Problem in showing ViewModel in Create form - asp.net-mvc

Question

I am learning how to use ViewModel to show the fields from 2 different models. I have one model containing the MsgTypeId, MsgType and MsgStatus and the another model OptStatus containing the StatusId, StatusName and StatusValue. The MsgStatus will be shown in form of drop down list and show all the values in OptStatus. Both models have a separate database table to store their values.
namespace theManager.Areas.Settings.Models
{
public class OptStatus
{
[Required]
[Key]
public int StatusId { get; set; }
[Required]
public string StatusName { get; set; }
[Required]
public char StatusValue { get; set; }
}
}
namespace theManager.Areas.Settings.Models
{
public class OptMsgType
{
[Required]
[Key]
public int MsgTypeId { get; set; }
[Required]
public string MsgType { get; set; }
[Required]
public string MsgStatus { get; set; }
}
}
I have created a ViewModel to show these fields in the Create form of OptMsgType. However, when I run the code, I got an error
"System.NullReferenceException: 'Object reference not set to an instance of an object.'"
I would like to ask if there is something wrong with my ViewModel. Thanks!
namespace theManager.Areas.Settings.ViewModels
{
public class OptMsgTypeCreateViewModel
{
public OptMsgType OptMsgType { get; set; }
public IEnumerable<SelectListItem> OptStatuses { get; set; }
}
}
OptMsgTypeController.cs
public IActionResult Create(int id)
{
var OptMsgTypeViewModel = new OptMsgTypeCreateViewModel();
OptMsgTypeViewModel.OptStatuses = _context.OptStatus.ToList().Select(x => new SelectListItem
{
Text = x.StatusName,
Value = x.StatusValue.ToString()
});
OptMsgTypeViewModel.OptMsgType = _context.OptMsgType.Where(a => a.MsgTypeId == id).FirstOrDefault();
//var v = _context.OptMsgType.Where(a => a.MsgTypeId == id).FirstOrDefault();
return View(OptMsgTypeViewModel);
}
I have problems in displaying the Create form which will show the fields declared in the ViewModel.
#model theManager.Areas.Settings.ViewModels.OptMsgTypeCreateViewModel
#{
ViewData["Title"] = "Create";
Layout = null;
}
<h2>Message Type Settings</h2>
#using (Html.BeginForm("Create","OptMsgType", FormMethod.Post, new { id= "popupForm" }))
{
if (Model != null && Model.OptMsgType.MsgTypeId > 0)
{
#Html.HiddenFor(a=>a.OptMsgType.MsgTypeId)
}
<div class="form-group">
<label>Message Type ID</label>
#Html.TextBoxFor(a=>a.OptMsgType.MsgTypeId,new { #class = "form-control" })
#Html.ValidationMessageFor(a=>a.OptMsgType.MsgTypeId)
</div>
<div class="form-group">
<label>Leave Type</label>
#Html.TextBoxFor(a => a.OptMsgType.MsgType, new { #class = "form-control" })
#Html.ValidationMessageFor(a => a.OptMsgType.MsgType)
</div>
<div class="form-group">
<label>Status</label>
#Html.DropDownListFor(model => model.OptStatuses, new SelectList(Model.OptStatuses, "Value", "Text"), htmlAttributes: new { #class = "form-control", id = "OptStatus" })
#Html.ValidationMessageFor(a => a.OptStatuses)
</div>
<div>
<input type="submit" value="Create" />
</div>
}

Answer 1

The System.NullReferenceException indicates that you are using a field without initializing it. It coulbe a problem with your view model or it could be a problem anywere else. For example from the code smaple is not possible to see where you initialize the context you are using to get the data, and that could be the cause of the exception you are getting.
Either way I would advise you to pay attention to yout IDE, it usualy indicates in which line adnd class the exception is being thown. If you navigate to that class at that line you will easily identify which field can be de cause of the exception.
Regarding your view model, its considered a good practice to always initialize the lists on your model on the constructor of your class. This way you can guarantee that they are already initialized when you try to use them.
So my sugestion would be to initialize your list on the constructor of your viewmodel
public OptMsgTypeCreateViewModel()
{
OptStatuses = new List<OptStatus>();
}

Answer 2

#George, thanks for the reply. Please try this then: instantiate your class in the viewmodel.
public class OptMsgTypeCreateViewModel
{
public OptMsgTypeCreateViewModel()
{
OptMsgType = new OptMsgType();
}
public OptMsgType OptMsgType { get; set; }
public IEnumerable<SelectListItem> OptStatuses { get; set; }
}

Answer 3

hi in action controller you should change this code:
OptMsgTypeViewModel.OptStatuses = _context.OptStatus.ToList().Select(x => new SelectListItem
{
Text = x.StatusName,
Value = x.StatusValue.ToString()
});
I think _context.OptStatus.ToList() in null so you get this exception. change code to this:
OptMsgTypeViewModel.OptStatuses =new list<SelectListItem>();
var temp= _context.OptStatus.ToList();
if(temp!=null&&temp.count()>0)
{
OptMsgTypeViewModel.OptStatuses = temp.Select(x => new SelectListItem
{
Text = x.StatusName,
Value = x.StatusValue.ToString()
}).tolist();
}
EDIT:
I think this object "Model.OptMsgType" is null
change code in view like this:
if (Model != null && Model.OptMsgType!=null && Model.OptMsgType.MsgTypeId > 0)
{
#Html.HiddenFor(a=>a.OptMsgType.MsgTypeId)
}