C# how can I save my input datas in list to .txt file

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C#

IO

i need to save data in list which i input in console.I have 3 function`Show(),Add(),Delete(). When i input 1 it must show me my data's list,when i input 2 it must ask me "Enter new user's data with comma",for example` John,25,then it must be saved in my list, that's to say when i input 1 it must show me my list + new adding data, and when i input 3 it must ask me "Please type the index of Person, whom you want to delete",for example i will input 2 and it must delete second index person, and then when i input 1 it must show me my list without second index person.
But in my code it's not working, after adding or deleting it's not show me that person whom i added or deleted.
Please help me,What's my mistake???????

What I have tried:

class Controller
    {
        public List<Person> all; 
        public void Show()
        {
            string path = "C:/Users/User/Desktop/mardik.txt";
            FileStream fs = new FileStream(path, FileMode.OpenOrCreate, FileAccess.Read);
            BinaryFormatter bf = new BinaryFormatter();
            List<Person> all = bf.Deserialize(fs) as List<Person>;
            foreach (Person item in all)
            {
                Console.WriteLine(item.name + " " + item.age);
            }
            fs.Close();
        }
        public void Add()
        {
            string path = "C:/Users/User/Desktop/mardik.txt";
            FileStream fs = new FileStream(path, FileMode.OpenOrCreate, FileAccess.ReadWrite);
            BinaryFormatter bf = new BinaryFormatter();
            List<Person> all = bf.Deserialize(fs) as List<Person>;
            
            Console.WriteLine("Enter new user's data with comma");
            string text = Console.ReadLine();
            string[] segments = text.Split(',');
            string a = segments[0];
            int b;
            bool hajoxvec = int.TryParse(segments[1], out b);
            if (!hajoxvec)
            {
                Console.WriteLine("Please try again");
                this.Add();
            }
            else
            {
                all.Add(new Person(a, b));

            }
                foreach (Person item in all)
                {
                    Console.WriteLine(item.name + " " + item.age);
                }
            
            fs.Close();
        }
        public void Delete()
        {
            string path = "C:/Users/User/Desktop/mardik.txt";
            FileStream fs = new FileStream(path, FileMode.OpenOrCreate, FileAccess.ReadWrite);
            BinaryFormatter bf = new BinaryFormatter();
            List<Person> all = bf.Deserialize(fs) as List<Person>;
            Console.WriteLine();
            Console.WriteLine("Please type the index of Person, whom you want to delete");
            int num = int.Parse(Console.ReadLine());
            if (num < 0 || num > 4)
            {
                Console.WriteLine("Please try again");
                this.Delete();
            }
            else
            {
                all.RemoveAt(num);
            }
            foreach (Person item in all)
            {
                Console.WriteLine(item.name + " " + item.age);
            }
            fs.Close();
        }
    }

[Serializable]
    class Person
    {
        public string name;
        public int age;
        public Person(string a, int b)
        {
            this.name = a;
            this.age = b;
        }
    }

static void Main(string[] args)
        {
            string path = "C:/Users/User/Desktop/mardik.txt";
            //List<Person> mardik = new List<Person>();
            //mardik.Add(new Person("Valod", 20));
            //mardik.Add(new Person("Petros", 23));
            //mardik.Add(new Person("Poghos", 25));
            //mardik.Add(new Person("Hranush", 22));
            //mardik.Add(new Person("Suren", 18));
            //FileStream fs = new FileStream(path, FileMode.OpenOrCreate, FileAccess.Write);
            //BinaryFormatter bf = new BinaryFormatter();
            //bf.Serialize(fs, mardik);
            //fs.Close();
            Controller c = new Controller();
            while (true)
            {
                Console.WriteLine();
                Console.WriteLine("Incheq uzum anel:\n1.Show all People\n2.Add new person\n3.Delete any person");
                int x = int.Parse(Console.ReadLine());
                switch (x)
                {
                    case 1:
                        c.Show();
                        break;
                    case 2:
                        c.Add();
                        break;
                    case 3:
                        c.Delete();
                        break;
                }
            }
        }