For what it's worth, I wrote methods in Java that will do what you want, and I understand C# is similar enough you will probably be able to read the code without much trouble. (The syntax, that is. If you are not comfortable with recursive functions then that could give you trouble.) My user name is @undefined over on this forum thread. To use the code, you could loop through k-values from 1 to the length of your string inclusive. Alternatively, you could generate all subsets (discarding the empty set) as described in this thread and then get the permutations from there. Another way is to write a kth-permutation function and use that. I don't have one posted online; the one I use is a bit messy and I should rewrite it sometime.
Edit: Worth mentioning that I wrote the methods in a way that seemed easy, but more efficient would be to use a stack, that way you don't have to create tons of new objects.
