declare offset array for x and y, Julia likes the idea.

ConnectedCellInaGrid6.cs

using System;
using System.Collections.Generic;
using System.IO;
class Solution {
    static void Main(String[] args) {
        /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution */
        
        int m = Convert.ToInt32(Console.ReadLine()); // row
        int n = Convert.ToInt32(Console.ReadLine()); // col
        
        string[,] grid = new string[m,n];
        
        for (int i = 0; i < m; i++) {            
            string row = Console.ReadLine();
            string[] r = row.Split(' ');
            
            for (int j = 0; j < n; j++) {
                grid[i,j] = r[j];
                //Console.Write(grid[i,j] + " ");
            }
            //Console.WriteLine();
        }
        
        int max = 0;
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                int res = CountRegion(i, j, grid);
                
                max = Math.Max(res,max);
            }
        }
        
        Console.WriteLine(max);
    }
    
    public static int CountRegion(int row, int col, string[,] grid) {
        int count = 0;
        
        int[] rowOffset = new int[] { -1, -1, -1, 0, 1, 1, 1, 0 };
        int[] colOffset = new int[] { -1, 0, 1, 1, 1, 0, -1, -1 };
        
        if (row >= 0 && col >= 0 && row < grid.GetLength(0) && col < grid.GetLength(1)) {
            if (grid[row,col] == "1") {
                count++;
                grid[row,col] = "X";
                for (int i = 0; i < rowOffset.Length; i++) {
                    count += CountRegion(row + rowOffset[i], col + colOffset[i], grid);
                }
            }
        }
        
        return count;
    }
}