asp.Net MVC view model is empty on post - asp.net

Question

I have a complex view model that I am passing to a create view. When I enter data on the page and post it the model is empty. Both the fields in the sub-object and the "test" field are empty. Why?
public class ContactIncident
{
[Key]
public int Id { get; set; }
[DataType(DataType.MultilineText)]
public string Description { get; set; }
[Display(Name = "Incident Date")]
[DataType(DataType.Date)]
public DateTime? IncidentDateTime { get; set; }
[Display(Name = "Follow Up Date")]
[DisplayFormat(ApplyFormatInEditMode = true, DataFormatString = "{0:dd/MM/yyyy}")]
[DataType(DataType.Date)]
public DateTime? FollowUpDate { get; set; }
}
public class IncidentManager
{
public ContactIncident Incident { get; set; }
public string Test { get; set; }
}
public ActionResult Create(int? id)
{
IncidentManager im = new IncidentManager();
ContactIncident ci = new ContactIncident();
ci.IncidentDateTime = DateTime.Now;
ci.FollowUpDate = DateTime.Now.AddDays(14);
im.Incident = ci;
return View(im);
}
[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Create(IncidentManager im)
{
if (ModelState.IsValid)
{
ContactIncident ci = new ContactIncident();
ci.IncidentDateTime = incident.Incident.IncidentDateTime;
ci.Description = im.Incident.Description;
return RedirectToAction("Index");
}
return View(incident);
}
View:
#model MyApp.Web.ViewModels.IncidentManager
#{
ViewBag.Title = "Edit Incident";
}
<h4>#ViewBag.Title</h4>
#using (Html.BeginForm())
{
#Html.AntiForgeryToken()
<div class="form-horizontal well">
#Html.ValidationSummary(true)
#Html.EditorFor(model=>model.Test)
<div class="row">
<div class="col-md-2">
#Html.LabelFor(model => model.Incident.IncidentDateTime)
</div>
<div class="col-md-2">
#Html.DisplayFor(model => model.Incident.IncidentDateTime)
</div>
</div>
<div class="row">
<div class="col-md-2">
#Html.LabelFor(model => model.Incident.Description)
</div>
<div class="col-md-10">
#Html.EditorFor(model => model.Incident.Description, new { htmlAttributes = new { #class = "form-control", rows = "5" }, })
</div>
<div class="col-md-2">
#Html.LabelFor(model => model.Incident.FollowUpDate)
</div>
<div class="col-md-2">
#Html.EditorFor(model => model.Incident.FollowUpDate, new { htmlAttributes = new { #class = "form-control"}, })
</div>
</div>
</div>
<div class="form-group">
<div class="col-md-offset-2 col-md-10">
<input type="submit" value="Save" class="btn btn-default" />
</div>
</div>
}

Answer 1

The problem is that the DefaultModelBinder won't be able to map nested models properly if you use a different parameter name. You must use the same parameter name as the ViewModel name.
public ActionResult Create(IncidentManager incidentManager)
As a general practice, always use the name of the model as the parameter name to avoid mapping problems.
UPDATE:
The DefaultModelBinder uses "convention based" mapping.
IncidentManager.Incident = incidentManager.Incident (will map)
IncidentManager.Incident = im.Incident //won't map because 'im' != 'incidentManager'