Destination Array not long enough?

Question

I have a class with the following method:

public List<Bike> bikesCopy 
{
     get 
     { 
       List<Bike> bs;
       lock (_bikes) bs = new List<Bike>(_bikes);
       return bs;
     }
}

Which makes a copy of another list, private List<Bike> _bikes;

The strange thing now is, that I get the following error:

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

What is the problem here?

Where exactly do you get the exception? Can you update the question with the exception stack trace and point out the exact code line where it is thrown? — Fredrik Mörk, Apr 28 '12 at 08:57
Yes, it is the following message (sorry for the mess): `at System.Array.Copy(Array sourceArray, Int32 sourceIndex, Array destinationArray, Int32 destinationIndex, Int32 length, Boolean reliable) at System.Array.Copy(Array sourceArray, Int32 sourceIndex, Array destinationArray, Int32 destinationIndex, Int32 length) at System.Collections.Generic.List'1.CopyTo(T[] array, Int32 arrayIndex) at System.Collections.Generic.List'1..ctor(IEnumerable'1 collection) at MyGame.Player.get_bikesCopy()` — Geert, Apr 28 '12 at 12:49
Did you every get to the root cause of this problem? I am running into something similar. — Klaus Nji, Oct 09 '12 at 21:03
If you didn't resolve this, I believe the problem could be that you have not put a lock around the _bikes.Add(_) method (or maybe another List method). This is because the Add method is not thread safe in a List. Anyway, hope that helps if anyone gets the same problem — Theo, Oct 22 '12 at 11:18

Wolf5 · Accepted Answer · 2018-02-16T08:21:37.773

126

I would say the error lies in the object _bikes not being thread safe. As commented, somewhere there is a modify of the _bikes object that is not being lock'ed.

It is a split second error where the variable bs is set up to a size X when the size of _bikes is measured. In the next split second as it is about to fill the list, the _bikes object has increased in size giving the error.

So go over your code. Find all references of your _bikes object and make sure they are thread safe handled (with lock).

edited Feb 16 '18 at 08:21

answered Jan 02 '13 at 08:23

Wolf5

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I am terribly sorry, I accidentally clicked on downvote but I actually wanted to upvote and now I cannot un-vote anymore :-/ Sorry!! – Xan-Kun Clark-Davis Feb 12 '18 at 15:17
4

That is ok. The worst is when you get down voted and have no clue to as of why. – Wolf5 Feb 15 '18 at 11:38
Yeah, I got a "fan" a couple of days ago (18 downvotes within 2 minutes) and learned about the anti-serial-downvoting bot. I really like that bot :-) P.S.: The website says, if you edit the answer, I can undo the downvote again. – Xan-Kun Clark-Davis Feb 15 '18 at 19:55
What did you edit?! I am confused... However: I like bikes :-) – Xan-Kun Clark-Davis Feb 18 '18 at 22:53

score 7 · Answer 2 · answered May 06 '20 at 05:01

This error occurs because multiple threads are adding items in a single list. Lists are by default not a thread-safe solution. In a multi-threaded code, it is only recommended to read from a list, and not write to it.

As described here:

If you are only reading from a shared collection, then you can use the classes in the System.Collections.Generic namespace.

Better use a thread-safe solution like System.Collections.Concurrent namespace which provides implementations like ConcurrentBag, ConcurrentDictionary, ConcurrentQueue, ConcurrentStack etc.

For example:

public ConcurrentBag<Bike> bikesCopy 
{
     get => new ConcurrentBag<Bike>()
}

score 6 · Answer 3 · edited May 23 '17 at 12:34

6

Well you could try:

using System.Linq; //ToList() is an extension function defined here
...
lock(_bikes)
    return _bikes.ToList();

The details of the exception are discussed here: Why doesn't a foreach loop work in certain cases?

edited May 23 '17 at 12:34

Community

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answered Apr 28 '12 at 08:59

edvaldig

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I am not capable using LINQ, as it is not whitelisted on my framework.... Anyway, it doesn't answer my problem either :) – Geert Apr 28 '12 at 12:42
24

You can't use LINQ?!?!? I'd quit that job as soon as I found that out! – Serj Sagan Jan 26 '16 at 18:16
1

Not sure why, but `.ToList()` works great versus `new List(YourList)`, which always throws an exception. – Jul 13 '16 at 17:29
1

@JamesM It's the line `_items = new T[capacity];` in the source code for List (https://referencesource.microsoft.com/mscorlib/system/collections/generic/list.cs.html) which makes all the difference. – Xan-Kun Clark-Davis Feb 18 '18 at 22:58

Xan-Kun Clark-Davis · Answer 4 · 2018-02-18T23:00:50.977

Not really an answer, more a research comment.

I ran into the same problem and did a quick test. I tried with the code below and could not get this code to throw the ArgumentException: Destination array was not long enough. But when I remove the .ToList() from the line

return allLines.ToList().ToArray();

it immediately crashes.

This is the demo code and even the IDE tells me, I should remove the ToList() call as it seems redundant.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;

namespace ConsoleApp1
{
    class Program
    {
        static void Main() {

            List<string> thelist = new List<string>();

            Thread producer = new Thread(() => {
                while (true) {
                    thelist.Add("a" + DateTime.Now);
                }
            });

            Thread transformer = new Thread(() => {
                while (true) {
                    string[] thearray = thelist.ToList().ToArray();
                    Console.WriteLine(thearray.Length);
                }
            });
            producer.Start();
            transformer.Start();
            Console.ReadKey(true);
        }
    }
}

I really wonder, why it would not crash, as the List is also backed by an array.

Destination Array not long enough?

4 Answers4

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