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工厂方法模式是对简单工厂模式的改进，它为每个对象增加了一个工厂类，专门用于生成该对象。
工厂方法实现加减乘除例子如下：
1 操作类
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("除数不能为0");
}
return NumberA / NumberB;
}
}
2 为每一个操作类添加一个工厂对象
/// <summary>
/// 工厂方法模式把简单工厂的内部逻辑判断移导了客户端代码来进行，增加方法时，不需要增加条件分支，只需增加对象和工厂
/// </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 在客户端使用工厂生产需要使用的对象
IFactory addFactory = new AddFactory(); Operation operation = addFactory.CreateOperation(); operation.NumberA = 1; operation.NumberB = 2; Console.WriteLine(operation.GetResult());
运行结果为：3
工厂方法模式把判断移到了客户端，并没有解决判断实例化哪个对象的问题，但这种模式在某些场景中是有很大的好处的，例如使用SQLServer数据库，代码开发完成后，如果数据改为Oracle，只需修改实例化工厂，不需要修改其它地方，更不需要修改被操作对象的实例化过程，下面比较使用工厂方法模式和不使用时代码的区别：
private void UseFactory()
{
//如果所有加法变减法，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()
{
//如果所有加法变减法，需要修改所有实例化的地方，简单工厂也存在该问题
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());
}
总体而言，工厂发放克服了简单工厂违反开放-封闭原则的缺点，又保持了封装对象创建过程的优点，封装对象创建的过程使得更换对象时，程序不需要做大的改动，降低了客户端与具体对象的耦合度。
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原文地址：http://www.cnblogs.com/angela217/p/5395534.html
