Visitor Pattern In C#

In this article I will explain the visitor pattern in detail, but before jumping to Visitor Pattern, let’s have a look at the following screenshot for Design Patterns and its types.



There are a lot of design patterns (around 35-40), I am not going to explain other design pattern here otherwise the topic will become too large.

Let’s go to Visitor Pattern

Visitor is one of the most important design pattern of C#. As you can see in the above  screenshot, it falls under the category of Behavioural design pattern that means it will work and change the behaviour of the class.

Visitor pattern is used for separation of Business logic and algorithm from an object data structure i.e. it is used for separating logic from actual data structure.

Due to the separation of data structure and its logic we can add new logic without changing the current data structure and vice-versa.

Visitor Pattern represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates (as per GOF).



To co-relate the example with real world let’s take an example of Employee Monthly Salary, investment and Tax calculation.



Let’s calculate Income Tax, Annual Investment, Annual Net Earning… with C#.

Without Visitor Pattern

using System;
using System.Collections.Generic;
using System.Globalization;
namespace Earning_TaxInIndiaWithoutVisitorPattern
{
    class Program
    {
        static void Main(string[] args)
        {
            Employee emp = new Employee
            {
                EmployeeId = "XYZ1001", EmployeeName = "Banketeshvar Narayan"
            };
            AddDataForEmployee(emp);#
            region Calculate Net Earning of the Year
            double NetEarningoftheYear = 0.0;
            foreach(var monthlySalary_Earning in emp.MonthlySalary_Earnings)
            {
                NetEarningoftheYear += (monthlySalary_Earning.BasicSalary + monthlySalary_Earning.ConveyanceAllowance + monthlySalary_Earning.FoodCard_Bill + monthlySalary_Earning.HRAExemption + monthlySalary_Earning.MedicalAllowance + monthlySalary_Earning.OtherBills + monthlySalary_Earning.PersonalAllowance + monthlySalary_Earning.TelephoneBill);
            }
            foreach(var monthlySalary_Deduction in emp.MonthlySalary_Deductions)
            {
                NetEarningoftheYear -= (monthlySalary_Deduction.ProvidentFund_EmployeeContribution + monthlySalary_Deduction.ProvidentFund_EmployerContribution + monthlySalary_Deduction.ProfessionTax + monthlySalary_Deduction.OtherDeduction);
            }#
            endregion# region Calculate TaxableAmount
            double TaxableAmount = 0.0;
            foreach(var monthlySalary_Earning in emp.MonthlySalary_Earnings)
            {
                TaxableAmount += (monthlySalary_Earning.BasicSalary + monthlySalary_Earning.HRAExemption + monthlySalary_Earning.MedicalAllowance + monthlySalary_Earning.PersonalAllowance);
                //Non Taxable parts
                //monthlySalary_Earning.FoodCard_Bill
                //monthlySalary_Earning.ConveyanceAllowance
                //monthlySalary_Earning.TelephoneBill
                //monthlySalary_Earning.OtherBills
            }
            foreach(var monthlySalary_Deduction in emp.MonthlySalary_Deductions)
            {
                TaxableAmount -= (monthlySalary_Deduction.ProvidentFund_EmployeeContribution + monthlySalary_Deduction.ProvidentFund_EmployerContribution + monthlySalary_Deduction.ProfessionTax + monthlySalary_Deduction.OtherDeduction);
            }
            foreach(var monthlyExpense in emp.MonthlyExpenses)
            {
                TaxableAmount -= monthlyExpense.MonthlyRent;
            }
            foreach(var annualInvestment in emp.AnnualInvestments)
            {
                TaxableAmount -= annualInvestment.InvestmentAmmount;
            }#
            endregion
            Console.WriteLine("Annual Net Earning Amount : {0}", NetEarningoftheYear);
            Console.WriteLine("Annual Taxable Amount : {0}", TaxableAmount);
        }
        private static void AddDataForEmployee(Employee emp)
        {
            for (int i = 1; i <= 12; i++)
            {
                emp.MonthlySalary_Earnings.Add(new MonthlySalary_Earning
                {
                    MonthName = DateTimeFormatInfo.CurrentInfo.GetMonthName(i),
                        BasicSalary = 120000,
                        HRAExemption = 50000,
                        ConveyanceAllowance = 1600,
                        PersonalAllowance = 45000,
                        MedicalAllowance = 1500,
                        TelephoneBill = 2500,
                        FoodCard_Bill = 3000,
                        OtherBills = 35000
                });
                emp.MonthlySalary_Deductions.Add(new MonthlySalary_Deduction
                {
                    MonthName = DateTimeFormatInfo.CurrentInfo.GetMonthName(i),
                        ProvidentFund_EmployeeContribution = 8000,
                        ProvidentFund_EmployerContribution = 8000,
                        OtherDeduction = 700,
                        ProfessionTax = 200,
                        TDS = 15000
                });
                emp.MonthlyExpenses.Add(new MonthlyExpense
                {
                    MonthName = DateTimeFormatInfo.CurrentInfo.GetMonthName(1), MonthlyRent = 10000
                });
            }
            emp.AnnualInvestments.Add(new AnnualInvestment
            {
                InvestmentDetails = "MediclaimPolicy", InvestmentAmmount = 15000
            });
            emp.AnnualInvestments.Add(new AnnualInvestment
            {
                InvestmentDetails = "MediclaimPolicyforParents", InvestmentAmmount = 25000
            });
            emp.AnnualInvestments.Add(new AnnualInvestment
            {
                InvestmentDetails = "HouseLoan", InvestmentAmmount = 0.0
            });
            emp.AnnualInvestments.Add(new AnnualInvestment
            {
                InvestmentDetails = "EducationLoan", InvestmentAmmount = 0.0
            });
            emp.AnnualInvestments.Add(new AnnualInvestment
            {
                InvestmentDetails = "OtherInvestment", InvestmentAmmount = 5000
            });
            emp.AnnualInvestments.Add(new AnnualInvestment
            {
                InvestmentDetails = "RGESS", InvestmentAmmount = 5500
            });
            emp.AnnualInvestments.Add(new AnnualInvestment
            {
                InvestmentDetails = "Section80Cn80CCD_ExceptPF", InvestmentAmmount = 100000
            });
            //emp.AnnualInvestments.Add(new AnnualInvestment
            //{
            // //MediclaimPolicy = 15000,
            // //MediclaimPolicyforParents = 25000,
            // //HouseLoan = 0.0,
            // //EducationLoan = 0.0,
            // //OtherInvestment = 5000,
            // //RGESS = 5500,
            // //Section80Cn80CCD_ExceptPF = 100000
            //});
        }
    }#
    region Employee
    public class Employee
    {
        public string EmployeeId
        {
            get;
            set;
        }
        public string EmployeeName
        {
            get;
            set;
        }
        public List < MonthlySalary_Earning > MonthlySalary_Earnings = new List < MonthlySalary_Earning > ();
        public List < MonthlySalary_Deduction > MonthlySalary_Deductions = new List < MonthlySalary_Deduction > ();
        public List < AnnualInvestment > AnnualInvestments = new List < AnnualInvestment > ();
        public List < MonthlyExpense > MonthlyExpenses = new List < MonthlyExpense > ();
    }#
    endregion# region MonthlySalary_Earning
    public class MonthlySalary_Earning
    {
        public string MonthName
        {
            get;
            set;
        }
        public double BasicSalary
        {
            get;
            set;
        }
        public double HRAExemption
        {
            get;
            set;
        }
        public double ConveyanceAllowance
        {
            get;
            set;
        }
        public double PersonalAllowance
        {
            get;
            set;
        }
        public double MedicalAllowance
        {
            get;
            set;
        }
        public double TelephoneBill
        {
            get;
            set;
        }
        public double FoodCard_Bill
        {
            get;
            set;
        }
        public double OtherBills
        {
            get;
            set;
        }
    }#
    endregion# region MonthlySalary_Deduction
    public class MonthlySalary_Deduction
    {
        public string MonthName
        {
            get;
            set;
        }
        public double ProvidentFund_EmployeeContribution
        {
            get;
            set;
        }
        public double ProvidentFund_EmployerContribution
        {
            get;
            set;
        }
        public double ProfessionTax
        {
            get;
            set;
        }
        public double TDS
        {
            get;
            set;
        }
        public double OtherDeduction
        {
            get;
            set;
        }
    }#
    endregion# region AnnualInvestment
    public class AnnualInvestment
    {
        public string InvestmentDetails
        {
            get;
            set;
        }
        public double InvestmentAmmount
        {
            get;
            set;
        }
        //public double MediclaimPolicy { get; set; }
        //public double MediclaimPolicyforParents { get; set; }
        //public double HouseLoan { get; set; }
        //public double EducationLoan { get; set; }
        //public double Section80Cn80CCD_ExceptPF { get; set; }
        //public double RGESS { get; set; }
        //public double OtherInvestment { get; set; }
    }#
    endregion# region MonthlyExpense
    public class MonthlyExpense
    {
        public string MonthName
        {
            get;
            set;
        }
        public double MonthlyRent
        {
            get;
            set;
        }
    }#
    endregion
}

With Visitor Pattern

using System;
using System.Collections.Generic;
using System.Globalization;
namespace Earning_TaxationInIndiaWithVisitorPattern
{
    class Program
    {
        static void Main(string[] args)
        {
            Employee emp = new Employee
            {
                EmployeeId = "XYZ1001", EmployeeName = "Banketeshvar Narayan Sharma"
            };
            AddDataForEmployee(emp);
            var netAnnualEarningVisitor = new NetAnnualEarningVisitor();
            var annualTaxableAmount = new TaxableAmountVisitor();
            emp.Accept(netAnnualEarningVisitor);
            emp.Accept(annualTaxableAmount);
            Console.WriteLine("Annual Net Earning Amount : {0}", netAnnualEarningVisitor.NetEarningoftheYear);
            Console.WriteLine("Annual Taxable Amount : {0}", annualTaxableAmount.TaxableAmount);
            Console.ReadKey();
        }
        private static void AddDataForEmployee(Employee emp)
        {
            for (int i = 1; i <= 12; i++)
            {
                emp.Salaries.Add(new MonthlySalary_Earning
                {
                    MonthName = DateTimeFormatInfo.CurrentInfo.GetMonthName(i),
                        BasicSalary = 120000,
                        HRAExemption = 50000,
                        ConveyanceAllowance = 1600,
                        PersonalAllowance = 45000,
                        MedicalAllowance = 1500,
                        TelephoneBill = 2500,
                        FoodCard_Bill = 3000,
                        OtherBills = 35000
                });
                emp.Salaries.Add(new MonthlySalary_Deduction
                {
                    MonthName = DateTimeFormatInfo.CurrentInfo.GetMonthName(i),
                        ProvidentFund_EmployeeContribution = 8000,
                        ProvidentFund_EmployerContribution = 8000,
                        OtherDeduction = 700,
                        ProfessionTax = 200,
                        TDS = 15000
                });
                emp.Salaries.Add(new MonthlyExpense
                {
                    MonthName = DateTimeFormatInfo.CurrentInfo.GetMonthName(1), MonthlyRent = 10000
                });
            }
            emp.Salaries.Add(new AnnualInvestment
            {
                InvestmentDetails = "MediclaimPolicy", InvestmentAmmount = 15000
            });
            emp.Salaries.Add(new AnnualInvestment
            {
                InvestmentDetails = "MediclaimPolicyforParents", InvestmentAmmount = 25000
            });
            emp.Salaries.Add(new AnnualInvestment
            {
                InvestmentDetails = "HouseLoan", InvestmentAmmount = 0.0
            });
            emp.Salaries.Add(new AnnualInvestment
            {
                InvestmentDetails = "EducationLoan", InvestmentAmmount = 0.0
            });
            emp.Salaries.Add(new AnnualInvestment
            {
                InvestmentDetails = "OtherInvestment", InvestmentAmmount = 5000
            });
            emp.Salaries.Add(new AnnualInvestment
            {
                InvestmentDetails = "RGESS", InvestmentAmmount = 5500
            });
            emp.Salaries.Add(new AnnualInvestment
            {
                InvestmentDetails = "Section80Cn80CCD_ExceptPF", InvestmentAmmount = 100000
            });
        }
    }
    public class NetAnnualEarningVisitor: IVisitor
    {
        public double NetEarningoftheYear
        {
            get;
            set;
        }
        public void Visit(MonthlySalary_Earning monthlySalary_Earning)
        {
            NetEarningoftheYear += (monthlySalary_Earning.BasicSalary + monthlySalary_Earning.ConveyanceAllowance + monthlySalary_Earning.FoodCard_Bill + monthlySalary_Earning.HRAExemption + monthlySalary_Earning.MedicalAllowance + monthlySalary_Earning.OtherBills + monthlySalary_Earning.PersonalAllowance + monthlySalary_Earning.TelephoneBill);
        }
        public void Visit(MonthlySalary_Deduction monthlySalary_Deduction)
        {
            NetEarningoftheYear -= (monthlySalary_Deduction.ProvidentFund_EmployeeContribution + monthlySalary_Deduction.ProvidentFund_EmployerContribution + monthlySalary_Deduction.ProfessionTax + monthlySalary_Deduction.OtherDeduction);
        }
        public void Visit(AnnualInvestment annualInvestment)
        {
            // do nothing
        }
        public void Visit(MonthlyExpense monthlyExpense)
        {
            //do nothing
        }
    }
    public class TaxableAmountVisitor: IVisitor
    {
        public double TaxableAmount
        {
            get;
            set;
        }
        public void Visit(MonthlySalary_Earning monthlySalary_Earning)
        {
            TaxableAmount += (monthlySalary_Earning.BasicSalary + monthlySalary_Earning.HRAExemption + monthlySalary_Earning.MedicalAllowance + monthlySalary_Earning.PersonalAllowance);
        }
        public void Visit(MonthlySalary_Deduction monthlySalary_Deduction)
        {
            TaxableAmount -= (monthlySalary_Deduction.ProvidentFund_EmployeeContribution + monthlySalary_Deduction.ProvidentFund_EmployerContribution + monthlySalary_Deduction.ProfessionTax + monthlySalary_Deduction.OtherDeduction);
        }
        public void Visit(MonthlyExpense monthlyExpense)
        {
            TaxableAmount -= monthlyExpense.MonthlyRent;
        }
        public void Visit(AnnualInvestment annualInvestment)
        {
            TaxableAmount -= annualInvestment.InvestmentAmmount;
        }
    }
    public interface ISalary
    {
        void Accept(IVisitor visitor);
    }
    public interface IVisitor
    {
        void Visit(MonthlySalary_Earning monthlySalary_Earning);
        void Visit(MonthlySalary_Deduction monthlySalary_Deduction);
        void Visit(MonthlyExpense monthlyExpense);
        void Visit(AnnualInvestment annualInvestment);
    }#
    region Employee
    public class Employee: ISalary
    {
        public string EmployeeId
        {
            get;
            set;
        }
        public string EmployeeName
        {
            get;
            set;
        }
        public List < ISalary > Salaries = new List < ISalary > ();
        public void Accept(IVisitor visitor)
        {
            foreach(var salary in Salaries)
            {
                salary.Accept(visitor);
            }
        }
    }#
    endregion# region MonthlySalary_Earning
    public class MonthlySalary_Earning: ISalary
    {
        public string MonthName
        {
            get;
            set;
        }
        public double BasicSalary
        {
            get;
            set;
        }
        public double HRAExemption
        {
            get;
            set;
        }
        public double ConveyanceAllowance
        {
            get;
            set;
        }
        public double PersonalAllowance
        {
            get;
            set;
        }
        public double MedicalAllowance
        {
            get;
            set;
        }
        public double TelephoneBill
        {
            get;
            set;
        }
        public double FoodCard_Bill
        {
            get;
            set;
        }
        public double OtherBills
        {
            get;
            set;
        }
        public void Accept(IVisitor visitor)
        {
            visitor.Visit(this);
        }
    }#
    endregion# region MonthlySalary_Deduction
    public class MonthlySalary_Deduction: ISalary
    {
        public string MonthName
        {
            get;
            set;
        }
        public double ProvidentFund_EmployeeContribution
        {
            get;
            set;
        }
        public double ProvidentFund_EmployerContribution
        {
            get;
            set;
        }
        public double ProfessionTax
        {
            get;
            set;
        }
        public double TDS
        {
            get;
            set;
        }
        public double OtherDeduction
        {
            get;
            set;
        }
        public void Accept(IVisitor visitor)
        {
            visitor.Visit(this);
        }
    }#
    endregion# region AnnualInvestment
    public class AnnualInvestment: ISalary
    {
        public string InvestmentDetails
        {
            get;
            set;
        }
        public double InvestmentAmmount
        {
            get;
            set;
        }
        public void Accept(IVisitor visitor)
        {
            visitor.Visit(this);
        }
    }#
    endregion# region MonthlyExpense
    public class MonthlyExpense: ISalary
    {
        public string MonthName
        {
            get;
            set;
        }
        public double MonthlyRent
        {
            get;
            set;
        }
        public void Accept(IVisitor visitor)
        {
            visitor.Visit(this);
        }
    }#
    endregion
}