In MVC why I cannot add a value to a session of a controller?

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In this code:

C#

CardexController controller = new CardexController();
controller.Session.Add("cmstr", id);
controller.Index();

I encounter the System.NullReferenceException error on the line:

C#

controller.Session.Add("cmstr", id);

What should I do while id has value and it is not empty?

What I have tried:

Session sounds like an interface so I cannot make a new instance of it. And also I cannot add a value to it.

Posted 24-Jun-16 22:52pm

bahman.01

Updated 25-Jun-16 0:30am

v2

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phil.o · Accepted Answer · 2016-06-25T00:30:00

Solution 1

Session property of you CardexController instance is null, undoubtably.
It's time for you to put a breakpoint on the line which initializes the controller, and press F5.

The fact that the Session property expects some instance of an object which implements a given interface (I just extrapolate from what you wrote, as I do not have the formal definition of the class), does not mean that you do not have to provide it with a valid instance.

Posted 25-Jun-16 0:30am

phil.o

Comments

bahman.01 26-Jun-16 2:11am

I cannot understand what you mean by "Session property expects some instance of an object which implements a given interface"

phil.o 26-Jun-16 4:28am

Which part of the sentence do you have trouble with?

bahman.01 26-Jun-16 2:15am

Here is the real code in the first controlller:
public void checkItem(string id)
{
if (Session["cmstr"] == null)
Session.Add("cmstr", id);
else
Session["cmstr"] = id;

CardexController controller = new CardexController();
controller.cmstr = id;
controller.Session.Add("cmstr", id);
controller.Index();
}

and this is the code in the second controller named "CardexController" :
public ActionResult Index()
{
var item = Session["cmstr"];
if (item == null)
return PartialView("~/Views/RcvAccount/RcvAccountCustomer/_RcvAccountCustomerPartial.cshtml");
else
return PartialView("~/Views/RcvAccount/RcvAccountCardex/_RcvAccountCardexPartial.cshtml");
}

What should I write in the below function?
public CardexController()
{

}

phil.o 26-Jun-16 4:38am

Please, avoid code in comments; that's hardly readable. You should use the green "Improve question" button instead, and place the code in there with correct formatting.
From what I see, the problem may be that you create a new CardexController in the checkItem method. Since you assign it to a local variable, this variable becomes null at the end of the method. So, basically your code just creates a new CardexController everytime the method is executed, throwing it away at the end.
Could it be possible that you already have a CardexController instance somewhere else, that you should use instead?
And public CardexController() { ... } is not a function declaration, this is a class definition. I don't know if this mistake is due to a bad wording, or to an incomplete knowledge. I have the feeling that there are some key concepts of OOP programming in general, and MVC specifically, that you still have to learn and understand.