Destination Array not long enough?

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?

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).

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: https://stackoverflow.com/questions/1167333/why-doesnt-a-foreach-loop-work-in-certain-cases

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>()
}

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.