Indexing into arrays of arbitrary rank in C#

Question

I need to iterate over an array of arbitrary rank. This is for both reading and writing, so GetEnumerator will not work.

Array.SetValue(object, int) doesn't work on multidimensional arrays. Array.SetValue(object, params int[]) would require excessive arithmetic for iterating through the multidimensional space. It would also require dynamic invocation to get around the params part of the signature.

I'm tempted to pin the array and iterate over it with a pointer, but I can't find any documentation that says that multidimensional arrays are guaranteed to be contiguous. If they have padding at the end of a dimension then that won't work. I'd also prefer to avoid unsafe code.

Is there an easy way to sequentially address a multidimensional array using only a single index?

Answer 1

Multidimensional arrays are guaranteed to be contiguous. From ECMA-335:

Array elements shall be laid out within the array object in row-major order (i.e., the elements associated with the rightmost array dimension shall be laid out contiguously from lowest to highest index).

So this works:

int[,,,] array = new int[10, 10, 10, 10];

fixed (int* ptr = array)
{
    ptr[10] = 42;
}

int result = array[0, 0, 1, 0];  // == 42

Answer 2

You can use the Rank and GetUpperBound property/method to create an indices array that you can pass to the array's SetValue and GetValue methods:

int[] Indices(Array a, int idx)
{
    var indices = new int[a.Rank];

    for (var i = 0; i < a.Rank; i++)
    {
        var div = 1;

        for (var j = i + 1; j < a.Rank; j++)
        {
            div *= a.GetLength(j);
        }

        indices[i] = a.GetLowerBound(i) + idx / div % a.GetLength(i);
    }

    return indices;
}

..and use it like so:

for (var i = 0; i < array.Length; i++)
{
    var indices = Indices(array, i);
    array.SetValue(i, indices);
    var val = array.GetValue(indices);
}