NullReferenceException was unhandled by user code

Question

I have image slider, and in IE I have an error.

This is my code:

*//Složka s obrázky partnerů
string slozka = HttpContext.Current.Server.MapPath("~/Partneri");

protected void Page_Load(object sender, EventArgs e)
{
    int cislo_obrazku = 0;
    if (!Page.IsPostBack)
    {
        ArrayList slide = new ArrayList();
        System.IO.DirectoryInfo inputDir = new System.IO.DirectoryInfo(slozka);
        foreach (FileInfo eachfile in inputDir.GetFiles())
        {
            slide.Add(eachfile.ToString());
            cislo_obrazku += 1;
        }
        Session["cislo_obrazku"] = cislo_obrazku;
        Session["obrazky"] = slide;
        Casovac(this, new EventArgs());
    }
}

protected void Casovac(object sender, EventArgs e)
{
    ArrayList slide = new ArrayList();
    slide = (ArrayList)Session["obrazky"];
    if ((Session["aktualni"] != null) && (Convert.ToInt32(Session["aktualni"]) != Convert.ToInt32(Session["cislo_obrazku"]) - 1))
    {
        //Posun na další
        Image1.ImageUrl = "~\\Partneri\\" + slide[Convert.ToInt32(Session["aktualni"]) + 1].ToString();
        Session["aktualni"] = Convert.ToInt32(Session["aktualni"]) + 1;
    }
    else if (Session["aktualni"] == Session["cislo_obrazku"])
    {
        //Pokud není další, začni znovu
        Image1.ImageUrl = "~\\Partneri\\" + slide[0].ToString();
        Session["aktualni"] = 0;
    }
    else
    {
        if (Convert.ToInt32(Session["cislo_obrazku"]) != 0)
        {
            //Initally load the first image in the image control
            Image1.ImageUrl = "~\\Partneri\\" + slide[0].ToString();
            Session["aktualni"] = 0;
        }
    }
}

Error: NullReferenceException was unhandled by user code

Error point to this:

Image1.ImageUrl = "~\\Partneri\\" + slide[0].ToString();

and

Image1.ImageUrl = "~\\Partneri\\" + slide[0].ToString();

Have you some idea?

possible duplicate of [What is a NullReferenceException and how do I fix it?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — John Saunders, Mar 04 '15 at 02:50

score 2 · Accepted Answer · answered Apr 04 '13 at 11:28

NRE is possibly the easiest exception to understand. It is also one of the most common problems when you have not coded defensively enough.

Quite simply, it means you are trying to access a property/method on an object which is null.

In your case

 slide[0].ToString();

either

slide is null
slide[0] is null.

Set a breakpoint, debug your app. Determine Which is null, and either account for that (eg, with a default) or throw a more meaningful exception if it truly is an exceptional circumstance for that object to be null.

score 0 · Answer 2 · answered Apr 04 '13 at 11:26

0

The session object obrazky has not been initialized.

Add a null check after:

slide = (ArrayList)Session["obrazky"];

answered Apr 04 '13 at 11:26

jgauffin

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`if (slide == null || slide[0] == null)` – jgauffin Apr 04 '13 at 12:53
Error again...i haven´t wrote all code, now the code on top is edited and its all... – koky Apr 04 '13 at 13:13
where did you add the check? – jgauffin Apr 04 '13 at 13:31
under this...slide = (ArrayList)Session["obrazky"]; But i have the same error – koky Apr 04 '13 at 13:47
and what do you do in the if statement? – jgauffin Apr 04 '13 at 13:56
you have to either assign a value to the slide or exit the method is the `if` condition evaluates to false. – jgauffin Apr 04 '13 at 14:35

NullReferenceException was unhandled by user code

2 Answers2