Why would OutOfMemoryException be thrown while using PLINQ Take()?

Question

After I upgraded to DotNet 4.5, a query started giving me OutOfMemoryExceptions.

The (distilled) query is:

var tests = new int[]{}
    .AsParallel()
    .GroupBy(_ => _)
    .Take(int.MaxValue)
    .ToArray();

I'm posting this for anyone with the same problem. I'll answer below.

Answer 1

It appears to be a change in the framework.

The Take() operator is implemented in the TakeOrSkipQueryOperator internal class. There is one branch in the code that goes through a WrapHelper() function that creates a FixedMapHeap instance which in turn creates an array of Key elements of the size passed originally to Take() (which would be an array of 8Gb in the given example).