Getting exception when working with list and parallel loops

Question

I have written a code like following below:

Parallel.ForEach(filteredList, (f) =>
{
    var conditionMatchCount = itm.AsParallel().Max(i =>
        // One point if ID matches
        ((i.ItemID == f.ItemID) ? 1 : 0) +
        // One point if ID and QuantitySold match
        ((i.ItemID == f.ItemID && i.QuantitySold == f.QuantitySold) ? 1 : 0)
    );
    // Item is missing
    if (conditionMatchCount == 0)
    {
        ListToUpdate.Add(f);
        missingList.Add(f);
    }
    // Item quantity is different
    else if (conditionMatchCount == 1)
    {
        ListToUpdate.Add(f);
    }
});

I basically have two lists:

// itm - list whos data comes from DB 
// filteredList => whos data comes from an external source

What I'm trying to achieve with the code above is to compare the two lists

and see which items from filteredList (new one) are not present in "itm" list...

If they are not present in "itm" list, they are added to

missingList.Add(f);

Also any items that have different QuantitySold property different than the one in "itm" list, I add them as well to ListToUpdate;

The error that I'm getting here is on the following line:

else if (conditionMatchCount == 1)
{
    ListToUpdate.Add(f);
}

And it says:

Destination array was not long enough. Check destIndex and length and the array's lower bounds

What am I doing wrong here?

P.S. Guys, both ListToUpdate and Missing list are plain simple lists, is that whats causing the issue here?

Possible duplicate of [Destination Array not long enough?](http://stackoverflow.com/questions/10362023/destination-array-not-long-enough) — Equalsk, Feb 02 '17 at 16:43

score 4 · Accepted Answer · answered Feb 02 '17 at 16:41

4

Don't use List<T> with multiple threads. They aren't thread safe.

Consider using a collection from System.Collections.Concurrent.

answered Feb 02 '17 at 16:41

Daniel A. White

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For my own benefit, would using a `lock` be an option as well? Agree that changing the datatype is best but if this was not possible... – Equalsk Feb 02 '17 at 16:43
yea thats possible but things could suffer. – Daniel A. White Feb 02 '17 at 16:43
@Equalsk good point... I'd mess up the entire code if i Converted it to concurrent bag or some other .. – User987 Feb 02 '17 at 16:43
depending how you want to consume from it @User987, you could use a `ConcurrentQueue` or `ConcurrentStack`. – Daniel A. White Feb 02 '17 at 16:44
@DanielA.White which one of these is the fastest when performance comes in question ? – User987 Feb 02 '17 at 16:45
@User987 you have to test it. i can't reasonably answer that. – Daniel A. White Feb 02 '17 at 16:47
@DanielA.White okay thx ! =) I'll accept ur asnwer as correct one – User987 Feb 02 '17 at 16:48
@DanielA.White P.S. I thought that because i use PLINQ that the piece of code I wrote is thread safe... Apparently not? – User987 Feb 02 '17 at 16:48
1

@User987 PLINQ only does certain things. it does not make unthread safe code thread safe. – Daniel A. White Feb 02 '17 at 16:49

Getting exception when working with list and parallel loops

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