pluralsight-csharp-fundamentals/gradebook/src/GradeBook/Book.cs

using System;
using System.Collections.Generic;

namespace GradeBook
{
    public delegate void GradeAddedDelegate(object sender, EventArgs args);

    public class Book
    {
        private List<double> grades;

        readonly string category;

        private string name;
        public string Name
        {
            get;
            set;
        }

        public Book(string name)
        {
            Name = name;
            grades = new List<double>();
        }

        public void AddGrade(double grade)
        {
            if (grade <= 100 && grade >= 0)
            {
                this.grades.Add(grade);

                if (GradeAdded != null)
                {
                    // Somebody is listening/subscribing
                    GradeAdded(this, new EventArgs()); // sender = this, pass; Raising event
                }
            } else
            {
                throw new ArgumentException($"Invalid {nameof(grade)}");
            }
        }

        public void AddGrade(char letter)
        {
            switch (letter)
            {
                case 'A':
                    AddGrade(90);
                    break;
                case 'B':
                    AddGrade(80);
                    break;
                case 'C':
                    AddGrade(70);
                    break;
                default:
                    AddGrade(0);
                    break;
            }
        }

        public event GradeAddedDelegate GradeAdded;

        public Statistics GetStatistics()
        {
            var result = new Statistics();
            result.Average = 0.0;
            result.highGrade = double.MinValue;
            result.lowGrade = double.MaxValue;

            foreach (var grade in this.grades)
            {
                result.lowGrade = Math.Min(grade, result.lowGrade);
                result.highGrade = Math.Max(grade, result.highGrade);
                result.Average += grade;
            }

            result.Average /= grades.Count;

            switch (result.Average)
            {
                case var d when d >= 90.0:
                    result.Letter = 'A';
                    break;
                case var d when d >= 80.0:
                    result.Letter = 'B';
                    break;
                case var d when d >= 70.0:
                    result.Letter = 'C';
                    break;
                case var d when d >= 60.0:
                    result.Letter = 'D';
                    break;
                default:
                    result.Letter = 'F';
                    break;
            }

            return result;
        }
    }
    
}