Q:

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?

A:

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).

A:

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?

