C#无法覆盖继承的成员

我正在从一本名叫切格维登·格拉迪斯的书中学习C。我正在制作与书中所写相同的程序和代码。但有一个问题。我无法重写父类中的方法。从第一章开始，我已经完整地阅读了这本书，读了5遍，所有内容都是一样的，但我不明白为什么我不能重写父类中的方法。下面是基类PassFailActivity.cs中的代码

using System;
namespace ProtectedMembers
{
    public class PassFailActivity : GradedActivity2
    {
        private double minPassingScore; // Minimum passing score

        /// <summary>
        /// The constructor sets the minimum passing score
        /// </summary>
        /// <param name="mps">The minimum passing score.</param>
        public PassFailActivity(double mps)
        {
            minPassingScore = mps;
        }

        /// <summary>
        /// The GetGrade method returns a letter grade determined
        /// from the score field. This methos overrides the base class method.
        /// </summary>
        /// <returns>The letter grade</returns>
        public override char GetGrade()
        {
            char letterGrade;

            if (base.GetScore() >= minPassingScore)
                letterGrade = 'P';
            else
                letterGrade = 'F';

            return letterGrade;
        }
    }
}

输出应该是

How many questions are on the exam? 100
How many questions did the student miss? 25
What is the minimum passing score? 60
Each question counts 1 points.
The exam score is 75
The exam grade is P

---

我尝试将基方法设置为虚拟，并在尝试重写时调用override，这只会导致以下错误“'ProtectedMembers.PassFailActivity.GetGrade（）”：无法重写继承的成员'ProtectedMembers.GradedActivity2.GetGrade（）'，因为它未标记为虚拟、抽象或重写”。我完全检查了

错误是不言自明的，请在

GradedActivity2

中标记函数：

public virtual char GetGrade()

从MSDN：

virtual关键字用于修改方法、属性、索引器或 事件声明，并允许在派生 班级。例如，此方法可以被任何 继承它：

---

基类中的方法

GetGrade

应该告诉您允许使用以下关键字进行派生：

---

这只是一种故障保护机制，也是一种优化，因为不必在派生类中的派生方法上检查非虚拟方法。

您需要在继承树的根类中使

GetGrade

虚拟。您需要尝试只包含与您的问题相关的代码，您似乎包含了所有代码，因此很难确定问题所在。很抱歉，我没有在MSDN上搜索它。显然，我不能说“+1 for

错误是自解释的

”如果您理解关键字Virtual you will not day错误不是自解释的，它确实适用于任何想要快速参考的人如果错误是自解释的，prouser135不会问这个问题，我也不会用谷歌搜索它。哦，FSCK，这就像turboPascal一样，完全是Java的后盾™ 因此☹ 如果您需要覆盖系统行为，则非常恼人…

using System;

namespace ProtectedMembers
{
    public class PassFailExamDemo
    {
        public static void Main111()
        {
            int questions, // Number of questions
                missed; // Number of questions missed
            double minPassing; // Minmum passing score

            // Get the number of questions on the exam
            Console.Write("How many questions are " + "on the exam? ");
            questions = Convert.ToInt32(Console.ReadLine());

            // Get the number of questions missed.
            Console.Write("How many questions did " + "the student miss? ");
            missed = Convert.ToInt32(Console.ReadLine());

            // Get the minimum passing score
            Console.Write("What is the minimum " + "passing score? ");
            minPassing = Convert.ToInt32(Console.ReadLine());

            // Create a PassFailExam project
            PassFailExam exam = new PassFailExam(questions, missed, minPassing);

            // Display the teset results.
            Console.WriteLine("Each questions counts {0} points.",
                exam.GetPointsEach());
            Console.WriteLine("The exam score is {0} ",
                exam.GetScore());
            Console.WriteLine("The exam grade is {0} ",
                exam.GetGrade());
            Console.ReadLine();
        }
    }
}

How many questions are on the exam? 100
How many questions did the student miss? 25
What is the minimum passing score? 60
Each question counts 1 points.
The exam score is 75
The exam grade is P

public virtual char GetGrade()

public virtual char GetGrade()