为什么在尝试构建时出现错误？

Question

Trying to build and then run my first program is giving the below error.

Book.cs(50,27): error CS0161: 'Book.GetStatistics()': not all code paths return a value [C:\\dev\\gradebook\\src\\GradeBook\\GradeBook.csproj]

Line 50 is: public Statistics GetStatistics()

Any help would be amazing!

Tried to rebuild and saved all changes

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;
    }
}

Answer 1

The return result; statement is inside the for loop. This means that if there are no grades, or if all grades are 42.1, the return statement will not be reached.

You probably meant to place the return statement below the closing } of the for-loop (most of the average computation should probably be moved there, too).