Try using the Parallels library (increases performance) due to the lack of side effects:

``` c#
Parallels.For(0, 1000, i => { // logic to enqueue });
```

This will require you to use the ConcurrentQueue<int> datatype instead of List<int>.  ConcurrentQueue is threadsafe, but List is not.  Also you'll want to reference System.Threading in your using block.