See the definition of System.Array class
public abstract class Array : IList, ...
Theoretically, I should be able to write this bit and be happy
int[] list = new int[] {};
IList iList = (IList)list;
I also should be able to call any method from the iList
ilist.Add(1); //exception here
My question is not why I get an exception, but rather why Array implements IList?
The remarks section of the documentation for IList says:
IList is a descendant of the ICollection interface and is the base interface of all non-generic lists. IList implementations fall into three categories: read-only, fixed-size, and variable-size. A read-only IList cannot be modified. A fixed-size IList does not allow the addition or removal of elements, but it allows the modification of existing elements. A variable-size IList allows the addition, removal, and modification of elements.
Obviously, arrays fall into the fixed-size category, so by the definition of the interface it makes sense.
Definition of IList interface is "Represents a non-generic collection of objects that can be individually accessed by index.". Array completely satisfies this definition, so must implement the interface. Exception when calling Add() method is "System.NotSupportedException: Collection was of a fixed size" and occurred because array can not increase its capacity dynamically. Its capacity is defined during creation of array object.