当我想通过模型访问它时，模型上的 MVC Null 异常在 ViewModel 中传递给 View。 不是@Html

Question

I am having problem in MVC with my Model which is passed in ViewModel to View. I have database with several tables on which I called entity framework to create models.

This is my ViewModel:

namespace BP_final.ViewModels
{
     public class AllDBModels
    {
        public Models.Graph AllGraphs { get; set; }
        public IEnumerable<NAMTAR_STUDYROOMS> Namtar_studyrooms { get; set; }
        public IEnumerable<NAMTAR_STATS_STUDYROOMS> Namtar_stats_studyrooms { get; set; }
        public List<bp_data> bp_data { get; set; }
        public Models.Report reports { get; set; }

        public AllDBModels()
        {
            Namtar_studyrooms = new List<NAMTAR_STUDYROOMS>();
            reports = new Models.Report() { RoomNumber = "1"};
            AllGraphs = new Models.Graph();

        }
    }
}

I create few properties for my tables and I initialize some of them. Then in my controller i call my ViewModel class so it should create instance:

public ActionResult Index()
    {
        AllDBModels allModels = new AllDBModels();
        allModels.AllGraphs = new Graph();
        allModels.AllGraphs.Charts = new List<Highcharts>();

        ChartsModel model = new ChartsModel();
        model.Charts = new List<Highcharts>();
...
...

return View(allModels);

In my View I get the ViewModel, then I want to use it. I call DropDownListFor:

@model BP_final.ViewModels.AllDBModels

@{

ViewBag.Title = "Create";

}
<h2>Create</h2>

@using (Html.BeginForm()) 
...
...

    <div class="form-group">
        @Html.LabelFor(model => model.reports.RoomNumber, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10 ">
            @Html.DropDownListFor(model => model.reports.RoomNumber, new SelectList(
/*this fails*/   Model.Namtar_studyrooms.Select(x => new SelectListItem{ Value = x.ID.ToString(), Text = x.NAME }), "Value", "Text", 2
            ), new { @class = "form-control" })
            @Html.ValidationMessageFor(model => model.reports.Graph, "", new { @class = "text-danger" })
        </div>
    </div>

I want to populate the dropdown options with data from namtar_studyrooms table/model but can't. It is working for the first labmda expression which is taking from @Html (as model.reports while model is my viewmodel) but the next lamda I use with Model.Namtar_studyrooms.Select( does not work and I am getting null exception on Model. I looked on the internet and it seems I have everything as everyone else and I cant seem to find what is wrong

Answer 1

Add the dropdown collection to ViewBag.

In Controller;

public ActionResult Index()
    {
    AllDBModels allModels = new AllDBModels();
    allModels.AllGraphs = new Graph();
    allModels.AllGraphs.Charts = new List<Highcharts>();

    ChartsModel model = new ChartsModel();
    model.Charts = new List<Highcharts>();
    ViewBag.Rooms = db.namtar_studyrooms.ToList() //the collection here (you should changeit, just an example)
}

In View;

 @Html.DropDownListFor(model => model.reports.RoomNumber, new SelectList(ViewBag.Rooms, "RoomID", "RoomNameForDisplay"), "Select Room for default", new { @class = "form-control" })

Hope Helps.