简单工厂模式

该模式不属于GOF的23个设计模式，违背了开放封闭原则（对扩展开放，对修改关闭）

简单的计算器
类图
制图工具：draw.io
SimpleFactoryPattern.drawio.xml

C#代码

    public abstract class Arithmetic
    {
        public double NumberA;
        public double NumberB;
        public abstract double GetResult();
    }
    public class SimpleFactory
    {
        public static Arithmetic CreateOperate(string operate)
        {
            Arithmetic arithmetic=null;
            switch (operate)
            {
                case "+":
                    arithmetic = new Add();
                    break;
                case "-":
                    arithmetic = new Subtract();
                    break;
                case "*":
                    arithmetic = new Multiply();
                    break;
                case "/":
                    arithmetic = new Divide();
                    break;
                default:
                    break;
            }
            return arithmetic;
        }
    }
    public class Add : Arithmetic
    {
        public override double GetResult()
        {
            return NumberA + NumberB;
        }
    }
    public class Subtract : Arithmetic
    {
        public override double GetResult()
        {
            return NumberA - NumberB;
        }
    }
    public class Multiply : Arithmetic
    {
        public override double GetResult()
        {
            return NumberA * NumberB;
        }
    }
    public class Divide : Arithmetic
    {
        public override double GetResult()
        {
            if (NumberB==0)
            {
                throw new Exception("除数不能为0");
            }
            return NumberA / NumberB;
        }
    }

使用

            Arithmetic arithmetic = SimpleFactory.CreateOperate("-");
            arithmetic.NumberA = 10;
            arithmetic.NumberB = 20;
            double result=arithmetic.GetResult();
            Console.WriteLine(result);