我正在实现asp.net核心MVC项目。在名为ApiApplicant的Create方法的控制器类中，我有3个选择列表，应从名为APIApplicantHistory的表中填充其项目。我的模型，创建方法和视图的实现如下：
using System.Collections.Generic;
namespace CSDDashboard.Models
{
public partial class Apiapplicant
{
public Apiapplicant()
{
ApiApplicantHistory = new HashSet<ApiApplicantHistory>();
}
public int Id { get; set; }
public string ApiRequestDate { get; set; }
public int? ApiRequestNo { get; set; }
public int? Apiid { get; set; }
public int? ApplicantId { get; set; }
public int? GateId { get; set; }
public string NocRequestDate { get; set; }
public string NocRequestNo { get; set; }
public string Url { get; set; }
public string Description { get; set; }
public bool? IsDeleted { get; set; }
public virtual Api Api { get; set; }
public virtual Applicant Applicant { get; set; }
public virtual Gate Gate { get; set; }
public virtual ICollection<ApiApplicantHistory> ApiApplicantHistory { get; set; }
}
}
using System;
using System.Collections.Generic;
namespace CSDDashboard.Models
{
public partial class ApiApplicantHistory
{
public int Id { get; set; }
public int? ApiApplicantId { get; set; }
public string Date { get; set; }
public int? SentResponseType { get; set; }
public int? UnconfirmedReason { get; set; }
public int LastReqStatus { get; set; }
public string Description { get; set; }
public virtual Apiapplicant ApiApplicant { get; set; }
public virtual EntityType LastReqStatusNavigation { get; set; }
public virtual EntityType SentResponseTypeNavigation { get; set; }
public virtual EntityType UnconfirmedReasonNavigation { get; set; }
}
}
using System;
using System.Collections.Generic;
namespace CSDDashboard.Models
{
public partial class EntityType
{
public EntityType()
{
ApiApplicantHistoryLastReqStatusNavigation = new HashSet<ApiApplicantHistory>();
ApiApplicantHistorySentResponseTypeNavigation = new HashSet<ApiApplicantHistory>();
ApiApplicantHistoryUnconfirmedReasonNavigation = new HashSet<ApiApplicantHistory>();
}
public int Id { get; set; }
public string Name { get; set; }
public string EntityKey { get; set; }
public virtual ICollection<ApiApplicantHistory> ApiApplicantHistoryLastReqStatusNavigation { get; set; }
public virtual ICollection<ApiApplicantHistory> ApiApplicantHistorySentResponseTypeNavigation { get; set; }
public virtual ICollection<ApiApplicantHistory> ApiApplicantHistoryUnconfirmedReasonNavigation { get; set; }
}
}
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
namespace CSDDashboard.Models
{
public class APIApplicantViewModel
{
public Apiapplicant apiApplicantvm { get; set; }
public ApiApplicantHistory apiApplicantHistoryvm { get; set; }
}
}
public class ApiapplicantsController : Controller
{
private readonly CSSDDashboardContext _context;
public ApiapplicantsController(CSSDDashboardContext context)
{
_context = context;
}
public IActionResult Create()
{
ViewData["sentResponseType"] = new SelectList(_context.EntityType.Where(g => g.EntityKey == "sentResponseType").ToList(), "ID", "name");
ViewData["unconfirmedReason"] = new SelectList(_context.EntityType.Where(g => g.EntityKey == "unconfirmedReason").ToList(), "ID", "name");
ViewData["lastReqStatus"] = new SelectList(_context.EntityType.Where(g => g.EntityKey == "lastRequestStatus").ToList(), "ID", "name");
return View();
}
}
以及创建视图实现的一部分：
@model CSDDashboard.Models.APIApplicantViewModel
@{
ViewData["Title"] = "create";
}
<div class="row">
<div class="col-md-4">
<form asp-action="Create">
<div asp-validation-summary="ModelOnly" class="text-danger"></div>
<div class="form-group">
<label asp-for="apiApplicantvm.GateId" class="control-label"></label>
<select asp-for="apiApplicantvm.GateId" class="form-control" asp-items="ViewBag.GateId"></select>
</div>
<div class="form-group">
<label asp-for="apiApplicantHistoryvm.SentResponseType" class="control-label"></label>
<select asp-for="apiApplicantHistoryvm.SentResponseType" class="form-control" asp-items="ViewBag.sentResponseType"></select>
</div>
<div class="form-group">
<label asp-for="apiApplicantHistoryvm.UnconfirmedReason" class="control-label"></label>
<select asp-for="apiApplicantHistoryvm.UnconfirmedReason" class="form-control" asp-items="ViewBag.unconfirmedReason"></select>
</div>
<div class="form-group">
<label asp-for="apiApplicantHistoryvm.LastReqStatus" class="control-label"></label>
<select asp-for="apiApplicantHistoryvm.LastReqStatus" class="form-control" asp-items="ViewBag.lastReqStatus"></select>
</div>
<div class="form-group">
<input type="submit" value="Create" class="btn btn-primary" />
</div>
</form>
</div>
</div>
<div>
<a asp-action="Index">Back</a>
</div>
@section Scripts {
@{await Html.RenderPartialAsync("_ValidationScriptsPartial");}
}
在create方法中，所有viewData均填充有正确的相关数据，但Create视图中存在问题，运行项目后，在Create页面中显示如下错误：
[处理请求时发生未处理的异常。NullReferenceException：对象引用未设置为对象的实例。
调试代码后，我了解到在create视图中，apiApplicantvm不为null，但apiApplicantHistoryvm返回null，因此出现上述错误。如果有人可以告诉我如何解决此问题，我将不胜感激。
我希望您将EF内核用作ORM，因为您正在使用Asp.net core。
要在EF core中加载相关数据。
可以通过两种方式完成，或者延迟加载
急切加载
_context.EntityType.Where(g=>g.EntityKey=="sentResponseType")
.Include(x=>x.ApiApplicantHistoryLastReqStatusNavigation).ToList()
OR
延迟加载在您的模态创建中
protected override void
OnConfiguring(DbContextOptionsBuilder optionsBuilder) => optionsBuilder .UseLazyLoadingProxies() .UseSqlServer(myConnectionString);
OR
在您的核心MVC项目的startup.cs中延迟加载示例。
.AddDbContext<BloggingContext>(
b => b.UseLazyLoadingProxies()
.UseSqlServer(myConnectionString));
有关更多信息docs.microsoft.com/en-us/ef/core/querying/related-data
https://docs.microsoft.com/en-us/ef/core/querying/related-data
非常感谢您的回答。我正在使用EF core，并且按照您的建议更改了代码。我添加了ViewData [“ sentResponseType”] =新的SelectList（_context.EntityType.Where（g => g.EntityKey ==“ sentResponseType”）.Include（x => x.ApiApplicantHistoryLastReqStatusNavigation）.ToList（），“ ID”，“ name”）;
在我的创建方法中。
而且我的问题在此处的创建视图中，在以下行中，apiApplicantHistoryvm中存在空值：
谢谢您的帮助。问题在我的代码中ViewData [“ sentResponseType”] =新的SelectList（_context.EntityType.Where（g => g.EntityKey ==“ sentResponseType”）。ToList（），“ ID”，“名称”）;
根据我的EntityType模型，我应该使用ID而不是ID。