Array implements the IsReadOnly property because it is required by the System.Collections.IList interface. An array that is read-only does not allow the addition, removal, or modification of elements after the array is created.
If you cast or convert an array to an IList interface object, the IList.IsReadOnly property returns false. However, if you cast or convert an array to a IList(T) interface, the IsReadOnly property returns true.
Retrieving the value of this property is an O(1) operation.