object - C# cannot override inherited member -

i'm learning c# book named chegwidden gladdis. i'm making same program , same code written in book. there problem. can't override method parent class. had readed book start of chapter, 5 times, everyhing same can't figure out why can't can't override method parent class. here's code base class passfailactivity.cs

using system; namespace protectedmembers {     public class passfailactivity : gradedactivity2     {         private double minpassingscore; // minimum passing score          /// <summary>         /// constructor sets minimum passing score         /// </summary>         /// <param name="mps">the minimum passing score.</param>         public passfailactivity(double mps)         {             minpassingscore = mps;         }          /// <summary>         /// getgrade method returns letter grade determined         /// score field. methos overrides 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;         }     } } 

and gradedactivity2.cs

using system;  namespace protectedmembers {     public class gradedactivity2     {         protected double score; // numberic score          /// <summary>         /// setscore method sets score field.         /// </summary>         /// <param name="s">the value store in score</param>         public void setscore(double s)         {             score = s;         }          /// <summary>         /// getscore method returns score.         /// </summary>         /// <returns>the value stored in score field</returns>         public double getscore()         {             return score;         }          /// <summary>         /// getgrade method returns letter grade determined score field.         /// </summary>         /// <returns>return letter grade</returns>         public char getgrade()         {             char lettergrade;              if (score >= 90)                 lettergrade = 'a';             else if (score >= 80)                 lettergrade = 'b';             else if (score >= 70)                 lettergrade = 'c';             else if (score >= 60)                 lettergrade = 'd';             else                 lettergrade = 'f';              return lettergrade;         }     } } 

and passfailexam

using system;  namespace protectedmembers {     public class passfailexam : passfailactivity     {         private int numquestions; // number of questions         private double pointseach; // points each question         private int nummissed; // number of questions missed          /// <summary>         /// constructor sets number of questions, number         /// of questions missed, , minimum passing score.         /// </summary>         /// <param name="questions">the number of questions</param>         /// <param name="missed">the number of questions missed</param>         /// <param name="minpassing">the minimum passing score</param>         public passfailexam(int questions, int missed, double minpassing) : base(minpassing)         {             // declare local variable score.             double numericscore;              // set numquestions , nummissed fields.             numquestions = questions;             nummissed = missed;              // calculate points each questions , numeric score exam.             pointseach = 100.0 / questions;             numericscore = 100.0 - (missed * pointseach);              // call base class's setscore method set mumeric score.             setscore(numericscore);         }          /// <summary>         /// getpointseach method returns number of points each questions worth         /// </summary>         /// <returns>the value in pointseach field</returns>         public double getpointseach()         {             return pointseach;         }          /// <summary>         /// getnummissed method returns number of questions missed         /// </summary>         /// <returns>the value in nummissed field</returns>         public int getnummissed()         {             return nummissed;         }     } } 

and here main

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              // number of questions on exam             console.write("how many questions " + "on exam? ");             questions = convert.toint32(console.readline());              // number of questions missed.             console.write("how many questions did " + "the student miss? ");             missed = convert.toint32(console.readline());              // minimum passing score             console.write("what minimum " + "passing score? ");             minpassing = convert.toint32(console.readline());              // create passfailexam project             passfailexam exam = new passfailexam(questions, missed, minpassing);              // display teset results.             console.writeline("each questions counts {0} points.",                 exam.getpointseach());             console.writeline("the exam score {0} ",                 exam.getscore());             console.writeline("the exam grade {0} ",                 exam.getgrade());             console.readline();         }     } } 

the output should be

how many questions on exam? 100 how many questions did student miss? 25 minimum passing score? 60 each question counts 1 points. exam score 75 exam grade p 

i have tried make base method virtual , calling override when trying override , gets me error "'protectedmembers.passfailactivity.getgrade()': cannot override inherited member 'protectedmembers.gradedactivity2.getgrade()' because not marked virtual, abstract, or override". checked

the error self-explanatory, mark function virtual in gradedactivity2:

public virtual char getgrade() 

from msdn:

the virtual keyword used modify method, property, indexer, or event declaration , allow overridden in derived class. example, method can overridden class inherits it: