Question

So I came across some code of mine from an old VSTO project, and noted this little bit:

Excel.Worksheet sheet = Globals.ThisAddIn.Application.Worksheets["Unique Hits Per URL"];
        Dictionary<int, string> ids = new Dictionary<int, string>();
        object[,] cellRange = (object[,])sheet.get_Range("E:E").Cells.Value;
        for (int i = 1; i < cellRange.GetUpperBound(0); i++)
            if (cellRange[i, 1] != null)
                ids.Add(i, cellRange[i, 1].ToString());

What does specifying [,] on a datatype mean? Looking at the code it appears to function as a matrix, but honestly I thought c# matrices were handled with notation like object[ ][ ].

Answer 1

object[,] refers to a rectangular array, that means it’s a grid.
Then you have object[][] which is a jagged array, an array of array.

The main difference is that object[,] will always have fixed dimensions, while with a jagged array (object[][]) all arrays can have different sizes.

This is an example that clearly shows the difference in usage (both do the same):

// Create and fill the rectangluar array
int[,] rectangularArray = new int[10, 20];
for (int i = 0; i < 200; i++)
    rectangularArray[i / 20, i % 20] = i;

// Next line is an error:
// int[][] jaggedArray = new int[10][20];
int[][] jaggedArray = new int[10][]; // Initialize it

// Fill the jagged array
for (int i = 0; i < 200; i++)
{
    if (i % 20 == 0)
        jaggedArray[i / 20] = new int[20]; // This size doesn't have to be fixed
    jaggedArray[i / 20][i % 20] = i;
}

// Print all items in the rectangular array
foreach (int i in rectangularArray)
    Console.WriteLine(i);

// Print all items in the jagged array
// foreach (int i in jaggedArray) <-- Error
foreach (int[] innerArray in jaggedArray)
    foreach (int i in innerArray)
        Console.WriteLine(i);

EDIT:
Warning, this code above is not real production code, it’s just the clearest way of doing it as example.
The intensive use of divisions through / and % makes it much slower. You could better use a nested for loop.