EitanB,

   You should cycle through the items using the indexer, and not foreach,
like so:

for (int index = 0; index < myList.Count; ++index)
{
   // Get the item.
   MyStruct myStruct = myList[index];

   // Modify the item here.

   // Assign the item back.
   myList[index] = myStruct;
}

   The reassignment is necessary as you are working with what I assume is a
value type, so accessing properties off instances of that type returned by
the indexer would not change the value in the list.