问当我试图构建时，为什么会出现错误？

EN

using System;
using System.Collections.Generic;

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

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

        public void AddGrade(double grade)
        {
            if (grade <= 100 && grade >= 0)
            {  
                grades.Add(grade);
            }
            else 
            {
                Console.WriteLine("Invalid Value");  
            }
        }

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

            for (var index =0; index < grades.Count; index++)
            {
                if (grades[index] == 42.1) 
                {
                    continue;
                }

                result.Low = Math.Min(grades[index], result.Low);
                result.High = Math.Max(grades[index], result.High);
                result.Average += grades[index];
                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;
            }
        }

        private List<double> grades;
        public string Name;
    }
}