How to output grades in a C# program?

Question

C++ / C#

namespace ConsoleApp1
{
    struct Student
    {
        public string Name;
        public string GroupName;
        public DateTime BirthDay;
        public Dictionary<string, byte> Marks;
        public Student(string name, string groupname, DateTime birthday)
        {
            Name = name;
            GroupName = groupname;
            BirthDay = birthday;
            Marks = new Dictionary<string, byte>();
        }
        public string ToString(bool isFullInfo)
        {
            string res = "";
                res += String.Format("{0,15} {3}.{2}.{1}r. {4}", Name, BirthDay.Year, BirthDay.Month, BirthDay.Day, GroupName);
            if (isFullInfo)
            {
                foreach (KeyValuePair<string, byte> m in Marks)
                    res += String.Format("\n{0} - {1}", m.Key, m.Value);
            }
            return res;
        }

        class Program
        {
            static void Main(string[] args)
            {
                string[] Subjects = new string[] { "Физика", "История", "Информатика" };
                List<Student> students = new List<Student>();
                Student n1 = new Student("Николай", "18-ИНН", new DateTime(2000, 02, 12));
                n1.Marks.Add("Физика", 4);
                n1.Marks.Add("История", 4);
                n1.Marks.Add("Информатика", 5);
                Student n2 = new Student("Денис", "18-ИАС", new DateTime(2000, 03, 25));
                students.Add(n2);
                n2.Marks.Add("Физика", 2);
                n2.Marks.Add("История", 4);
                n2.Marks.Add("Информатика", 4);

                Student n3 = new Student("Александр", "18-ИСТ", new DateTime(2000, 05, 19));
                students.Add(n3);
                n3.Marks.Add("Физика", 5);
                n3.Marks.Add("История", 5);
                n3.Marks.Add("Информатика", 5);

                Console.WriteLine("Меню: \n1) Список студентов \n2) Список задолжников \n3) Список студентов с повышенной стипендией");
                int menu = Convert.ToByte(Console.ReadLine());
                switch (menu)
                {
                    case 1:
                        for (int i = 0; i < students.Count; i++)
                        {
                            Console.WriteLine("Имя: {0} Группа: {1} Дата рождения: {2}", students[i].Name + "\t", students[i].GroupName + "\t", students[i].BirthDay + "\t");
                        }
                        break;
                        case 2:
                        foreach (Student s in students)
                        {
}
                        break;
                }
                Console.ReadKey();
            }

        }
    }
}

There is a program written in C#. It is required to display students and their grades for each subject. Also identify debtors and students receiving an increased scholarship. Debtors are students who have received at least one "2".

Answer 1

It's not entirely clear what caused the problem. But let's figure it out.
1. You already have a ToString method definition for your Student object . I propose to change it a little - instead of passing a variable indicating the request for information in a detailed or abbreviated form, we will divide it into two separate methods. ShortFormat , which will return the short version, while overriding the default ToString() to get the full data.

public string ShortFormat()
{
    return string.Format("{0,15} {1} {2}", Name, BirthDay.ToShortDateString(), GroupName);
}

public override string ToString()
{
    StringBuilder sb = new StringBuilder(ShortFormat());
    sb.AppendLine("\nОценки:");
    foreach (var m in Marks)
        sb.AppendFormat("{0} - {1}\n", m.Key, m.Value);
    return sb.ToString();
}

2. To determine if our student has debts, we will use the ContainsValue method : Now it remains only to correctly call the methods / properties:

switch (menu)
{
    case 1:
        foreach (var student in students)
            Console.WriteLine(student);
        break;
    case 2:
        foreach (var student in students.Where(s => s.HasDebt))
            Console.WriteLine(student.ShortFormat());
        break;
}

Note that we used the Linq Where method to filter the debtors .
And in addition a few comments:
1. Exposing a mutable collection (in your case Dictionary<_,_>) is usually not a good idea, it's better to add a few helper methods / properties that will work with a private field.
2. In the current implementation, the menu allows you to select only one option.