How can I create an ArrayList with a starting index of 1 (instead of 0)

Question

How can I start the index in an ArrayList at 1 instead of 0? Is there a way to do that directly in code?

(Note that I am asking for ArrayList, for plain arrays please see Initializing an array on arbitrary starting index in c#)

Answer 1

One legitimate reason to do this is in writing unit tests. Certain 3rd party SDKs (for example Excel COM interop) expect you to work with arrays that have one-based indices. If you're working with such an SDK, you can and should stub out this functionality in your C# test framework.

The following should work:

object[,] comInteropArray = Array.CreateInstance(
    typeof(object),
    new int[] { 3, 5 },
    new int[] { 1, 1 }) as object[,];

System.Diagnostics.Debug.Assert(
     1 == comInteropArray.GetLowerBound(0),
    "Does it look like a COM interop array?");
System.Diagnostics.Debug.Assert(
     1 == comInteropArray.GetLowerBound(1),
    "Does it still look like a COM interop array?");

I don't know if it's possible to cast this as a standard C# style array. Probably not. I also don't know of clean way to hard-code the initial values of such an array, other than looping to copy them from a standard C# array with hard-coded initial values. Neither of these shortcomings should be a big deal in test code.

Answer 2

Here is something I am using right now to quickly port a VBA application to C#:

It's an Array class that starts with 1 and accepts multiple dimensions.

Although there is some additional work to do (IEnumerator, etc...), it's a start, and is enough to me as I only use indexers.

public class OneBasedArray<T>
{
    Array innerArray;

    public OneBasedArray(params int[] lengths)
    {
        innerArray = Array.CreateInstance(typeof(T), lengths, Enumerable.Repeat<int>(1, lengths.Length).ToArray());
    }

    public T this[params int[] i]
    {
        get
        {
            return (T)innerArray.GetValue(i);
        }
        set
        {
            innerArray.SetValue(value, i);
        }
    }
}