Program.cs

using System;
using System.Collections.Generic;

namespace StudentManagementSystem
{
    class Program
    {
        static List<Student> students = new List<Student>();

        static void Main(string[] args)
        {
            while (true)
            {
                Console.WriteLine("1. Add a student");
                Console.WriteLine("2. Assign grade");
                Console.WriteLine("3. Retrieve student info");
                Console.WriteLine("4. Exit");

                try
                {
                    int choice = int.Parse(Console.ReadLine());

                    switch (choice)
                    {
                        case 1:
                            AddStudent();
                            break;
                        case 2:
                            AssignGrade();
                            break;
                        case 3:
                            RetrieveStudentInfo();
                            break;
                        case 4:
                            return;
                        default:
                            Console.WriteLine("Invalid choice. Please enter a number between 1 and 4.");
                            break;
                    }
                }
                catch (FormatException)
                {
                    Console.WriteLine("Invalid input. Please enter a number.");
                }
                catch (Exception ex)
                {
                    Console.WriteLine($"An unexpected error occurred: {ex.Message}");
                }
                finally
                {
                    Console.WriteLine("Instruction executed. Press Enter to continue...");
                    Console.ReadLine();
                }
            }
        }

        static void AddStudent()
        {
            try
            {
                Console.Write("Enter student name: ");
                string name = Console.ReadLine();
                if (string.IsNullOrWhiteSpace(name) || !char.IsUpper(name[0]))
                {
                    throw new ArgumentException("Name must start with an uppercase letter.");
                }

                Console.Write("Enter student age: ");
                int age = int.Parse(Console.ReadLine());
                if (age < 18 || age > 65)
                {
                    throw new ArgumentException("Age must be between 18 and 65.");
                }

                students.Add(new Student { Name = name, Age = age });
                Console.WriteLine("Student added successfully.");
            }
            catch (FormatException)
            {
                Console.WriteLine("Age must be a number.");
            }
            catch (ArgumentException ex)
            {
                Console.WriteLine($"Validation error: {ex.Message}");
            }
            catch (Exception ex)
            {
                Console.WriteLine($"An unexpected error occurred: {ex.Message}");
            }
        }

        static void AssignGrade()
        {
            try
            {
                Console.Write("Enter student name: ");
                string name = Console.ReadLine();
                Student student = students.Find(s => s.Name == name);
                if (student == null)
                {
                    throw new StudentNotFoundException("Student not found.");
                }

                Console.Write("Enter student grade: ");
                int grade = int.Parse(Console.ReadLine());
                if (grade < 0 || grade > 100)
                {
                    throw new ArgumentException("Grade must be between 0 and 100.");
                }

                student.Grade = grade;
                Console.WriteLine("Grade assigned successfully.");
            }
            catch (FormatException)
            {
                Console.WriteLine("Grade must be a number.");
            }
            catch (ArgumentException ex)
            {
                Console.WriteLine($"Validation error: {ex.Message}");
            }
            catch (StudentNotFoundException ex)
            {
                Console.WriteLine(ex.Message);
            }
            catch (Exception ex)
            {
                Console.WriteLine($"An unexpected error occurred: {ex.Message}");
            }
        }

        static void RetrieveStudentInfo()
        {
            try
            {
                Console.Write("Enter student name: ");
                string name = Console.ReadLine();
                Student student = students.Find(s => s.Name == name);
                if (student == null)
                {
                    throw new StudentNotFoundException("Student not found.");
                }

                Console.WriteLine($"Name: {student.Name}, Age: {student.Age}, Grade: {student.Grade}");
            }
            catch (StudentNotFoundException ex)
            {
                Console.WriteLine(ex.Message);
            }
            catch (Exception ex)
            {
                Console.WriteLine($"An unexpected error occurred: {ex.Message}");
            }
        }
    }

    class Student
    {
        public string Name { get; set; }
        public int Age { get; set; }
        public int Grade { get; set; }
    }

    class StudentNotFoundException : Exception
    {
        public StudentNotFoundException(string message) : base(message) { }
    }
}