array declaration in c#

// Define and Initialize
int[] arr = {1, 2, 3};

// Buuuut:
// Initial defining
int[] arr;

// This works
arr = new int[] {1, 2, 3};   

// This will cause an error
// arr = {1, 2, 3}; 

using System;
using System.Linq;
using System.Runtime.InteropServices;

namespace ConsoleApplication4
{
    public static class Program
    {
        private static void Main()
        {
			/*
				C# supports multidimensional arrays up to 32 dimensions.
    			The multidimensional array can be declared by adding commas in the
    			square brackets.
    			For example, [,] declares two-dimensional array,
    			[, ,] declares three-dimensional array,
    			[, , ,] declares four-dimensional array, and so on.
    			So, in a multidimensional array, number of commas = Number of Dimensions.
			*/

			// For instance:
			int[,] arr2d; // two-dimensional array
			int[, ,] arr3d; // three-dimensional array
			int[, , ,] arr4d ; // four-dimensional array

          // declare and instantiate a 2d array with 4 rows and 5 columns
          	var array = new int[4,5](); 
          	/* The [4, 5] defines the number of rows and columns.
          	   The first rank (4) denotes the number of rows, and the second rank (5)
               defines number of columns.
               The code below instantiate and illustrate the 2d array divide into rows and columns.
          	*/
          
          	var array = new [,] // or var array = new int[4,5]
            {
              // col0,col1,col2,col3,col4
                {0.1, 0.2, 0.3, 0.4, 0.5}, // row0
                {1.1, 1.2, 1.3, 1.4, 1.5}, // row1
                {2.1, 2.2, 2.3, 2.4, 2.5}, // row2
                {3.1, 3.2, 3.3, 3.4, 3.5}, // row3
            };
			
          	// Get the element 0.2 (that is row0, col1)
          	array[0, 1]; //returns 0.2
          	// Get the element 2.3 (that is row2, col2)
          	array[2, 2]; //returns 2.3          

			//array[4, 1]; //throws run-time error as there is no 4th row
          	//array[2, 5]; //throws run-time error as there is no 5th column
          // get the third row
            var row = array.GetRow(2);

            // This prints 2.1, 2.2, 2.3, 2.4, 2.5
            Console.WriteLine(string.Join(", ", row.Select(element => element.ToString())));
        }
    }
}

int[] arr = Enumerable.Range(0, X+1).ToArray();