定义： 策略模式定义了一系列算法，并将每个算法封装起来，使他们可以相互替换，且算法的变化不会影响到使用算法的客户。
优点： 1、算法可以自由切换。 2、避免使用多重条件判断。 3、扩展性良好。
缺点： 1、策略类会增多。 2、所有策略类都需要对外暴露。

namespace ConsoleApp1
{//定义抽象策略public interface IStrategy{float calc(int num1, int num2);}//定义具体策略//加法public class AddStrategy : IStrategy{public float calc(int num1, int num2){return num1 + num2;}}//减法public class SubstractStrategy : IStrategy{public float calc(int num1, int num2){return num1 - num2;}}//乘法public class MultiplicationStrategy : IStrategy{public float calc(int num1, int num2){return num1 * num2;}}//除法public class DivisionStrategy : IStrategy{public float calc(int num1, int num2){return num1 / num2;}}//处理环境public class Environment{private IStrategy strategy;public Environment(IStrategy strategy){this.strategy = strategy;}public float calculate(int a, int b){return strategy.calc(a, b);}}class Program{static void Main(string[] args){Environment environment = new Environment(new AddStrategy());float result = environment.calculate(20, 5);Console.WriteLine("计算结果：" + result);Environment environment1 = new Environment(new SubstractStrategy());float result1 = environment1.calculate(20, 5);Console.WriteLine("计算结果：" + result1);Environment environment2 = new Environment(new MultiplicationStrategy());float result2 = environment2.calculate(20, 5);Console.WriteLine("计算结果：" + result2);Environment environment3 = new Environment(new DivisionStrategy());float result3 = environment3.calculate(20, 5);Console.WriteLine("计算结果：" + result3);}}
}