How to handle NullReferenceException in a foreach?

Question

foreach (string s in myField.getChilds()) {
    if (s == null)
        //handle null
    else
        //handle normal value 
}

When I run my program i get a NullReferenceException because getChilds may return null. How can I make my program to continue anyway and handle the exception? I can't handle it outside of the foreach, can't explain why because it will take too much time (and I am sure you guys are busy :P). Any ideas?

I already tryed that way:

foreach (string s in myField.getChilds() ?? new ArrayList(1)) {
        if (s == null)
            //handle null
        else
            //handle normal value 
    }

But it does not work, program just jump at the end of the foreach but I want it to enter the foreach instead!

I don't understand your comment regarding the null coalescing operator: "it does not work, program just jump at the end of the foreach but I want it to enter the foreach instead!". It's possible because you are not giving a string that it is just jumping. You have to give it a value it can assign to s. — Matt Mitchell
– Matt Mitchell, Commented Dec 20, 2010 at 8:30

Gabe · Accepted Answer · 2010-12-20 08:31:10Z

10

One way to do this (though not the best way) is:

foreach (string s in myField.getChilds() ?? new string[] { null })

or

foreach (string s in myField.getChilds() ?? new ArrayList { null })

The reason new ArrayList(1) doesn't work is that it creates a list that has the capacity to hold 1 element, but is still empty. However new string[] { null } creates a string array with a single element which is just null, which is what you appear to want.

edited Dec 20, 2010 at 8:31

answered Dec 20, 2010 at 8:24

Gabe

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4 Comments

Jon Skeet Over a year ago

I see no indication that the OP wants to actually go into the loop if the return value is null... I have assumed he wants to treat null in the same way as an empty collection.

Gabe Over a year ago

Jon: When OP says "program just jump at the end of the foreach but I want it to enter the foreach instead!" I take that as an indication he wants a null collection to be treated as a one-element collection containing null.

Jon Skeet Over a year ago

Whoops, you're absolutely right. Hadn't seen that. +1, and deleting my answer :)

raz3r Over a year ago

That worked! I should have thinked about that, thank you guys so much, all of you. Sorry if I am noob :(

Darin Dimitrov · Accepted Answer · 2010-12-20 08:23:28Z

3

var children = myField.getChilds();
if (children == null)
{
    // Handle the null case
}
else
{
    foreach (string s in children)
    {

    }
}

or simply use the null coalescing operator:

foreach (string s in myField.getChilds() ?? Enumerable.Empty<string>())
{

}

answered Dec 20, 2010 at 8:23

Darin Dimitrov

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7 Comments

raz3r Over a year ago

As I said I can't handle it outside, it must be done in the foreach. Otherwise I would have done it in that way :P I am noob but not stupid LOL. Anyway I tried your suggestion but I had to add a cast since getChilds() returns an ArrayList. So i modified it like that: string s in (string[])source.getChilds().ToArray() ?? Enumerable.Empty<string>() but again, NullReferenceException :(

Darin Dimitrov Over a year ago

@raz3r, ArrayList??? Please don't. Use strongly typed collections. Also are you sure that it is not the myField variable that is null?

raz3r Over a year ago

Looks like I am an idiot instead because I try to call the ToArray() method on a null object. Ofcourse I get NullReferenceException anyway xD Beside that I can give it a try with a collection but then if I implement an Interface I must override all Count, GetEnumerator etc. Am I wrong? What type do you suggest? myField is absolutely not null because I use it before and I have debugged the application don't worry.

Jon Skeet Over a year ago

@raz3r: You really should explain why you can't "handle this outside the foreach" - it's a natural answer, and a very unnatural restriction.

jgauffin Over a year ago

It does not make any sense that you should handle stuff inside a loop if the collection you are looping is empty or null. There are nothing to process

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TalentTuner · Accepted Answer · 2010-12-20 08:36:12Z

2

if it is the myField.getChilds() which may contain null

than

foreach (string s in myField.getChilds()) {
if (string.IsNullOrEmpty(s))
    //handle null
else
    //handle normal value

}

this way ,you can handel null or empty strings.

answered Dec 20, 2010 at 8:36

TalentTuner

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