private Cell[,] mapCells = new Cell[10, 10];

bool cellExists = index.x >= 0 && // check left
              index.y >= 0 && // check bottom
              index.x < mapCells.GetLength(0) && // check right
              index.y < mapCells.GetLength(1); // check top

Vector2 dir = new Vector2(/* this can be
     (0,1) // up
     (0,-1) // down
     (-1,0) // left
     (1,0) // right
  */);

public static class Extensions
{
    public static bool IsExist(this Cell[,] mapCells, Cell index)
    {
        bool cellExists = index.x >= 0 &&
               index.y >= 0 &&
               index.x < mapCells.GetLength(0) &&
               index.y < mapCells.GetLength(1);

        return cellExists;
    }
}

mapCells.IsExist(index);