Question

I am trying to understand something a bit better with being new to C#, .NET 3.5 and MVC.

I am running through the MVC NerdDinner example and if you look at the ViewModel here: http://nerddinnerbook.s3.amazonaws.com/Part6.htm#highlighter_662935

You can see the Country list and how it gets populated, this seems to work fine but I tried to do a similar thing below using LINQ and I am having problems, with the SelectList approach even though it inherits from the IEnumerable interface.

I have got a task table with a foreign key to a status table. The below code gives me a NullReferenceException when I do a GET on a create action. I can see that an anonymous task object would not have a status set.. so I probably need to check for it, but I dont understand how this is not done for the NerdDinner example??

        public class TaskViewModel {
        // Properties

        public Task Task { get; private set; }
        public SelectList Status { get; private set; }

        // Constructor
        public TaskViewModel(Task task) {
            TaskRepository taskRepo = new TaskRepository();
            Task = task;
            Status = new SelectList(taskRepo.GetStatus(), Task.Status.description);
        }

    }

        //
        // GET: /Tasks/Create

        public ActionResult Create()
        {
            Task task = new Task();

            return View(new TaskViewModel(task));
        } 


//Code from TaskRepository 

        private TaskManagerDataContext db = new TaskManagerDataContext();

        public IQueryable<Status> GetStatus() {
            return from status in db.Status
                   orderby status.description
                   select status;
        }

I did another approach using LINQ for the type dropdown and the population of the drop down works but I am yet to test if it selects the correct value once a post is made and the details view is returned. I am also wondering whether this should some how be moved into my repository rather than have a class in my controller doing this sort of thing??

Here is the code:

//In TaskViewModel Class
        public IEnumerable<SelectListItem> Types { get; private set; }

//In TaskViewModel constructor

                IList<NPType> types = taskRepo.GetTypes().ToList();

            Types =
                from type in types
                select new SelectListItem {
                    Selected = (type.typeId == task.typeId),
                    Text = type.description,
                    Value = type.typeId.ToString()
                };

//The TaskForm partial View that is used for the Create action of the TaskController
            <p>
                <label for="type">type:</label>
                <%= Html.DropDownList("Type", Model.Types)%>
                <%= Html.ValidationMessage("type", "*") %>
            </p>
            <p>
                <label for="status">status:</label>
                <%= Html.DropDownList("Status", Model.Status)%>
                <%= Html.ValidationMessage("status", "*") %>
            </p>

and the TaskForm view inherits System.Web.Mvc.ViewUserControl

Answer 1

My attempt at trying to understand this better.

    //controller code creating a select list in the viewmodel class.
//taskRepo.GetStatus() returns an IQueryable<Status>
        Status = new SelectList(taskRepo.GetStatus(), Task.Status);

//MVC Framework SelectList class constructor and ToEnumerable method
            public SelectList(IEnumerable items, string dataValueField, string dataTextField, object selectedValue)
            : base(items, dataValueField, dataTextField, ToEnumerable(selectedValue)) {
            SelectedValue = selectedValue;
        }

        private static IEnumerable ToEnumerable(object selectedValue) {
            return (selectedValue != null) ? new object[] { selectedValue } : null;
        }

I can see that SelectList uses its base class of MultiSelectList and that constructor is here:

        public MultiSelectList(IEnumerable items, string dataValueField, string dataTextField, IEnumerable selectedValues) {
        if (items == null) {
            throw new ArgumentNullException("items");
        }

        Items = items;
        DataValueField = dataValueField;
        DataTextField = dataTextField;
        SelectedValues = selectedValues;
    }

When I run this project the html is given as:

    <select id="Status" name="Status"><option>NPTaskManager.Models.Status</option>
<option>NPTaskManager.Models.Status</option>
<option>NPTaskManager.Models.Status</option>
<option>NPTaskManager.Models.Status</option>
</select>

Which is to be expected.

If I change the controller code to:

Status = new SelectList(taskRepo.GetStatus(), Task.Status.statusId.ToString(), Task.Status.description);

Then I get a NullReferenceException. Since this is not an ArgumentNullException It seems to me that the root of the exception is not the first SelectList argument. What I am trying to understand is how this all occurs?
Is it because Task.Status needs to be added to Task in the create action of the controller?

I will change this code to use the LINQ approach that I used for the task above, all I am trying to achieve now is some understanding of what is going on.