ConcurrentDict doesnt prevent items to be null inside an loop

Question

foreach (KeyValuePair<uint, Character> kvp in World.AllChars)
{
    Character Charr = kvp.Value;
    Charr.Do(); <-- NullrefException: Charr is null
}

Code with the check:

foreach (KeyValuePair<uint, Character> kvp in World.AllChars)
{
    Character Charr = kvp.Value;
    if(Charr != null)
    {
       Charr.Do(); <-- NullrefException: Charr is null
    }
}

EDIT:

Nothing inside Do() causes it to be null, as it doesnt even get to execute Do() because Charr is null.

Thats how my loop looks like. I already tried to do a if != null check but it still goes past that and throws an nullref exception.

How can I prevent that? Im using a Concurrent Dictionary and a highly multithreaded server architecture.

We don't have nearly enough context here. Yes, the values within a concurrent dictionary can be null... — Jon Skeet, Jan 20 '14 at 14:21

score 1 · Answer 1 · answered Jan 20 '14 at 14:39

1

The question mentions that you check for null but it's unclear on which value you are doing it. Consider the following code

Character Charr = kvp.Value;
if (Charr != null) { 
  Charr.Do();
}

It is not possible in this scenario for Char to be null. It's possible that a member of Charr is null and hence the Do method throws. If that is the case though it's likely that Character type is not suitable for multi-threaded scenarios (at least the way they are being used)

answered Jan 20 '14 at 14:39

JaredPar

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Haha Indeed, thats how I did it when I said I checked if its null. It still happens to be null some times! – Yuki Jan 21 '14 at 14:47
@Yuki thats not really possible barring a bug in the CLR itself. – JaredPar Jan 21 '14 at 18:02

score 0 · Accepted Answer · answered Jan 20 '14 at 14:33

0

If you have a null check for Charr added and still get a null reference exception something in the Do() Method will cause this exception. Check the call stack of the exception or/and use a debugger to go through your code step by step.

answered Jan 20 '14 at 14:33

Roland Bär

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No, as I said, Charr itselfe is null. I checked it on debug (mouse over charr - shows null) – Yuki Jan 21 '14 at 14:49
I don't see a way how Charr can get null as it is a stack variable that is not influenced by other threads. But if you have it as a member variable it could of course be changed by other threads, and no concurrent list will take care of that. – Roland Bär Jan 21 '14 at 15:09

ConcurrentDict doesnt prevent items to be null inside an loop

2 Answers2