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?

Answer 1

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).

Answer 2

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?

Answer 3

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>()
}