填充下拉列表返回 null 指针异常

Question

A small project consisting of 2 classes Appointments and Department , I need to be able to select the department name from a dropdown box.

I have tried <select> and asp-items , both are returning nullpointerexcpetion or something similar like ArgumentNullException: Value cannot be null. (Parameter 'items') ArgumentNullException: Value cannot be null. (Parameter 'items')

Appointment Class

    public class Appointment
    {
        [Key]
        public virtual int Id { get; set; }
 
        [Display(Name = "Department")]

        public virtual int Dep_Id { get; set; }

        public Department Departments{ get; set; }

    }

Department class
    public class Department
    {
        [Key]
        public virtual int ID { get; set; }
        [Required]
        public virtual string Name { get; set; }
    }

Two controllers using Entity Framework (Web API and MVC) were scaffolded based on these classes and localdb Department table was filled with some values. In the MVC controller generated from Appointment , I created a method

        public void PopulateDepartmentsDropDownList()
        {
            var departmentsQuery = from d in _context.Departments
                                   orderby d.Name
                                   select d;
            ViewBag.DepartmentID = new SelectList(departmentsQuery.AsNoTracking(), "ID", "Name");
        }

And finally in the Create.cshtml also generated from Appointment , these attempts do not work.

<select asp-for="Dep_Id" class="control-label" asp-items=ViewBag.DepartmentID></select>

generates an empty dropdown and

@Html.DropDownList("dep", new SelectList(ViewBag.DepartmentID, "ID", "Name")) crashes the page.

I am not sticking to any method, any solution that works based on any controller is fine.

Answer 1

I think you lack typecasting like in this question. I can not test it now, but i think this is the solution. @Html.DropDownListFor("dep", ViewBag.DepartmentID as SelectList)

Answer 2

I think you are missing to convert variable to List

In the controller:

var departmentsQuery = from d in _context.Departments
                                   orderby d.Name
                                   select d;
    List<Department> department = departmentsQuery.ToList();
    
    ViewBag.DepartmentID = new SelectList(department, "ID", "Name");

In the View:

@Html.DropDownList("DepartmentID", (IEnumerable<SelectListItem>)ViewBag.DepartmentID, null, new { @class ="form-control" })

or you can replace the "null" with whatever you want display as default selector, ie "Select Department".