LINQ Quicksort is Unstable Except When Cascading

Question

On page 64 of "LINQ To Objects Using C# 4.0" (Tony Magennis) he states that LINQ's quicksort ordering algorithm is unstable...

...although this is simply solved by cascading the result into a ThenBy or ThenByDescending operator.

Huh? Why would cascading an unstable sortation into another sortation fix the result? In fact, I'd say that isn't possible. The original order, once passed through an unstable sort, is simply lost. What am I missing here?

Answer 1

Put simply, he's wrong. The Linq OrderBy et al. methods are documented as performing a stable sort:

This method performs a stable sort; that is, if the keys of two elements are equal, the order of the elements is preserved. In contrast, an unstable sort does not preserve the order of elements that have the same key.