Question

Does an ArrayList internally use an Array? Is this an empty array if we use the default constructor (new ArrayList())? Thanks.

Answer 1

Yes it does. One of the easiest ways to verify this is to look at the source. You can either get your hands on the reference source, or you can simply use .NET Reflector to decompile the .NET DLLs.

Here’s the relevant part of ArrayList, from Reflector:

public class ArrayList : IList, ICollection, IEnumerable, ICloneable
{
    static ArrayList()
    {
        emptyArray = new object[0];
    }

    public ArrayList()
    {
        this._items = emptyArray;
    }

    private object[] _items;

    private static readonly object[] emptyArray;

    // etc...
}

You shouldn’t rely on this always being the case. It is an implementation detail and could change in future versions of .NET (although it probably won’t). Also for new code you should consider using List<T> instead of ArrayList.