设计模式

// 工厂生成对象类
public class OperationFactory
{
    public static OperationClass GetOperation(string operationType)
    {
        OperationClass opera = null;
        switch (operationType)
        {
            case "+":
                opera = new AddOperation();
                break;
            case "-":
                opera = new SubOperation();
                break;
            case "*":
                opera = new MulOperation();
                break;
            case "/":
                opera = new DivOperation();
                break;
            case "sqrt":
                opera = new SqrtOperation();
                break;
            case "squa":
                opera = new SquareOperation();
                break;
            default:
                 break;
        }
        return opera;
    }
}

// 运算符基类
public abstract class OperationClass
{
    public double NumA { get; set; }
    public double NumB { get; set; }

    public abstract double GetResult();        
}

// 加号类
public class AddOperation : OperationClass
{
    public override double GetResult()
    {
        return NumA + NumB;
    }
}

// 减号类
public class SubOperation : OperationClass
{
    public override double GetResult()
    {
        return NumA - NumB;
    }
}

// 除号类
public class DivOperation : OperationClass
{
    public override double GetResult()
    {
        try
        {
            if (NumB.Equals(0))
            {
                throw new DivideByZeroException("分母为零");
            }
            return NumA / NumB;
        }
        catch (DivideByZeroException ex)
        {
            Console.WriteLine(ex);
        }

        return 0;           
    }
}

// 乘号类
public class MulOperation : OperationClass
{
    public override double GetResult()
    {            
        return NumA * NumB;            
    }
}

// 开根号类
public class SqrtOperation : OperationClass
{
    public override double GetResult()
    {
        return Math.Sqrt(NumA);
    }
}

// 业务运用类
public class OperationOutput
{
    public void Show()
    {            
        OperationClass ope;
        do
        {
            Console.WriteLine("请输入正确的运算符： * / + - sqrt squa");
            ope = OperationFactory.GetOperation(Console.ReadLine());
        }
        while (ope == null);
        
        Console.WriteLine("请输入第一个数字");
        ope.NumA = double.Parse(Console.ReadLine());

        if (!(ope is SqrtOperation))
        {
                Console.WriteLine("请输入第二个数字");
                ope.NumB = double.Parse(Console.ReadLine());
        }            

        Console.WriteLine("结果是：" + ope.GetResult());
        Console.WriteLine();
    }
}

1）缺点：每新增一种操作，就需要修改 OperationFactory 类的 GetOperation(string operationType) 方法，这违法了对扩展开放，对修改封闭的原则。
2）可通过工厂模式、抽象工厂的方式得到适当改善，可通过反射彻底改善。