Why can't I update my variables from my Model in my Controller using C# MVC

Question

I'm trying to run an update process in my Controller and in doing so trying to access values from my Model, this is the code in my Controller

    [HttpGet]
    [Authorize]
    public void updateNewGroup(int id)
    {
        int incomingID = id;
        TaskViewModel model = new TaskViewModel();                       
        model.groupIdx.Add(id);            
    }

This is how I've set up my variable in my Model

    public List<int> groupIdx { get; set; }

When the code runs and it attempts to update groupIdx list I get an error 'NullReferenceException unhandled by user' what am I doing wrong?

Because `groupIdx` is `null`. You need to initialize it - `model.groupIdx = new List();` — , Dec 01 '15 at 09:49

score 1 · Accepted Answer · answered Dec 01 '15 at 09:49

1

As far as I can see - you're not initializing model.groupIdx thus it remains null - that's why you're getting NullReferenceException

It should be something like

TaskViewModel model = new TaskViewModel();                       
model.groupIdx = new List<int>();
model.groupIdx.Add(id);

answered Dec 01 '15 at 09:49

Andrey Korneyev

26,353
15
70
71

Both answers were correct, thank you. The only other problem is that each time the updateNewGroup method is called groupIdx is always null and never retains it's value. How do I get it to persist? – user616076 Dec 01 '15 at 10:15
@user616076 of course `groupIdx` is null each time you're calling `updateNewGroup` because you're creating new model each time in the very beginning of this method. If you need to somehow store this `groupIdx` - it can be done by variable ways. You can store it in DB, some static field or something else - it's a broad question and it's hard to say more without knowing your application architecture and goals. – Andrey Korneyev Dec 01 '15 at 11:11

score 0 · Answer 2 · answered Dec 01 '15 at 09:50

0

Your groupIdx is still null, it's uninitialized. Either add a constructor to the Model where you initialize it or set it when you create the object.

model.groupIdx = new List<int>();

answered Dec 01 '15 at 09:50

Peter

171
4

Why can't I update my variables from my Model in my Controller using C# MVC

2 Answers2