How to cast a ReadOnlyCollection<T> to T[]?

No, it's not possible to convert a ReadOnlyCollection to an array without iterating it. That would turn the collection to a writable collection, breaking the contract of being read-only.

There are different ways of iterating the collection that spares you of writing the loop yourself, for example using the CopyTo method

int[] collection = new int[theObject.TheProperty.Count];
theObject.TheProperty.CopyTo(collection, 0);

Or the extension method ToArray:

int[] collection = theObject.TheProperty.ToArray();