The soap in dishwashers is harsh and that's one of the reasons that dishwashers dulls knifes because the harshness and abrasiveness of the dishwasher soap dulls the knifes.
Another reason for a dishwasher dulling knives is the water jets from the dishwasher causing the knives to move around and clash into other things inside the dishwasher which then causes the knife to lose it's sharpness and become dull.
You should always just gently wash your knives by hand to keep the sharp edge on them because when I last washed knives in the dishwasher the knives did become dull.
I had to resharpen the knives so now I just wash the knives gently by hand which is good enough to clean them.