NullReferenceException on ConcurrentBag.Add with threads

Question

I was originally using a List for this but I changed to a ConcurrentBag after seeing that Lists where not "thread-safe".

The following is my code so far. When it runs, it will not add to the ConcurrentBag, I get a NullReferenceException - "Object reference not set to an instance of an object." and I am not sure what the problem is as I do not have much experience with threads.

static ConcurrentBag<String> urls;

// ...

static void buildThreads()
{
    for (int i = 0; i < threads; i++)
    {
        Thread thread = new Thread(buildTokens);
        thread.Start();
    }
}

static void buildTokens()
{
    while (true)
    {
        if (numSockets < tokens)
        {
            getUrl();

            numSockets++;
            Console.WriteLine(numSockets + "/" + tokens);
        }
    }
}

static void getUrl()
{
    urls.Add("test");
}

I would appreciate any help. Thanks.

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) — nvoigt, Mar 08 '14 at 18:36

score 2 · Accepted Answer · answered Mar 08 '14 at 18:33

2

Perform this instantiating before accessing this object, it's contains a null reference right now:

static ConcurrentBag<string> urls = new ConcurrentBag<string>();

answered Mar 08 '14 at 18:33

Yair Nevet

12,725
14
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Oops, I knew it was going to be something little like that. Thanks! – Dan Mar 08 '14 at 18:34

score 2 · Answer 2 · answered Mar 08 '14 at 18:35

You have to "new" your concurrent bag (create an instance):

static ConcurrentBag<String> urls = new ConcurrentBag<String>()

If you are using Visual Studio to build and debug your code you should be able to place a break point on the line where you add to the bag and notice the value of the variable is "null"

Sudhakar Tillapudi · Answer 3 · 2014-03-08T18:40:05.410

1

Problem : Your ConcurrentBag variable urls is null as you have not initialised it properly.

Solution: You need to initialise your ConcurrentBag variable urls properly using new keyword.

Replace This:

static ConcurrentBag<String> urls;

With This:

static ConcurrentBag<String> urls = new ConcurrentBag<String>();

edited Mar 08 '14 at 18:40

answered Mar 08 '14 at 18:33

Sudhakar Tillapudi

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5
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NullReferenceException on ConcurrentBag.Add with threads

3 Answers3