design the function to return the value - it is also a good idea.

ConnectedCellInaGrid3.cs

using System;
using System.Collections.Generic;
using System.IO;
class Solution {
    static void Main(String[] args) {
        int rows = Convert.ToInt32(Console.ReadLine());
        int columns = Convert.ToInt32(Console.ReadLine());
        
        string[][] matrix = new string[rows][];
        
        for(int i=0;i<rows;i++)
            matrix[i] = Console.ReadLine().Split(' ');
        
        int maxScore = 0;
        for(int i=0;i<rows;i++){
            for(int j=0;j<columns;j++){
                int score = findArea(matrix, i, j, rows, columns);
                if(score>maxScore)
                    maxScore = score;
            }
        }
        
        Console.WriteLine(maxScore);
    }
    
    static int findArea(string[][] matrix, int x, int y, int rows, int columns){
        int result = 0;
        
        if(x>=0 && x<rows && y>=0 && y<columns && matrix[x][y]=="1"){
            result++;
            matrix[x][y]="X";
            
            result += findArea(matrix, x + 1, y, rows, columns);
            result += findArea(matrix, x + 1, y-1, rows, columns);
            result += findArea(matrix, x + 1, y+1, rows, columns);
            result += findArea(matrix, x - 1, y, rows, columns);
            result += findArea(matrix, x - 1, y-1, rows, columns);
            result += findArea(matrix, x - 1, y+1, rows, columns);
            result += findArea(matrix, x, y-1, rows, columns);
            result += findArea(matrix, x, y+1, rows, columns);              
        }
        
        return result;
    }
}