Question

I have a drop down list written in Razor for a MVC app I am working on as:

@Html.DropDownList("BillId", "")

However the user does not have to select anything according to the logic of my program (the list is populated with ‘Bill’ objects in my controller). If they do not select any thing I get an error

The ViewData item that has the key 'BillId' is of type 'System.Int32' but must be of type 'IEnumerable<SelectListItem>'.

How do I write a statement in Razor to return a BillId of 0 if nothing is selected?

I am not sure of the syntax as I have a background in straight java and VB but something alongs the line of

 If DropdownBox.SelectedIndex = 0
 Else 
 BillId = DropdownBox.SelectedIndex
 End

Controller as follows:

Function Create(id As Integer) As ViewResult
        ViewBag.id = id
        Dim job As Job = New Job
        job.CustomerId = id
        job.JobAmount = 0
        job.JobDate = Date.Now()
        job.JobStatus = "Active"

        Dim BillList = New List(Of Bill)()

        Dim BillQuery = From s In db.Bills
                        Select s

        BillList.AddRange(BillQuery)

        ViewBag.BillIdList = New SelectList(BillList, "BillId", "BillDate")

        ViewBag.BillId = New SelectList(BillList, "BillId", "BillDate")

        Return View(job)
    End Function

The POST function for create is as below:

    <HttpPost()>
    Function Create(job As Job) As ActionResult
        If ModelState.IsValid Then
            db.Jobs.Add(job)
            db.SaveChanges()

            Dim customer As Customer = db.Customers.Find(job.CustomerId)
            Dim customerNumber As String = customer.CustCellphone.ToString()
            Dim messageSender As SendMessage = New SendMessage
            Dim smsMessage As String = "LAUNDRY: Job Number " & job.JobId & " has been booked in. You will be notified when individual services within it are ready for collection."
            messageSender.SendMessage(smsMessage, customerNumber)

            Dim url As String = "/RequestedService/AddService/" + job.JobId.ToString()
            Return Redirect(url)
        End If
        Return View(job)
    End Function

EDIT

I was wondering too how this is passed back as in the POST I may be able to check for a ‘null’? However I feel the problem may be the moment the submit button is pressed

Answer 1

In your POST controller action you forgot to populate the ViewCrap (oops, I meant ViewBag) before returning the view:

<HttpPost()>
Function Create(job As Job) As ActionResult
    If ModelState.IsValid Then
        ...
    End If

    ' Here you must populate the ViewCrap before returning the view the same 
    ' way you did in your GET action because your view depend on it
    Dim BillQuery = From s In db.Bills
                    Select s
    ViewBag.BillId = New SelectList(BillQuery.ToList(), "BillId", "BillDate")

    Return View(job)
End Function

But I would hyper strongly recommend you to use view models and forget about the existence of the …… (the word that I don’t want to pronounce).

---

UPDATE:

Now let’s look at the correct way to implement this (which is by using view models). A view model is a class that you should define for each of your views and which will represent its specific requirements. So from what you have said in the comments section to my answer you want to have a dropdown list in your view where the user has to select a bill from a dropdown and which is required.

So let’s roll the view model:

public class JobViewModel
{
    [Required(ErrorMessage = "Please select a bill")]
    [Display(Name = "Bill")]
    public int? SelectedBillId { get; set; }
    public IEnumerable<SelectListItem> Bills 
    { 
        get 
        {
            return db.Bills.ToList().Select(x => new SelectListItem
            {
                Value = x.BillId.ToString(),
                Text = x.BillDate.ToString()
            });
        } 
    }

    public int CustomerId { get; set; }

    ... here you could put any other properties that you want 
        to display on the view, things like JobId, ...
}

then we define our controller with the 2 actions:

public ActionResult Create(int id)
{
    var model = new JobViewModel
    {
        CustomerId = id
    };
    return View(model);
}

[HttpPost]
public ActionResult Create(JobViewModel model)
{
    if (ModelState.IsValid)
    {
        // Using AutoMapper here to map between the domain model
        // and the view model (http://automapper.org/)
        var job = Mapper.Map<JobViewModel, Job>(model);

        // Now call your service layer to do the necessary processings
        // on this job domain model including saving the job and sending 
        // messages and stuff. This avoids polluting your controller with
        // business logic code which belongs to your service layer
        ServiceLayer.ProcessJob(job);

        return RedirectToAction("AddService", "RequestedService", new { id = job.JobId });
    } 

    return View(model);
}

and finally you will have a corresponding view which will be strongly typed to the view model:

@model JobViewModel

@using (Html.BeginForm())
{
    <div>
        @Html.LabelFor(x => x.SelectedBillId)
        @Html.DropDownListFor(x => x.SelectedBillId, Model.Bills, "-- select --")
        @Html.ValidationMessageFor(x => x.SelectedBillId)
    </div>

    ... some other input fields

    <p><button type="submit">OK</button></p>
}

---

And now, as promised in the comments section let me show what I dubbed the absolute pornographic approach to solve this and which if you implemented in your application I will have to ask you to no longer come back and ask any ASP.NET MVC related question on StackOverflow 🙂

The pornographic approach consisted into manually inserting an item with id = 0 and text = empty string into the beginning of the list and then inside the controller verifying if the selected id equals 0 in order to check whether the model is valid or not:

So in your GET action:

Function Create(id As Integer) As ViewResult
    ViewBag.id = id
    Dim job As Job = New Job
    job.CustomerId = id
    job.JobAmount = 0
    job.JobDate = Date.Now()
    job.JobStatus = "Active"

    Dim Bills = db.Bills.ToList().Select(Function(s) New SelectListItem With { .Value = s.BillId.ToString(), .Text = s.BillDate.ToString() })
    Bills.Insert(0, New SelectListItem With { .Value = "0", .Text = "" })
    ViewBag.BillId = Bills

    Return View(job)
End Function

<HttpPost()>
Function Create(job As Job, BillId as Integer) As ActionResult
    If BillId > 0 Then
        db.Jobs.Add(job)
        db.SaveChanges()

        Dim customer As Customer = db.Customers.Find(job.CustomerId)
        Dim customerNumber As String = customer.CustCellphone.ToString()
        Dim messageSender As SendMessage = New SendMessage
        Dim smsMessage As String = "LAUNDRY: Job Number " & job.JobId & " has been booked in. You will be notified when individual services within it are ready for collection."
        messageSender.SendMessage(smsMessage, customerNumber)

        Dim url As String = "/RequestedService/AddService/" + job.JobId.ToString()
        Return Redirect(url)
    End If
    ModelState.AddModelError("BillId", "Please select a bill")
    Return View(job)
End Function

and inside the view:

@Html.DropDownList("BillId")