario from controller side but this one I think more less code and easiest way. <td>@item.CustomerType?.CustomerTypeCode</td> this also would work but I prefer to use ternary operator here

输出：

希望对您有所帮助。

Answer 2

您只需添加，

[必填]

属性

EF 外键为 NULL

EF foreign key is NULL

c#

entity-framework

asp.net-core

razor-pages