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Create sample showing how to implement short circuiting priority rules.
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‎DesignPatternsInCSharp/DesignPatternsInCSharp.csproj

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<ItemGroup>
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<None Remove="Adapter\People.json" />
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<None Remove="RulesEngine\DiscountsShortCircuit\README.md" />
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</ItemGroup>
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<ItemGroup>
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<Content Include="RulesEngine\DiscountsShortCircuit\README.md" />
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</ItemGroup>
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<ItemGroup>

‎DesignPatternsInCSharp/RulesEngine/Discounts/DiscountCalculate_CalculateDiscountPercentage.cs renamed to ‎DesignPatternsInCSharp/RulesEngine/Discounts/DiscountCalculator_CalculateDiscountPercentage.cs

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namespace DesignPatternsInCSharp.RulesEngine.Discounts
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{
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public class DiscountCalculate_CalculateDiscountPercentage
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public class DiscountCalculator_CalculateDiscountPercentage
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{
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private DiscountCalculator _calculator = new DiscountCalculator();
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const int DEFAULT_AGE = 30;

‎DesignPatternsInCSharp/RulesEngine/DiscountsShortCircuit/Customer.cs

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using System;
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namespace DesignPatternsInCSharp.RulesEngine.DiscountsShortCircuit
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{
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public class Customer
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{
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public DateTime? DateOfFirstPurchase { get; set; }
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public DateTime? DateOfBirth { get; set; }
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public bool IsVeteran { get; set; }
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public ReferredBy Referrer { get; set; }
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}
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// Customers who are referred by others get a one time 30% discount on their first purchase.
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// We can determine if they are eligible by seeing if they have a referrer and if DateOfFirstPurchase is null.
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public class ReferredBy
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{
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public ReferredBy(Customer customer)
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{
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Customer = customer;
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}
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public Customer Customer { get; private set; }
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public DateTime DateReferred { get; set; } = DateTime.Today;
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}
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}

‎DesignPatternsInCSharp/RulesEngine/DiscountsShortCircuit/DiscountCalculator.cs

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using System;
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using System.Collections.Generic;
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using System.Linq;
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namespace DesignPatternsInCSharp.RulesEngine.DiscountsShortCircuit
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{
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// View History
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// https://github.githistory.xyz/ardalis/DesignPatternsInCSharp/blob/master/DesignPatternsInCSharp/RulesEngine/Discounts/DiscountCalculator.cs
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public interface IDiscountRule
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{
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decimal CalculateDiscount(Customer customer, decimal currentDiscount);
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}
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// Priority rules short-circuit all other rules if they match.
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public interface IExclusivePriorityRule : IDiscountRule
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{
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int PriorityLowestExecutesFirst { get; }
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}
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public class ReferredCustomerRule : IExclusivePriorityRule
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{
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public int PriorityLowestExecutesFirst => 0;
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public decimal CalculateDiscount(Customer customer, decimal currentDiscount)
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{
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if(customer.Referrer != null)
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{
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// here we can either duplicate the logic in FirstTimeCustomerRule or use that rule
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bool firstTimeCustomer = new FirstTimeCustomerRule().CalculateDiscount(customer, 0) > 0;
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if (firstTimeCustomer) return .30m;
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}
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return 0;
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}
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}
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public class FirstTimeCustomerRule : IDiscountRule
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{
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public decimal CalculateDiscount(Customer customer, decimal currentDiscount)
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{
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if (!customer.DateOfFirstPurchase.HasValue)
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{
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return .15m;
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}
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return 0;
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}
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}
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public class LoyalCustomerRule : IDiscountRule
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{
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public decimal CalculateDiscount(Customer customer, decimal currentDiscount)
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{
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if (customer.DateOfFirstPurchase.HasValue)
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{
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if (customer.DateOfFirstPurchase.Value < DateTime.Now.AddYears(-15))
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{
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return .15m;
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}
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if (customer.DateOfFirstPurchase.Value < DateTime.Now.AddYears(-10))
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{
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return .12m;
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}
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if (customer.DateOfFirstPurchase.Value < DateTime.Now.AddYears(-5))
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{
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return .10m;
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}
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if (customer.DateOfFirstPurchase.Value < DateTime.Now.AddYears(-2))
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{
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return .08m;
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}
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if (customer.DateOfFirstPurchase.Value < DateTime.Now.AddYears(-1))
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{
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return .05m;
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}
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}
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return 0;
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}
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}
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public class VeteranRule : IDiscountRule
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{
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public decimal CalculateDiscount(Customer customer, decimal currentDiscount)
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{
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if (customer.IsVeteran)
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{
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return .10m;
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}
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return 0;
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}
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}
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public class SeniorRule : IDiscountRule
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{
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public decimal CalculateDiscount(Customer customer, decimal currentDiscount)
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{
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if (customer.DateOfBirth < DateTime.Now.AddYears(-65))
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{
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return .05m;
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}
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return 0;
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}
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}
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public class BirthdayRule : IDiscountRule
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{
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public decimal CalculateDiscount(Customer customer, decimal currentDiscount)
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{
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bool isBirthday = customer.DateOfBirth.HasValue && customer.DateOfBirth.Value.Month == DateTime.Today.Month && customer.DateOfBirth.Value.Day == DateTime.Today.Day;
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if (isBirthday) return currentDiscount + 0.10m;
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return currentDiscount;
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}
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}
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/// <summary>
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/// Adding exclusive priority rules we could just use a single list
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/// but then we would have LSP violations and we would need to sort it
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/// and treat the priority rules separately. Using two lists from the start
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/// yields a simpler overall design.
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/// </summary>
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public class DiscountRuleEngine
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{
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List<IDiscountRule> _rules = new List<IDiscountRule>();
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List<IExclusivePriorityRule> _priorityRules = new List<IExclusivePriorityRule>();
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public DiscountRuleEngine(IEnumerable<IDiscountRule> rules, IEnumerable<IExclusivePriorityRule> priorityRules)
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{
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_rules.AddRange(rules);
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_priorityRules.AddRange(priorityRules);
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}
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public decimal CalculateDiscountPercentage(Customer customer)
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{
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decimal discount = 0m;
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foreach(var priorityRule in _priorityRules.OrderBy(pr => pr.PriorityLowestExecutesFirst))
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{
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var result = priorityRule.CalculateDiscount(customer, 0);
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if (result > 0) return result;
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}
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foreach(var rule in _rules)
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{
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discount = Math.Max(discount, rule.CalculateDiscount(customer, discount));
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}
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return discount;
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}
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}
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public class DiscountCalculator
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{
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public decimal CalculateDiscountPercentage(Customer customer)
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{
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// We need to produce separate list of priority rules now
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var ruleType = typeof(IDiscountRule);
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IEnumerable<IDiscountRule> rules = this.GetType().Assembly.GetTypes()
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.Where(p => ruleType.IsAssignableFrom(p) && !p.IsInterface)
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.Select(r => Activator.CreateInstance(r) as IDiscountRule);
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// create a strongly-typed list of just the priority rules
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var priorityRules = rules.Where(r => r is IExclusivePriorityRule)
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.Select(r => (IExclusivePriorityRule)r)
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.ToList();
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// create a list of all the other rules
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var baseRules = rules.Where(r => !(r is IExclusivePriorityRule)).ToList();
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var engine = new DiscountRuleEngine(baseRules, priorityRules);
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return engine.CalculateDiscountPercentage(customer);
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}
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}
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}

‎DesignPatternsInCSharp/RulesEngine/DiscountsShortCircuit/DiscountCalculator_CalculateDiscountPercentage.cs

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using FluentAssertions;
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using System;
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using Xunit;
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namespace DesignPatternsInCSharp.RulesEngine.DiscountsShortCircuit
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{
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public class DiscountCalculator_CalculateDiscountPercentage
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{
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private DiscountCalculator _calculator = new DiscountCalculator();
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const int DEFAULT_AGE = 30;
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[Fact]
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public void Returns0PctForBasicCustomer()
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{
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var customer = CreateCustomer(DEFAULT_AGE, DateTime.Today.AddDays(-1));
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var result = _calculator.CalculateDiscountPercentage(customer);
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result.Should().Be(0m);
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}
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[Fact]
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public void Returns5PctForCustomersOver65()
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{
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var customer = CreateCustomer(65, DateTime.Today.AddDays(-1));
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var result = _calculator.CalculateDiscountPercentage(customer);
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result.Should().Be(.05m);
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}
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[Theory]
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[InlineData(20)]
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[InlineData(70)]
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public void Returns15PctForCustomerFirstPurchase(int customerAge)
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{
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var customer = CreateCustomer(customerAge);
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var result = _calculator.CalculateDiscountPercentage(customer);
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result.Should().Be(.15m);
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}
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[Theory]
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[InlineData(20)]
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[InlineData(70)]
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public void Returns10PctForCustomersWhoAreVeterans(int customerAge)
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{
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var customer = CreateCustomer(customerAge, DateTime.Today.AddDays(-1));
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customer.IsVeteran = true;
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var result = _calculator.CalculateDiscountPercentage(customer);
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result.Should().Be(.10m);
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}
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[Theory]
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[InlineData(1, .05)]
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[InlineData(2, .08)]
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[InlineData(5, .10)]
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[InlineData(10, .12)]
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[InlineData(15, .15)]
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public void ReturnsCorrectLoyaltyDiscountForLongtimeCustomers(int yearsAsCustomer, decimal expectedDiscount)
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{
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var customer = CreateCustomer(DEFAULT_AGE,
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DateTime.Today.AddYears(-yearsAsCustomer).AddDays(-1));
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var result = _calculator.CalculateDiscountPercentage(customer);
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result.Should().Be(expectedDiscount);
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}
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[Theory]
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[InlineData(1, .15)]
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[InlineData(2, .18)]
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[InlineData(5, .20)]
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[InlineData(10, .22)]
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[InlineData(15, .25)]
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public void ReturnsCorrectLoyaltyDiscountForLongtimeCustomersOnTheirBirthday(int yearsAsCustomer, decimal expectedDiscount)
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{
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var customer = CreateBirthdayCustomer(DEFAULT_AGE, DateTime.Today.AddYears(-yearsAsCustomer).AddDays(-1));
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var result = _calculator.CalculateDiscountPercentage(customer);
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result.Should().Be(expectedDiscount);
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}
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[Theory]
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[InlineData(1)]
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[InlineData(2)]
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public void ReturnsVeteransDiscountForLoyal1And2YearCustomers(int yearsAsCustomer)
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{
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var customer = CreateCustomer(DEFAULT_AGE,
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DateTime.Today.AddYears(-yearsAsCustomer).AddDays(-1));
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customer.IsVeteran = true;
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var result = _calculator.CalculateDiscountPercentage(customer);
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result.Should().Be(.10m);
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}
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[Theory]
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[InlineData(1)]
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[InlineData(2)]
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public void ReturnsVeteransDiscountForLoyal1And2YearCustomersOnBirthday(int yearsAsCustomer)
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{
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var customer = CreateBirthdayCustomer(DEFAULT_AGE,
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DateTime.Today.AddYears(-yearsAsCustomer).AddDays(-1));
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customer.IsVeteran = true;
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var result = _calculator.CalculateDiscountPercentage(customer);
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result.Should().Be(.20m);
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}
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[Fact]
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public void Returns10PctForCustomerSecondPurchaseOnBirthday()
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{
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var customer = CreateBirthdayCustomer(20, DateTime.Today.AddDays(-1));
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var result = _calculator.CalculateDiscountPercentage(customer);
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result.Should().Be(.10m);
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}
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[Fact]
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public void Returns30PctForCustomerFirstPurchaseWithReferrer()
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{
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var customer = CreateCustomer();
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customer.Referrer = new ReferredBy(CreateCustomer());
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var result = _calculator.CalculateDiscountPercentage(customer);
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result.Should().Be(.30m);
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}
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private Customer CreateCustomer(int age = DEFAULT_AGE, DateTime? firstPurchaseDate = null)
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{
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return new Customer
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{
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DateOfBirth = DateTime.Today.AddYears(-age).AddDays(-1),
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DateOfFirstPurchase = firstPurchaseDate
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};
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}
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private Customer CreateBirthdayCustomer(int age = DEFAULT_AGE, DateTime? firstPurchaseDate = null)
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{
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return new Customer
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{
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DateOfBirth = DateTime.Today.AddYears(-age),
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DateOfFirstPurchase = firstPurchaseDate
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};
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}
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}
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}

‎DesignPatternsInCSharp/RulesEngine/DiscountsShortCircuit/README.md

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# Discount Rules with Short-Circuiting
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In response to a reader question found here:
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https://github.com/ardalis/DesignPatternsInCSharp/issues/15
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This folder shows how to add a new ReferralDiscount that yields a 30% discount and immediately stops checking all other rules. There are two parts to this:
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- This rule needs to run first
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- If it matches, no other rules should run
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Our design will allow for multiple short-circuiting rules which can be run in a priority order.

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