CSharp: Visitor Pattern in donet core 3

    /// <summary>
    /// 访问者模式  Visitor Pattern
    /// geovindu,Geovin Du eidt    
    /// </summary>
    interface IEmployee
    {
        /// <summary>
        /// To set an employee name
        /// </summary>
        string Name { get; set; }
        /// <summary>
        /// To set an employee department
        /// </summary>
        string Dept { get; set; }
        /// <summary>
        /// To set an employee designation
        /// </summary>
        string Designation { get; set; }

        /// <summary>
        /// To display an employee details
        /// </summary>
        void DisplayDetails();


        /// <summary>
        ///Newly added for this example
        ///To set years of Experience        
        ///</summary>
        double Experience { get; set; }
        /// <summary>
        /// Newly added for this example
        /// </summary>
        /// <param name="visitor"></param>
        void Accept(IVisitor visitor);
    }
    /// <summary>
    /// Leaf node
    /// </summary>
    class Employee : IEmployee
    {

        /// <summary>
        /// 
        /// </summary>
        public string Name { get; set; }
        /// <summary>
        /// 
        /// </summary>
        public string Dept { get; set; }
        /// <summary>
        /// 
        /// </summary>
        public string Designation { get; set; }
        /// <summary>
        /// 
        /// </summary>
        public double Experience { get; set; }
        /// <summary>
        /// Details of a leaf node
        /// </summary>
        public void DisplayDetails()
        {
            Console.WriteLine($"姓名：{Name} 工作在 {Dept} 部门.描述:{Designation}.经历 : {Experience} 年.");
        }
        /// <summary>
        /// 
        /// </summary>
        /// <param name="visitor"></param>
        public void Accept(IVisitor visitor)
        {
            visitor.VisitEmployees(this);
        }

    }
    /// <summary>
    /// Non-leaf node
    /// </summary>
    class CompositeEmployee : IEmployee
    {

        /// <summary>
        /// 
        /// </summary>
        public string Name { get; set; }

        /// <summary>
        /// 
        /// </summary>
        public string Dept { get; set; }
        /// <summary>
        /// 
        /// </summary>
        public string Designation { get; set; }
        /// <summary>
        /// 
        /// </summary>
        public double Experience { get; set; }

        //The container for child objects
        //private List<IEmployee> subordinateList = new List<IEmployee>();
        //Making it public now
        /// <summary>
        /// 
        /// </summary>
        public List<IEmployee> subordinateList = new List<IEmployee>();

        /// <summary>
        /// To add an employee
        /// </summary>
        /// <param name="e"></param>
        public void AddEmployee(IEmployee e)
        {
            subordinateList.Add(e);
        }

        /// <summary>
        /// To remove an employee
        /// </summary>
        /// <param name="e"></param>
        public void RemoveEmployee(IEmployee e)
        {
            subordinateList.Remove(e);
        }

        /// <summary>
        /// Details of a composite node
        /// </summary>
        public void DisplayDetails()
        {
            Console.WriteLine($"\n姓名：{Name} 工作在 {Dept} 部门.描述:{Designation}.经历 : {Experience} 年.");
            foreach (IEmployee e in subordinateList)
            {
                e.DisplayDetails();
            }
        }

        public void Accept(IVisitor visitor)
        {
            visitor.VisitEmployees(this);
        }
    }
    /// <summary>
    /// Visitor interface    
    /// </summary>
    interface IVisitor
    {
        //To visit leaf nodes
        void VisitEmployees(Employee employee);

        //To visit composite nodes
        void VisitEmployees(CompositeEmployee employee);
    }
    /// <summary>
    /// Concrete visitor class-PromotionCheckerVisitor
    /// </summary>
    class PromotionCheckerVisitor : IVisitor
    {

       /// <summary>
       /// 
       /// </summary>
        string eligibleForPromotion = String.Empty;
        /// <summary>
        /// 工作经历大于15年
        /// </summary>
        /// <param name="employee"></param>
        public void VisitEmployees(CompositeEmployee employee)
        {
            //We'll promote them if experience is greater than 15 years
            eligibleForPromotion = employee.Experience > 15 ? "是" : "否";
            Console.WriteLine($"{employee.Name} 来自于 {employee.Dept} 工作长于15年是否有资格升职? :{eligibleForPromotion}");

        }
        /// <summary>
        /// 工作经历在于12年
        /// </summary>
        /// <param name="employee"></param>
        public void VisitEmployees(Employee employee)
        {
            //We'll promote them if experience is greater than 12 years
            eligibleForPromotion = employee.Experience > 12 ? "Yes" : "No";
            Console.WriteLine($"{employee.Name} 来自于 {employee.Dept} 工作长于12年是否有资格升职? :{eligibleForPromotion}");
        }
    }

 using DictType = Dictionary<Type, Action<Expression, StringBuilder>>;
    /// <summary>
    /// 访问者模式  Visitor Pattern
    /// geovindu,Geovin Du eidt    
    /// </summary>
    public abstract class Expression
    {
    }
    /// <summary>
    /// 
    /// </summary>
    public class DoubleExpression : Expression
    {

        /// <summary>
        /// 
        /// </summary>
        public double Value;
        /// <summary>
        /// 
        /// </summary>
        /// <param name="value"></param>
        public DoubleExpression(double value)
        {
            Value = value;
        }
    }
    /// <summary>
    /// 
    /// </summary>
    public class AdditionExpression : Expression
    {
        /// <summary>
        /// 
        /// </summary>
        public Expression Left;
        /// <summary>
        /// 
        /// </summary>
        public Expression Right;
        /// <summary>
        /// 
        /// </summary>
        /// <param name="left"></param>
        /// <param name="right"></param>
        /// <exception cref="ArgumentNullException"></exception>
        public AdditionExpression(Expression left, Expression right)
        {
            Left = left ?? throw new ArgumentNullException(paramName: nameof(left));
            Right = right ?? throw new ArgumentNullException(paramName: nameof(right));
        }
    }

    /// <summary>
    /// 
    /// </summary>

    public static class ExpressionPrinter
    {
        /// <summary>
        /// 
        /// </summary>
        private static DictType actions = new DictType
        {
            [typeof(DoubleExpression)] = (e, sb) =>
            {
                var de = (DoubleExpression)e;
                sb.Append(de.Value);
            },
            [typeof(AdditionExpression)] = (e, sb) =>
            {
                var ae = (AdditionExpression)e;
                sb.Append("(");
                Print(ae.Left, sb);
                sb.Append("+");
                Print(ae.Right, sb);
                sb.Append(")");
            }
        };
        /// <summary>
        /// 
        /// </summary>
        /// <param name="e"></param>
        /// <returns></returns>
        public static string Print3(this Expression e)
        {
            var sb = new StringBuilder();
            Print2(e, sb);
            return sb.ToString();
        }
        /// <summary>
        /// 
        /// </summary>
        /// <param name="e"></param>
        /// <param name="sb"></param>
        private static void Print2(Expression e, StringBuilder sb)
        {
            actions[e.GetType()](e, sb);
        }
        /// <summary>
        /// 
        /// </summary>
        /// <param name="e"></param>
        /// <param name="sb"></param>
        private static void Print(Expression e, StringBuilder sb)
        {
            if (e is DoubleExpression de)
            {
                sb.Append(de.Value);
            }
            else
            if (e is AdditionExpression ae)
            {
                sb.Append("(");
                Print(ae.Left, sb);
                sb.Append("+");
                Print(ae.Right, sb);
                sb.Append(")");
            }
            // breaks open-closed principle
            // will work incorrectly on missing case
        }
    }

调用：

           //访问者模式
            Console.WriteLine("***访问者模式 Visitor Pattern with Composite Pattern Demo. ***");

            #region Mathematics department
            //2 lecturers work in Mathematics department
            Employee mathTeacher1 = new Employee { Name = "老大", Dept = "应用数学", Designation = "讲师" ,Experience=13.7};
            Employee mathTeacher2 = new Employee { Name = "老二", Dept = "应用数学", Designation = "讲师", Experience = 6.5 };

            //The college has a Head of Department in Mathematics
            CompositeEmployee hodMaths = new CompositeEmployee { Name = "老三", Dept = "数学", Designation = "数学系主任", Experience = 14 };

            //Lecturers of Mathematics directly reports to HOD-Maths
            hodMaths.AddEmployee(mathTeacher1);
            hodMaths.AddEmployee(mathTeacher2);
            #endregion

            #region Computer Science department
            //3 lecturers work in Computer Sc. department
            Employee cseTeacher1 = new Employee { Name = "老四", Dept = "计算机科学", Designation = "讲师", Experience = 10.2 };
            Employee cseTeacher2 = new Employee { Name = "老五", Dept = "计算机科学.", Designation = "讲师", Experience = 13.5 };
            Employee cseTeacher3 = new Employee { Name = "老六", Dept = "计算机科学", Designation = "讲师", Experience = 7.3 };

            //The college has a Head of Department in Computer science
            CompositeEmployee hodCompSc = new CompositeEmployee { Name = "老幺", Dept = "计算机科学.", Designation = "计算机科学系主任.", Experience = 16.5 };

            //Lecturers of Computer Sc. directly reports to HOD-CSE
            hodCompSc.AddEmployee(cseTeacher1);
            hodCompSc.AddEmployee(cseTeacher2);
            hodCompSc.AddEmployee(cseTeacher3);
            #endregion

            #region Top level management
            //The college also has a Principal
            CompositeEmployee principal = new CompositeEmployee { Name = "族长", Dept = "规划监督管理部门", Designation = "校长", Experience = 21 };

            //Head of Departments's of Maths and Computer Science directly reports to Principal.
            principal.AddEmployee(hodMaths);
            principal.AddEmployee(hodCompSc);
            #endregion

            /* 
             * Printing the leaf-nodes and branches in the same way.
             * i.e. in each case, we are calling DisplayDetails() method.
            */           
            Console.WriteLine("\nDetails of a college structure is as follows:");
            //Prints the complete structure
            principal.DisplayDetails();            
          
          

            List<IEmployee> participants = new List<IEmployee>();

            //For employees who directly reports to Principal
            foreach (IEmployee e in principal.subordinateList)
            {
                // e.Accept(aVisitor);
                participants.Add(e);
            }
            //For employees who directly reports to HOD-Maths
            foreach (IEmployee e in hodMaths.subordinateList)
            {
                //e.Accept(aVisitor);
                participants.Add(e);
            }
            //For employees who directly reports to HOD-Comp.Sc
            foreach (IEmployee e in hodCompSc.subordinateList)
            {
                //e.Accept(aVisitor);
                participants.Add(e);
            }
            Console.WriteLine("\n***访问者模式 Visitor starts visiting our composite structure***\n");
            IVisitor visitor = new PromotionCheckerVisitor();
            /*
           * Principal is already holding the highest position.
           * We are not checking whether he is eligible 
           * for promotion or not.
           */
            //principal.Accept(visitor);
            //Visitor is traversing the participant list
            foreach ( IEmployee  emp in participants)
            {
                emp.Accept(visitor);
            }

            //
            var geovindu = new AdditionExpression(
           left: new DoubleExpression(1),
           right: new AdditionExpression(
           left: new DoubleExpression(2),
            right: new DoubleExpression(3)));
            var sb = new StringBuilder();
            //ExpressionPrinter.Print2(e, sb);
            Console.WriteLine(geovindu.Print3());

输出：

***访问者模式 Visitor Pattern with Composite Pattern Demo. ***

Details of a college structure is as follows:

姓名：族长 工作在 规划监督管理部门 部门.描述:校长.经历 : 21 年.

姓名：老三 工作在 数学 部门.描述:数学系主任.经历 : 14 年.
姓名：老大 工作在 应用数学 部门.描述:讲师.经历 : 13.7 年.
姓名：老二 工作在 应用数学 部门.描述:讲师.经历 : 6.5 年.

姓名：老幺 工作在 计算机科学. 部门.描述:计算机科学系主任..经历 : 16.5 年.
姓名：老四 工作在 计算机科学 部门.描述:讲师.经历 : 10.2 年.
姓名：老五 工作在 计算机科学. 部门.描述:讲师.经历 : 13.5 年.
姓名：老六 工作在 计算机科学 部门.描述:讲师.经历 : 7.3 年.

***访问者模式 Visitor starts visiting our composite structure***

老三 来自于 数学 工作长于15年是否有资格升职? :否
老幺 来自于 计算机科学. 工作长于15年是否有资格升职? :是
老大 来自于 应用数学 工作长于12年是否有资格升职? :Yes
老二 来自于 应用数学 工作长于12年是否有资格升职? :No
老四 来自于 计算机科学 工作长于12年是否有资格升职? :No
老五 来自于 计算机科学. 工作长于12年是否有资格升职? :Yes
老六 来自于 计算机科学 工作长于12年是否有资格升职? :No
(1+(2+3))