How to Error Handle a NullReferenceException - asp.net-mvc-4

Question

My website went down for a few days, therefore I am trying to produce some error handling while the MVC app doesnt have access to certain resources so if something doesnt become unavailable again the WHOLE THING doesnt have to go down.
At the moment a controller is trying to access viewbag.moreNewProducts that isnt available.
public ActionResult Index(string search)
{
string[] newProductLines = this.getMoreNewProducts();
string[] newNews = this.getMoreNews();
string[] newPromotions = this.getMorePromotions();
string[] fewerProductLines = this.getLessNewProducts(newProductLines);
ViewBag.moreNewProducts = newProductLines;
ViewBag.moreNews = newNews;
ViewBag.morePromotions = newPromotions;
ViewBag.lessNewProducts = fewerProductLines;
bool disableShowMore = false;
This is where I run into an error: " foreach (string line in newProductLines)"
public string[] getLessNewProducts(string[] newProductLines)
{
int charCount = 0;
int arrayCount = 0;
string[] displayProductLines = new string[6];
bool continueWriting;
if (newProductLines == null)
{
foreach (string line in newProductLines)
{
continueWriting = false;
for (int i = 0; charCount < 250 && i < line.Length && arrayCount < 5; i++)
{
string index = newProductLines[arrayCount].Substring(i, 1);
displayProductLines[arrayCount] += index;
charCount++;
continueWriting = true;
}
if (continueWriting == true)
{
arrayCount++;
}
}
string[] LessNewProducts = new string[arrayCount];
for (int d = 0; d < arrayCount; d++)
{
LessNewProducts[d] = displayProductLines[d];
}
return LessNewProducts;
}
else
{
return null;
}
}
how do I get around an if else statement so the whole thing doesnt have to crash?

Answer 1

Two things.
Your if (newProductLines == null) statement has the wrong condition on it. I don't believe that you want to enter that if newProductLines is null. You can inverse this condition to get the desired result(if (newProductLines != null)).
If you run into another situation later where you need to catch an error, you can always use the try-catch block to catch exceptions that you are expecting.
try
{
//code that could cause the error here
}
catch(NullReferenceException nullRefExcep)
{
//what you want it to do if the null reference exception occurs
}

Answer 2

if (newProductLines == null)
should be replaced with if (newProductLines != null) so you don't have to handle the code with newProductLines as null. Basically, with this condition you will always have the NullReferenceException unless you manage your exception with a try catch block.

Answer 3

The real question to ask yourself is:
Why would newProductLines be null?
Presumably getMoreNewProducts() found a situation where it thought it would be appropriate to return a null value.
If this is happening because the system has an error that would make your page meaningless, then you may just want to change getMoreNewProducts() so that it throws an exception when that error state occurs. Typically it's safest and easiest to debug programs that fail as soon as they run into an unexpected situation.
If this is happening because there are no new products, then you should just return an empty collection, rather than null. All your code should work just fine after that, without the need for an if/else statement: it will return an empty array for LessNewProducts, which is probably correct.
However, let's assume that there's a situation that you're anticipating will occur from time to time, which will make it impossible for you to retrieve newProductLines at that time, but which you would like the system to handle gracefully otherwise. You could just use null to indicate that the value isn't there, but it's really hard to know which variables might be null and which never should be. It may be wiser to use an optional type to represent that getMoreNewProducts() might not return anything at all, so you can force any consuming code to recognize this possibility and figure out how to deal with it before the project will even compile:
public ActionResult Index(string search)
{
Maybe<string[]> newProductLines = this.getMoreNewProducts();
string[] newNews = this.getMoreNews();
string[] newPromotions = this.getMorePromotions();
Maybe<string[]> fewerProductLines = newProductLines.Select(this.getLessNewProducts);
Disclaimer: I am the author of the Maybe<> class referenced above.
Here are some additional improvements I'd suggest:
Don't use ViewBag. Instead, create a strongly-typed ViewModel so that you can catch errors in your code at compile-time more often:
var viewModel = new ReportModel {
newProductLines = this.getMoreNewProducts(),
newNews = this.getMoreNews(),
...
};
...
return View(viewModel);
Learn to use LINQ. It will simplify a lot of your very complicated code. For example, instead of:
string[] LessNewProducts = new string[arrayCount];
for (int d = 0; d < arrayCount; d++)
{
LessNewProducts[d] = displayProductLines[d];
}
return LessNewProducts;
... you can say:
string[] LessNewProducts = displayProductLines.Take(arrayCount).ToArray();
In fact, I think your entire getLessNewProducts() method can be replaced with this:
return newProductLines
.Where(line => line.Length > 0)
.Select(line => line.Substring(0, Math.Min(line.Length, 250)))
.Take(5);