Processing MVC combobox value

Question

I thought it would be an easy task, but I'm struggling for a while now already.

I have Sport object which belongs to some SportGroup object.

I want to represent groups inside a combobox, and I created SportViewModel.cs with the following properties

public Guid Id {get; set;}
public string Name {get; set;}
public Guid? SportGroupId { get;set; }
public IEnumerable<SportGroup> SportGroups {get; set; }

HttpGet create action

SportViewModel newSport = new SportViewModel();
newSport.SportGroups = new SelectList(GetAllSportGroups(), "Id", "Name");

//GetAllSportGroups() returns list of SportGroups domain objects as you can imagine.

My create view looks like this

@model Models.SportViewModel
<div>Sport group</div>
<div>@Html.DropDownListFor(x => x.SportGroupId, Model.SportGroups)</div>

Now on the controller's httpPost action I receive the data and try to save it:

[HttpPost]
public ActionResult Create(SportViewModel newSport)
{
   // if model state is valid and other checking omitted
   // session and transaction omitted
   Sport sport = new Sport();
   sport.Id = newSport.Id;
   sport.Name = newSport.Name;
   SportGroup sportGroup = session.Load<SportGroup>(SportGroupId);
   sport.SportGroups.Add(sportGroup); // here is where I'm getting an error
}

After submitting I'm getting these error

Object reference not set to an instance of an object.
Referencing line sport.SportGroups.Add(sportGroup);

Please help. Thanks

on debugging SportGroup sportGroup = session.Load<SportGroup>(SportGroupId); is properly filled with SportGroup object. — Grunf
– Grunf, Commented Aug 25, 2012 at 11:43
Yes OK - but how about the .SportGroups property on the sport object?? — marc_s
– marc_s, Commented Aug 25, 2012 at 11:44

marc_s · Accepted Answer · 2012-08-25 12:02:27Z

2

Try this:

// if model state is valid and other checking omitted
// session and transaction omitted
Sport sport = new Sport();

if(sport.SportGroups != null)
{
   // everything OK !
}
else
{
   // you need to instantiate the .SportGroups property!
   sport.SportGroups = new .........();
}

Because otherwise:

sport.SportGroups.Add(sportGroup); // here is where I'm getting an error
     *************

If this .SportGroups property is NOT properly instantiated, it will be NULL and you cannot call .Add() on it! You'll get that exact exception that you have.

This is typically something you would best do in the constructor of the Sport class - make sure that all contained collections etc. are also initalized and instantiated when you create a new object of type Sport

edited Aug 25, 2012 at 12:02

answered Aug 25, 2012 at 11:56

marc_s

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Comments

user45886 · Accepted Answer · 2012-08-25 11:40:50Z

2

You may not have instantiated a collection instance that is behind the SportsGroups property of the Sport class. Check it's constructor/initialization.

answered Aug 25, 2012 at 11:40

user45886

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