Calculating GetHashCode efficiently with unordered list
If the order does not matter, use an kommutative operator to combine the hashCodes of the elements.

Possible candidates are:
^ binary xor   // THIS WOULD BE MY CHOICE
+ addition       // problem may be the overflow
* multiplication         // problem may be the overflow
| binary or    // not recommended, because after some of this operations it is likely that all bits are set, so the same hashCode would appear for quite different instances)