Why we have both jagged array and multidimensional array?

Solution 1:

1. A jagged array is an array-of-arrays, so an int[][] is an array of int[], each of which can be of different lengths and occupy their own block in memory. A multidimensional array (int[,]) is a single block of memory (essentially a matrix).

2. You can't create a MyClass[10][20] because each sub-array has to be initialized separately, as they are separate objects:

   MyClass[][] abc = new MyClass[10][];
   
   for (int i=0; i<abc.Length; i++) {
       abc[i] = new MyClass[20];
   }
   

   A MyClass[10,20] is ok, because it is initializing a single object as a matrix with 10 rows and 20 columns.

3. A MyClass[][,][,] can be initialized like so (not compile tested though):

   MyClass[][,][,] abc = new MyClass[10][,][,];
   
   for (int i=0; i<abc.Length; i++) {
       abc[i] = new MyClass[20,30][,];
   
       for (int j=0; j<abc[i].GetLength(0); j++) {
           for (int k=0; k<abc[i].GetLength(1); k++) {
               abc[i][j,k] = new MyClass[40,50];
           }
       }
   }
   

Bear in mind, that the CLR is heavily optimized for single-dimension array access, so using a jagged array will likely be faster than a multidimensional array of the same size.

Solution 2:

A jagged array is an array of arrays. Each array is not guaranteed to be of the same size. You could have

int[][] jaggedArray = new int[5][];
jaggedArray[0] = new[] {1, 2, 3}; // 3 item array
jaggedArray[1] = new int[10];     // 10 item array
// etc.

It's a set of related arrays.

A multidimensional array, on the other hand, is more of a cohesive grouping, like a box, table, cube, etc., where there are no irregular lengths. That is to say

int i = array[1,10];
int j = array[2,10]; // 10 will be available at 2 if available at 1