工廠方法模式

工廠方法模式是對簡單工廠模式的改進，它為每個對象增加了一個工廠類，專門用於生成該對象。

工廠方法實現加減乘除例子如下：

1 操作類

 public class Operation
    {
        public double NumberA { get; set; }
        public double NumberB { get; set; }

        public virtual double GetResult()
        {
            double result = 0;
            return result;
        }
    }

    public class OperationAdd : Operation
    {
        public override double GetResult()
        {
            return NumberA + NumberB;
        }
    }

    public class OperationSub : Operation
    {
        public override double GetResult()
        {
            return NumberA - NumberB;
        }
    }

    public class OperationMul : Operation
    {
        public override double GetResult()
        {
            return NumberA * NumberB;
        }
    }

    public class OperationDiv : Operation
    {
        public override double GetResult()
        {
            if (NumberB == 0)
            {
                throw new Exception("除數不能為0");
            }
            return NumberA / NumberB;
        }
    }

　　2 為每一個操作類添加一個工廠對象

     /// <summary>
    /// 工廠方法模式把簡單工廠的內部邏輯判斷移導了客戶端代碼來進行，增加方法時，不需要增加條件分支，只需增加對象和工廠
    /// </summary>
    interface IFactory
    {
        Operation CreateOperation();
    }

    public class AddFactory:IFactory
    {
        public Operation CreateOperation()
        {
            return new OperationAdd();
        }
    }

    public class SubFactory : IFactory
    {
        public Operation CreateOperation()
        {
            return new OperationSub();
        }
    }

    public class MulFactory : IFactory
    {
        public Operation CreateOperation()
        {
            return new OperationMul();
        }
    }

    public class DivFactory : IFactory
    {
        public Operation CreateOperation()
        {
            return new OperationDiv();
        }
    }

　　3 在客戶端使用工廠生產需要使用的對象

IFactory addFactory = new AddFactory();

Operation operation = addFactory.CreateOperation();
operation.NumberA = 1;
operation.NumberB = 2;
Console.WriteLine(operation.GetResult());

　　運行結果為：3

      工廠方法模式把判斷移到了客戶端，並沒有解決判斷實例化哪個對象的問題，但這種模式在某些場景中是有很大的好處的，例如使用SQLServer資料庫，代碼開發完成後，如果數據改為Oracle，只需修改實例化工廠，不需要修改其它地方，更不需要修改被操作對象的實例化過程，下麵比較使用工廠方法模式和不使用時代碼的區別：

        private void UseFactory()
        {
            //如果所有加法變減法，只修改一句
            IFactory addFactory = new AddFactory();

            Operation operation1 = addFactory.CreateOperation();
            operation1.NumberA = 1;
            operation1.NumberB = 2;
            Console.WriteLine(operation1.GetResult());

            Operation operation2 = addFactory.CreateOperation();
            operation2.NumberA = 5;
            operation2.NumberB = 3;
            Console.WriteLine(operation1.GetResult());

            Operation operation3 = addFactory.CreateOperation();
            operation3.NumberA = 6;
            operation3.NumberB = 1;
            Console.WriteLine(operation1.GetResult());
        }


        private void unUseFactory()
        {
            //如果所有加法變減法，需要修改所有實例化的地方，簡單工廠也存在該問題
            Operation operation1 = new OperationAdd();
            operation1.NumberA = 1;
            operation1.NumberB = 2;
            Console.WriteLine(operation1.GetResult());

            Operation operation2 = new OperationAdd();
            operation2.NumberA = 5;
            operation2.NumberB = 3;
            Console.WriteLine(operation1.GetResult());

            Operation operation3 = new OperationAdd();
            operation3.NumberA = 6;
            operation3.NumberB = 1;
            Console.WriteLine(operation1.GetResult());
        }

總體而言，工廠發放剋服了簡單工廠違反開放-封閉原則的缺點，又保持了封裝對象創建過程的優點，封裝對象創建的過程使得更換對象時，程式不需要做大的改動，降低了客戶端與具體對象的耦合度。