Checkboxlist - on Post: Create - NullReferenceException - nullreferenceexception

Question

StudentModel.
namespace mvcApp.Models
{
public class StudentModel
{
[Display(Name = "First Name")]
public string FirstName { get; set; }
[Display(Name = "Last Name")]
public string LastName { get; set; }
[Display(Name = "Email Address")]
public string EmailAddress { get; set; }
public List<SchoolOrganization> Organizations { get; set; }
}
public class SchoolOrganization
{
public string Name { get; set; }
public bool IsInvolved { get; set; }
}
}
Student is involved in multiple organizations.
Controller
namespace mvcApp.Controllers
{
public class HomeController : Controller
{
public ActionResult Index()
{
ViewBag.Message = "Modify this template to jump-start your ASP.NET MVC application.";
return View();
}
public ActionResult StudentInformation()
{
// populate student with data
var student = new StudentModel() { FirstName = "Joe", LastName = "Doe", EmailAddress = "jdoe#hotmail.com"};
// Populate with Organizations
student.Organizations = new List<SchoolOrganization>();
student.Organizations.Add(new SchoolOrganization() { Name = "Math Club", IsInvolved = true});
student.Organizations.Add(new SchoolOrganization() { Name = "Chess Club", IsInvolved = false });
student.Organizations.Add(new SchoolOrganization() { Name = "Football", IsInvolved = true });
return View(student);
}
**[HttpPost]
public ActionResult StudentInformation(StudentModel student)
{
Response.Write("Name: " + student.FirstName);
foreach (var o in student.Organizations)
{
Response.Write(o.Name + " : " + o.IsInvolved.ToString());
}
return View();
}**
}
}
Data will be eventually populated from database.
View
#model mvcApp.Models.StudentModel
#{
ViewBag.Title = "StudentInformation";
}
<h2>StudentInformation</h2>
#using (Html.BeginForm()) {
#Html.AntiForgeryToken()
#Html.ValidationSummary(true)
<fieldset>
<legend>StudentModel</legend>
<div class="editor-label">
#Html.LabelFor(model => model.FirstName)
</div>
<div class="editor-field">
#Html.EditorFor(model => model.FirstName)
#Html.ValidationMessageFor(model => model.FirstName)
</div>
<div class="editor-label">
#Html.LabelFor(model => model.LastName)
</div>
<div class="editor-field">
#Html.EditorFor(model => model.LastName)
#Html.ValidationMessageFor(model => model.LastName)
</div>
<div class="editor-label">
#Html.LabelFor(model => model.EmailAddress)
</div>
<div class="editor-field">
#Html.EditorFor(model => model.EmailAddress)
#Html.ValidationMessageFor(model => model.EmailAddress)
</div>
<div>
<table>
<tr>
<td>Organization Name</td><td>Is Involved</td>
</tr>
#for (int i = 0; i < Model.Organizations.Count; i++) <== System.NullReferenceException here
{
#Html.HiddenFor(m => m.Organizations[i].IsInvolved)
<tr>
<td>#Html.DisplayFor(m => m.Organizations[i].Name)</td>
<td>#Html.CheckBoxFor(m => m.Organizations[i].IsInvolved)</td>
</tr>
}
</table>
</div>
<p>
<input type="submit" value="Save" />
</p>
</fieldset>
}
The above code displays fine with HttGet. However, when i try to update i get System.NullReferenceException. https://www.dropbox.com/s/dz0bg3hkd0yq8e3/studentInformation.png?dl=0
Can anyone please help figure it what's going on?
Thank you.

Answer 1

In the code sample you have provided; the HomeController's [HttpPost] ActionResult for StudentInformation does not create and pass a new instance of the updated object model to the view, but instead runs a basic debug.writeline routine. As a result the dependent view for the [HttpPost] view of "StudentInformation.cshtml", does not receive a populated instance of the updated model...
Setting a breakpoint on the first line of the model for the "StudentInformation.cshtml" page and running the application will demonstrate that the model referenced at the top of the page, has no data within it...
This is why the [HttpPost] version of the page simply renders the blank model, without any altered data values you may have created, UNTIL it gets to the section where the view is dependent on a count of new data values which must be present within the model that is called at first line of the page... to continue.
An empty data model set results in a null reference because there are no values to count within it.
In order to view an updated group of settings, The [HttpPost] version of the view model must be passed an instance of the model that returns the new information as in "return View(nameOfViewDataSet)" (you build a data set again, and pass it as a new version of the model, with revised form data present).
Until there is data passed via the return View statement relative to the [HttpPost] version of the StudentInformation ActionResult, to the actual view, there will be no display data, and the count function will continue to return a null value.
I hope that is helpful.