Brain Teaser #2

This brain teaser comes courtesy of a co-worker named Simeon Cran.

Using C# and no branches, and no method calls, no allocations, and no unsafe code, write a method that takes a ulong and clears all the bits in it except the highest bit that was set. Use as few operations as possible.

e.g.:
0 -> 0
1 -> 1
2 -> 2
3 -> 2
4 -> 4
5 -> 4
6 -> 4
7 -> 4
8 -> 8
9 -> 8
10 -> 8
…