I think we should prefer to use the Positions constructors whenever possible (rather than assign directly to PositionsArray), since we may be implementing logic to e.g. restrict the number of input elements in the array, etc.

In fact we may just want to make PositionsArray { get; private set; } to enforce that.

We want to make this decision (with the array length, throwing exceptions, etc.) inside the Positions class, to keep the logic all in one place - since we'll want to make the same decision whether we are reading JSON or just using the Positions class directly.