Circular lists in C#

With utilization of the % (remainder) operator your code gets quite simple:

void nextItem() {
    index++; // increment index
    index %= items.Count; // clip index (turns to 0 if index == items.Count)
    // as a one-liner:
    /* index = (index + 1) % items.Count; */

    setItem();
}

void previousItem() {
    index--; // decrement index
    if(index < 0) {
        index = items.Count - 1; // clip index (sadly, % cannot be used here, because it is NOT a modulus operator)
    }
    // or above code as a one-liner:
    /* index = (items.Count+index-1)%items.Count; */ // (credits to Matthew Watson)

    setItem();
}