策略模式：灵活的算法封装与切换

策略模式是一种行为型设计模式，它将一组算法封装成独立的类，使它们可以相互替换。策略模式让算法的变化独立于使用算法的客户端。本文将以一个收银系统为例，详细介绍策略模式的实现和应用。

什么是策略模式？

策略模式定义了算法族，分别封装起来，让它们之间可以互相替换，此模式让算法的变化独立于使用算法的客户端。

核心组件：

策略接口/抽象类：定义了算法的公共接口
具体策略类：实现策略接口的具体算法
上下文类：维护一个对策略对象的引用，负责将客户端请求委派给策略对象

收银系统中的策略模式实现

1. 策略抽象类

首先，我们定义一个策略抽象类CashStrategy，它规定了所有收费策略必须实现的方法：

public abstract class CashStrategy
{public abstract decimal AcceptCash(decimal originalAmount);
}

2. 具体策略实现

接下来，实现几种具体的收费策略：

// 正常收费策略
public class NormalCash : CashStrategy
{public override decimal AcceptCash(decimal originalAmount) => originalAmount;
}// 打折策略
public class DiscountCash : CashStrategy
{private readonly decimal _discountRate;public DiscountCash(decimal rate) => _discountRate = rate;public override decimal AcceptCash(decimal originalAmount)=> originalAmount * _discountRate;
}// 满减策略
public class ReturnCash : CashStrategy
{private readonly decimal _condition;private readonly decimal _returnAmount;public ReturnCash(decimal condition, decimal returnAmount)=> (_condition, _returnAmount) = (condition, returnAmount);public override decimal AcceptCash(decimal originalAmount)=> originalAmount - Math.Floor(originalAmount / _condition) * _returnAmount;
}// 增收策略
public class RevenueGrowth : CashStrategy
{private readonly decimal _surchargeAmount;public RevenueGrowth(decimal amount) => _surchargeAmount = amount;public override decimal AcceptCash(decimal originalAmount)=> originalAmount + _surchargeAmount;
}

3. 上下文类

然后，创建一个上下文类来管理策略：

public class CashContext
{private CashStrategy _strategy;public void SetStrategy(CashStrategy strategy) => _strategy = strategy;public decimal GetResult(decimal money) => _strategy.AcceptCash(money);
}

4. 组合策略实现

策略模式的一个强大扩展是组合策略模式，它可以将多个策略组合使用：

public class CompositeCash : CashStrategy
{private readonly List<CashStrategy> _strategies;private readonly ExecutionOrder _order;public enum ExecutionOrder { Sequential, Priority }public CompositeCash(List<CashStrategy> strategies, ExecutionOrder order = ExecutionOrder.Sequential){_strategies = strategies;_order = order;}public override decimal AcceptCash(decimal originalAmount){var result = originalAmount;foreach (var strategy in _strategies.OrderBy(s => _order == ExecutionOrder.Priority ? 1 : 0)){result = strategy.AcceptCash(result);}return result;}
}

使用配置文件实现策略的动态加载

在实际应用中，我们希望能够通过配置文件动态加载不同的策略，而不是硬编码。这里我们使用JSON配置文件来实现。

1. JSON配置文件

{"Strategies": [{"Name": "正常收费","Type": "CashSystem.NormalCash, CashSystem"},{"Name": "八折优惠","Type": "CashSystem.DiscountCash, CashSystem","Params": [{"Name": "rate","Value": 0.8}]},{"Name": "增收","Type": "CashSystem.RevenueGrowth, CashSystem","Params": [{"Name": "amount","Value": 100}]},{"Name": "组合策略-折上折","Type": "CashSystem.CompositeCash, CashSystem","ExecutionOrder": "Sequential","Strategies": [{"Type": "CashSystem.DiscountCash, CashSystem","Params": [{"Name": "rate","Value": 0.9}]},{"Type": "CashSystem.DiscountCash, CashSystem","Params": [{"Name": "rate","Value": 0.95}]}]}]
}

2. 配置模型类

为了支持JSON配置，我们需要创建相应的数据模型：

public class StrategiesRoot
{public List<StrategyConfig> Strategies { get; set; }
}public class StrategyConfig
{public string Name { get; set; }public string Type { get; set; }public List<ParamConfig> Params { get; set; }public string ExecutionOrder { get; set; }public List<StrategyConfig> Strategies { get; set; }
}public class ParamConfig
{public string Name { get; set; }[JsonNumberHandling(JsonNumberHandling.AllowReadingFromString)]public decimal Value { get; set; }
}

3. 策略加载器和工厂

策略加载器负责从配置文件读取策略配置：

public class StrategyLoader
{public Dictionary<string, CashStrategy> LoadStrategies(){string jsonString = File.ReadAllText("Strategies.json");var options = new JsonSerializerOptions{PropertyNameCaseInsensitive = true};var strategiesRoot = JsonSerializer.Deserialize<StrategiesRoot>(jsonString, options);return strategiesRoot.Strategies.ToDictionary(s => s.Name,s => StrategyFactory.CreateStrategy(s));}
}

策略工厂负责根据配置创建具体的策略实例：

public class StrategyFactory
{public static CashStrategy CreateStrategy(StrategyConfig config){if (config.Type.StartsWith("CashSystem.CompositeCash")){// 添加程序集加载逻辑 var typeName = config.Type;var type = Type.GetType(typeName)?? throw new TypeLoadException($"无法加载类型: {typeName}");var order = Enum.Parse<CompositeCash.ExecutionOrder>(config.ExecutionOrder ?? "Sequential");// 递归创建子策略 var strategies = config.Strategies?.Select(CreateStrategy).ToList() ?? new List<CashStrategy>();return new CompositeCash(strategies, order);}else{var type = Type.GetType(config.Type);var parameters = config.Params?.ToDictionary(p => p.Name, p => p.Value)?? new Dictionary<string, decimal>();return type.Name switch{"NormalCash" => (CashStrategy)Activator.CreateInstance(type),"DiscountCash" => (CashStrategy)Activator.CreateInstance(type, parameters.Values.FirstOrDefault()),"ReturnCash" => (CashStrategy)Activator.CreateInstance(type, parameters["condition"], parameters["return"]),"RevenueGrowth" => (CashStrategy)Activator.CreateInstance(type, parameters.Values.FirstOrDefault()),_ => throw new ArgumentException("不支持的参数数量")};}}
}

4. 使用策略

最后，在客户端代码中使用这些策略：

static void Main(string[] args)
{// 组合策略调用 var context = new CashContext();var strategies = new StrategyLoader().LoadStrategies();context.SetStrategy(strategies["增收"]);// 500元消费场景计算 var amount = 500m;var result = context.GetResult(amount);Console.WriteLine($"应收金额：{result}元");
}