c# - MVC View 模型抛出的 NullReferenceException

Question

我正在努力解决这个问题，但是随着我的学习，我学到了很多这些东西，如果有人能解释我哪里出错了和/或一些好的资源，我将非常感激我可以阅读。

因此，我有一个基于我的数据库 Entity Framework 模型的模型和一个表示该模型中属性的 View 模型。我已经构建了一个 Kendo 网格来显示数据(在 js 文件中定义)并且 Controller 中的方法返回一个 Json 结果集。问题是，当我尝试在连接的数据库表中显示一个值时，如果没有设置键值，我会收到 nullreferenceexception 错误。显然我在这里遗漏了部分难题，因为必须有一种编码方法来阻止它发生。如有任何帮助，我们将不胜感激!

我的模型是这样的:

namespace TrainingKendoUI.Models
{
    using System;
    using System.Collections.Generic;

    public partial class TRAINING_EMPLOYEE_COURSES
    {
        public int EMP_COURSE_ID { get; set; }
        public int EMPLOYEE_ID { get; set; }
        public int COURSE_ID { get; set; }
        public Nullable<System.DateTime> DATE_ATTENDED { get; set; }
        public Nullable<decimal> COURSE_COST { get; set; }
        public string COURSE_RESITS { get; set; }
        public Nullable<int> PROVIDER_ID { get; set; }
        public Nullable<int> EMP_COURSE_STATUS_ID { get; set; }
        public Nullable<int> VENUE_ID { get; set; }

        public virtual TRAINING_COURSES TRAINING_COURSES { get; set; }
        public virtual TRAINING_EMPLOYEE_COURSE_STATUS TRAINING_EMPLOYEE_COURSE_STATUS  { get; set; }
        public virtual TRAINING_EMPLOYEES TRAINING_EMPLOYEES { get; set; }
        public virtual TRAINING_PROVIDERS TRAINING_PROVIDERS { get; set; }
        public virtual TRAINING_VENUES TRAINING_VENUES { get; set; }
    }
}

我的 Controller 方法如下所示:

    public JsonResult EmployeeCourses_Read()
    {
        var model = db.TRAINING_EMPLOYEE_COURSES;
        var ViewModel = new List<EmployeeCoursesIntersectionViewModel>();

        foreach (var employee in model)
        {
            ViewModel.Add(new EmployeeCoursesIntersectionViewModel(employee));
        }

        return Json(ViewModel, JsonRequestBehavior.AllowGet);
    }

我的 View 模型是这样的:

namespace TrainingKendoUI.ViewModels
{
    public class EmployeeCoursesIntersectionViewModel
    {

        #region Constructors

            public EmployeeCoursesIntersectionViewModel()
            {

            }

            public EmployeeCoursesIntersectionViewModel(TRAINING_EMPLOYEE_COURSES  model)
            {
                this.empCourseId = model.EMP_COURSE_ID;
                this.employee = model.TRAINING_EMPLOYEES.FIRST_NAME;
                this.course = model.TRAINING_COURSES.COURSE_NAME;
                this.dateAttended = model.DATE_ATTENDED;
                this.cost = model.COURSE_COST;
                this.resits = model.COURSE_RESITS;
                //These lines will produce a NullReference error if not set through the front end...
                this.provider = model.TRAINING_PROVIDERS.PROVIDER_NAME;
                this.status =    model.TRAINING_EMPLOYEE_COURSE_STATUS.EMP_COURSE_STATUS;
                this.venue = model.TRAINING_VENUES.VENUE_NAME;

        }

        #endregion

        #region Properties

        public int empCourseId { get; set; }
        public string employee { get; set; }
        public string course { get; set; }
        public Nullable<System.DateTime> dateAttended { get; set; }
        public Nullable<decimal> cost { get; set; }
        public string resits { get; set; }
        public string provider { get; set; }
        public string status { get; set; }
        public string venue { get; set; }

        #endregion

    }
}

Answer 1

在设置之前对对象进行空检查，即

this.provider = model.TRAINING_PROVIDERS == null ? "" 
                        : model.TRAINING_PROVIDERS.PROVIDER_NAME;

你必须对状态和地点做类似的事情

this.status =    model.TRAINING_EMPLOYEE_COURSE_STATUS== null ? ""
                        model.TRAINING_EMPLOYEE_COURSE_STATUS.EMP_COURSE_STATUS;
this.venue = model.TRAINING_VENUES== null ? ""
                        model.TRAINING_VENUES.VENUE_NAME;