Array check: Object reference not set to an instance of an object

Question

I am struggling to find a solution to this problem. I see lots of simlilar entries on this site that touch upon this subject, but I can't seem to arrive at a solution. I am trying to check a table in cache to see if it already exists and if not, populate it. Below is my code for that check and it errors on the 'if' statement telling me 'System.NullReferenceException: Object reference not set to an instance of an object'. This is puzzling because shouldn't the '.IsNullOrEmpty' catch this? I figure if the 1st element in the array is null or empty then it hasn't been cached yet and therefore take action.

            string[] saveCatList = Cache["Categories" + Session["sessopnID"]] as string[];
            if (string.IsNullOrEmpty(saveCatList[0]))
            {
                WBDEMOReference.getcatlist_itemcategories[] categories;
                strResult = callWebServ.getcatlist(Session["sessionID"].ToString(),
                            out strResultText, out dNumOfCat, out categories);

                for (int i = 0; i < categories.Length; i++)
                {
                    //ddCat is the ID number of the category drop down list
                    ddCat.Items.Add(new ListItem(categories[i].categorydesc.ToString(),
                                                 categories[i].categorynumber.ToString()));
                }
            }

Thank you for your answers! Good to know that the array itself is a check to make before looking at elements inside it. I still have old COBOL in me ;) — Anthony Darden
– Anthony Darden, Commented Jul 9, 2012 at 17:29

Zbigniew · Accepted Answer · 2012-07-09 16:19:42Z

5

With string.IsNullOrEmpty(saveCatList[0]) you check if first element of array is empty or null. It seems that your array is null, so you should firstly check your array:

if(saveCatList == null || string.IsNullOrEmpty(saveCatList[0]))

answered Jul 9, 2012 at 16:19

Zbigniew

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tmesser · Accepted Answer · 2012-07-09 16:20:35Z

1

Cache["Categories" + Session["sessopnID"]] as string[];

This cast is failing and 'as string' is returning null. Therefore, when you try to access the associated variable as an array, you're essentially doing null[0], which is a NullReferenceException.

If you add a check to first ensure that the array is not null this will work fine.

answered Jul 9, 2012 at 16:20

tmesser

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Tilak · Accepted Answer · 2012-07-09 16:23:09Z

0

Change

if (string.IsNullOrEmpty(saveCatList[0]))

To

if (saveCatList != null && saveCatList.Length>0 && string.IsNullOrEmpty(saveCatList[0]))

Also, Change

  ddCat.Items.Add(new ListItem(categories[i].categorydesc.ToString(),
                                             categories[i].categorynumber.ToString()));

To

if (categories[i].categorydesc != null && categories[i].categorynumber!= null)
{
  ddCat.Items.Add(new ListItem(categories[i].categorydesc.ToString(),
                                             categories[i].categorynumber.ToString()));

}

answered Jul 9, 2012 at 16:23

Tilak

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