alpha mask dll help(should be easy)

Question

im a hobbyist game programmer, i mostly excel at ruby. ive been trying to write a dll in c#(though i dont care what lang its in) that allows me to create an alpha mask. all i really need is the one method eg:

public Bitmap make_image(Bitmap image, Bitmap mask) { //code here return new_bitmap }

if you are wondering why dont i just do it in ruby its because ruby is too slow to do what i ask. if anyone could please help me get a working dll in any lang that would be amazing. if not i understand, but a point in the right direction would be nice.