What Is The Difference Between int [,] And int [] [] ?

The following declaration creates a two-dimensional array of four rows and two columns.

int[,] array = new int[4, 2];
// 2d array
int[,] 2dArray = new int[,] { { 1, 2 }, { 3, 4 }, { 5, 6 }, { 7, 8 } };

The following is a declaration of a jagged-array : single-dimensional array that has three elements, each of which is a single-dimensional array of integers:

int[][] jaggedArray = new int[3][];

Before you can use jaggedArray, its elements must be initialized. You can initialize the elements like this:

jaggedArray[0] = new int[5];
jaggedArray[1] = new int[4];
jaggedArray[2] = new int[2];

Each of the elements is a single-dimensional array of integers. The first element is an array of 5 integers, the second is an array of 4 integers, and the third is an array of 2 integers.

From here:
Multidimensional Arrays[^]
Jagged Arrays[^]

-KR

The difference is that [,] is a 2D fixed size array containing exactly x * y elements, where x and y are given when you allocate the space in the new assignment:

int[,] arr = new int[x,y];

[][] creates a jagged array, where you only assign one dimension at a time:

int[][] arr = new int[x][];
for(int i = 0; i < x; i++)
   {
   arr[i] = new int[(i + 1) * 2];
   }

Each element of the jagged array can be a different size, and needs to be allocated with a separate new statement.

While you can initialize fixed size arrays with this syntax:

int[,] arr = new int[5, 2] { { 0, 0 }, { 1, 2 }, { 2, 4 }, { 3, 6 }, { 4, 8 } };

You need this syntax to do the same with a jagged array:

int[][] arr = new int[5][] { new int[] { 0, 0 }, 
                             new int[] { 1, 2 }, 
                             new int[] { 2, 4 }, 
                             new int[] { 3, 6 }, 
                             new int[] { 4, 8 } };