c# - Count the number of bits that are "on" in a byte

Possible Duplicates:
Count the number of set bits in an integer
Best algorithm to count the number of set bits in a 32-bit integer?

That's an exam question and that is all I have - "Count the number of bits that are "on" in a byte" "On" means 1, I assume. Do I need to create a BitArray, randomly populate it and then iterate through it or is there a different way?