@ Html.DropDownListFor和viewmodel不发回到控制器？

Question

It's time to ask the internet. I'm a student and really new to MVC + coding in general, but I can't for the life of me figure out why this isn't working. Probably something really obvious. :/

I have a View ( AddMemberToGroup ) that is strongly-typed to a viewmodel ( PersonGroupingViewModel ). What am I trying to do is add a Person to a Group, with the user selecting a group from a dropdown list in the View.

Viewmodel:

public class PersonGroupingViewModel
{
    public int PersonId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set;}

    public List<Group> Memberships { get; set; }

    public SelectList AllGroups { get; set; }
    public int SelectedGroupId { get; set; }

}

Controller:

//GET People/AddToGroup
public ActionResult AddMemberToGroup(int? PersonId)
{
    if (PersonId == null)
    {
        return new HttpStatusCodeResult(HttpStatusCode.BadRequest); 
    }

    Person person = db.People.Find(PersonId);
    if (person == null)
    {
        return HttpNotFound();
    }

    SelectList allthegroups = new SelectList(db.Groups, "GroupId", "Name");

    PersonGroupingViewModel viewmodel = new PersonGroupingViewModel();
    viewmodel.PersonId = person.PersonId;
    viewmodel.FirstName = person.FirstName;
    viewmodel.LastName = person.LastName;
    viewmodel.AllGroups = allthegroups;

    //viewmodel.Memberships cannot be empty
    if (person.Memberships == null)
    {
        viewmodel.Memberships = new List<Group>();
    }
    else 
    { 
        viewmodel.Memberships = person.Memberships.ToList(); 
    }

    return View(viewmodel);
}

//POST: People/AddToGroup 
[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult AddMemberToGroup(PersonGroupingViewModel vm)
{
    SelectList allthegroups = new SelectList(db.Groups, "GroupId", "Name");
    vm.AllGroups = allthegroups; 

    int PersonId = vm.PersonId;
    int GroupId = vm.SelectedGroupId;

    Person person = db.People.Find(PersonId);
    Group group = db.Groups.Find(GroupId);
    group.Members.Add(person);
    db.SaveChanges();

    return View(vm);
}

View form (strongly-typed to PersonGroupingViewModel):

Add Member To Group

---

@using (Html.BeginForm())
{
@Html.AntiForgeryToken()

<div>
    <h4>Groups @Html.DisplayFor(model => model.FirstName) is already in:</h4>
    <ul>
        @foreach (var group in Model.Memberships)
        {
            <li>@Html.DisplayFor(modelItem => group.Name)</li>
        }
    </ul>
</div>

<div class="form-horizontal">
    <h4>Add @Html.DisplayFor(model => model.FirstName) @Html.DisplayFor(model => model.LastName) To Group:</h4>

    @Html.HiddenFor(model => model.PersonId)

    <div class="form-group">
        <label>Group: </label>
        @Html.DropDownListFor(m => m.SelectedGroupId, Model.AllGroups, String.Empty)
    </div>

    <div class="form-group">
        <div class="col-md-offset-2 col-md-10">
            <input type="submit" value="Add To Group" class="btn btn-default" />
        </div>
    </div>
</div>
}

When I click 'submit' in the browser I get back a URL: localhost:54696/People/AddMemberToGroup?PersonId=1

Along with exception "Object reference not set to an instance of an object" on the line Group group = db.Groups.Find(GroupId); in the controller.

Is the viewmodel coming back empty or not at all? I am not sure what is going on and would appreciate someone attempting explaining it in small words. I've done a lot of Googling and tried several suggestions from that but in the end I feel as though I'm going in circles.

Answer 1

Solved it...

group.Members was null so group.Members.Add(person) didn't work. Wrote in an if/else statement to instantiate a new member list if the list is null, and it works fine:

if (group.Members == null)
            {
                group.Members = new List<Person>();
                group.Members.Add(person);
            }
            else {
                group.Members.Add(person);
            }