为了尽可能演示出Visitor 设计模式的强大之处，在此举一个开发中的场景
例如 开发A组 负责做log功能，而B组需要A组暴露一个API，可以拿到所有的log。

A组的代码实现可能是如下这样的：

public abstract class OrderLog{
public string Content {get;set;}
public OrderLog(string content){
Content = content;
}


}

public class PlaceOrderLog :OrderLog{
public PlaceOrderLog(string content,string orderedBy):base(content)
{
OrderedBy = orderedBy;
}
public string OrderedBy {get;set;}


}


public class MakePaymentLog :OrderLog{
public MakePaymentLog(string content, string paymentGateway, string payedBy):base(content)
{
PaymentGateway = paymentGateway;
PayedBy = payedBy;
}
public string PaymentGateway {get;set;}
public string PayedBy {get;set;}


}


public class OrderCompleteLog : OrderLog{
public OrderCompleteLog(string content, DateTime completeDate):base(content)
{
OrderCompleteDate = completeDate;
}
public DateTime OrderCompleteDate {get;set;}


}


public class OrderLogger{
public static OrderLogger Do
{
get
{
return new OrderLogger();
}
}


public IEnumerable<OrderLog> GetLogs(){
return new List<OrderLog>{
new PlaceOrderLog("place order log","ordered by"),
new MakePaymentLog("make payment log","paypal", "payedBy"),
new OrderCompleteLog("order complete log",DateTime.Now)
};


}


}

B组用法：

var logs = OrderLogger.Do.GetLogs();

B组新需求来了，需要A组再做一个API，可以把颜色信息加入LOG实体中，比如类型A返回红色，类型B返回绿色
A组有两个选择，再加一个API，还是把这个责任分发回B组，因为显然显示颜色的逻辑B组更熟悉。当然是分发责任，可如何分发，访问者模式来了。

1.在Log基类中添加一行：

public abstract T Visit<T>(ILogVisitor<T> visitor);

2.其他log子类中相应的实现它：

public override T Visit<T>(ILogVisitor<T> visitor){
return visitor.Visit(this);
}

为什么加这个方法？相当于给外界开一个入口，这样它们可以进来，然后把自身的信息传递给它们完成相应逻辑。

3.给B组一个接口（如果没有返回值，可以把T拿掉）：

public interface ILogVisitor<T>{
T Visit(PlaceOrderLog log);
T Visit(MakePaymentLog log);
T Visit(OrderCompleteLog log);
}

为什么加这个接口？
1. 外部的访问者实现这个接口，就可以获取每个子类
2. 如果类层次变化，外部可以知道
3. 外部调用不用写大量的switch-case 或 if else臃肿的判断逻辑了

A组工作完成。接下来是B组要做的事情了——实现访问者接口。
实现也许是这样的：

public enum OrderLogColor{Red,Green}


public class OrderLogColorVisitor:ILogVisitor<OrderLogColor>{
public OrderLogColor Visit(PlaceOrderLog log){
return OrderLogColor.Red;
}


public OrderLogColor Visit(MakePaymentLog log){
return OrderLogColor.Green;
}


public OrderLogColor Visit(OrderCompleteLog log){
return OrderLogColor.Green;
}


}

逻辑很简单，对于不同的类型返回不同颜色。接下来就是调用部分：

var logs = OrderLogger.Do.GetLogs();
	show colors
	var colorVisitor = new OrderLogColorVisitor();
	foreach(var log in logs){
	Console.WriteLine(log.Visit<OrderLogColor>(colorVisitor));
	}

可以看到，拿到一个日志集合后，循环过程只需调用Visit函数，就进入了相应的log类，然后在log类中，调用Visit函数完成Visit逻辑。执行结果：

Red
Green
Green

接下来，B组需要一个新的任务，对不同的日志类型，格式化显示不同的字符串。

于是只需要添加另一个Visitor：

public class OrderLogFormattedVisitor : ILogVisitor <string> {


public string Visit(PlaceOrderLog log){
return "this is place order log formatted information ";
}


public string Visit(MakePaymentLog log){
return "this is make payment log formatted information ";
}


public string Visit(OrderCompleteLog log){
return "this is order complete log formatted information ";
}


}

调用：

var logs = OrderLogger.Do.GetLogs();
var formatVisitor = new OrderLogFormattedVisitor();
	foreach(var log in logs){
	Console.WriteLine(log.Visit<string>(formatVisitor));
	}

看出访问者模式的作用了吗？它把职责隔离的同时，有效的封装了变化。在以上例子中，A组只负责做LOG API，而B组负责拿到LOG后的事情。 在开发中会经常遇到相互调用的情况，每当这个时候，首先考虑职责分配，用接口隔离工作，决定调用的入口。这便是访问者模式的强大之处。

以下是完整的代码：

void Main()
{
	var logs = OrderLogger.Do.GetLogs();
	show colors
	var colorVisitor = new OrderLogColorVisitor();
	foreach(var log in logs){
	Console.WriteLine(log.Visit<OrderLogColor>(colorVisitor));
	}
	show formatted logs
	var formatVisitor = new OrderLogFormattedVisitor();
	foreach(var log in logs){
	Console.WriteLine(log.Visit<string>(formatVisitor));
	}
}


team A job
public abstract class OrderLog{
public string Content {get;set;}
public OrderLog(string content){
Content = content;
}


public abstract T Visit<T>(ILogVisitor<T> visitor);


}


public class PlaceOrderLog :OrderLog{
public PlaceOrderLog(string content,string orderedBy):base(content)
{
OrderedBy = orderedBy;
}
public string OrderedBy {get;set;}


public override T Visit<T>(ILogVisitor<T> visitor){
return visitor.Visit(this);
}


}


public class MakePaymentLog :OrderLog{
public MakePaymentLog(string content, string paymentGateway, string payedBy):base(content)
{
PaymentGateway = paymentGateway;
PayedBy = payedBy;
}
public string PaymentGateway {get;set;}
public string PayedBy {get;set;}


public override T Visit<T>(ILogVisitor<T> visitor){
return visitor.Visit(this);
}
}


public class OrderCompleteLog : OrderLog{
public OrderCompleteLog(string content, DateTime completeDate):base(content)
{
OrderCompleteDate = completeDate;
}
public DateTime OrderCompleteDate {get;set;}


public override T Visit<T>(ILogVisitor<T> visitor){
return visitor.Visit(this);
}


}


public class OrderLogger{
public static OrderLogger Do
{
get
{
return new OrderLogger();
}
}


public IEnumerable<OrderLog> GetLogs(){
return new List<OrderLog>{
new PlaceOrderLog("place order log","ordered by"),
new MakePaymentLog("make payment log","paypal", "payedBy"),
new OrderCompleteLog("order complete log",DateTime.Now)
};


}




}


now team B want a API , return different log color based on different type
team A give team B a visitor interface , ask them to implement




public interface ILogVisitor<T>{
T Visit(PlaceOrderLog log);
T Visit(MakePaymentLog log);
T Visit(OrderCompleteLog log);
}


team B come out with the implementations


public enum OrderLogColor{Red,Green}


public class OrderLogColorVisitor:ILogVisitor<OrderLogColor>{
public OrderLogColor Visit(PlaceOrderLog log){
return OrderLogColor.Red;
}


public OrderLogColor Visit(MakePaymentLog log){
return OrderLogColor.Green;
}


public OrderLogColor Visit(OrderCompleteLog log){
return OrderLogColor.Green;
}


}


in future 
if team B want more , just implement the ILogVisitor interface 
for example : formatted log


public class OrderLogFormattedVisitor : ILogVisitor <string> {


public string Visit(PlaceOrderLog log){
return "this is place order log formatted information ";
}


public string Visit(MakePaymentLog log){
return "this is make payment log formatted information ";
}


public string Visit(OrderCompleteLog log){
return "this is order complete log formatted information ";
}


}


whats next 
whenever team B want , he just add more visitor to accept different log types returned from teamA