<p>Is the sorting algorithm used by .NET's <code>Array.Sort()</code> method a <a href="http://en.wikipedia.org/wiki/Stable_sort#Classification" rel="noreferrer">stable</a> algorithm?</p>

From
MSDN
:
This implementation performs an unstable sort; that is, if two elements are equal, their order might not be preserved. In contrast, a stable sort preserves the order of elements that are equal.
The sort uses introspective sort. (Quicksort in version 4.0 and earlier of the .NET framework).
If you need a stable sort, you can use
Enumerable.OrderBy
.