I have created an item swapper control consisting in two listboxes and some buttons that allow me to swap items between the two lists. The swapping is done using javascript. I also move items up and down in the list. Basically when I move the items to the list box on the right I store the datakeys of the elements (GUIDs) in a hiddenfield. On postback I simply read the GUIDs from the field. Everything works great but on postback, I get the following exception:I have created an item swapper control consisti