static void Main(string[] args)
{
    //如果想執行新增加的操作
    ShapeVisitor visitor = new CustomVisitor();
    AppStructure app = new AppStructure(visitor);

    Shape shape = new Rectangle();
    shape.Draw();//執行自己的操作
    app.Process(shape);//執行新的操作


    shape = new Circle();
    shape.Draw();//執行自己的操作
    app.Process(shape);//執行新的操作


    shape = new Line();
    shape.Draw();//執行自己的操作
    app.Process(shape);//執行新的操作

}

//抽象圖形定義---相當于“抽象節點角色”Element
public abstract class Shape
{
    //畫圖形
    public abstract void Draw();
    //外界注入具體訪問者
    public abstract void Accept(ShapeVisitor visitor);
}

//抽象訪問者 Visitor
public abstract class ShapeVisitor
{
    public abstract void Visit(Rectangle shape);

    public abstract void Visit(Circle shape);

    public abstract void Visit(Line shape);

    //這里有一點要說：Visit方法的參數可以寫成Shape嗎？就是這樣 Visit(Shape shape)，當然可以，但是ShapeVisitor子類Visit方法就需要判斷當前的Shape是什么類型，是Rectangle類型，是Circle類型，或者是Line類型。
}

//具體訪問者 ConcreteVisitor
public sealed class CustomVisitor : ShapeVisitor
{
    //針對Rectangle對象
    public override void Visit(Rectangle shape)
    {
        Console.WriteLine("針對Rectangle新的操作！");
    }
    //針對Circle對象
    public override void Visit(Circle shape)
    {
        Console.WriteLine("針對Circle新的操作！");
    }
    //針對Line對象
    public override void Visit(Line shape)
    {
        Console.WriteLine("針對Line新的操作！");
    }
}

//矩形----相當于“具體節點角色” ConcreteElement
public sealed class Rectangle : Shape
{
    public override void Draw()
    {
        Console.WriteLine("矩形我已經畫好！");
    }

    public override void Accept(ShapeVisitor visitor)
    {
        visitor.Visit(this);
    }
}

//圓形---相當于“具體節點角色”ConcreteElement
public sealed class Circle : Shape
{
    public override void Draw()
    {
        Console.WriteLine("圓形我已經畫好！");
    }

    public override void Accept(ShapeVisitor visitor)
    {
        visitor.Visit(this);
    }
}

//直線---相當于“具體節點角色” ConcreteElement
public sealed class Line : Shape
{
    public override void Draw()
    {
        Console.WriteLine("直線我已經畫好！");
    }

    public override void Accept(ShapeVisitor visitor)
    {
        visitor.Visit(this);
    }
}

//結構對象角色
internal class AppStructure
{
    private ShapeVisitor _visitor;

    public AppStructure(ShapeVisitor visitor)
    {
        this._visitor = visitor;
    }

    public void Process(Shape shape)
    {
        shape.Accept(_visitor);
    }
}

static void Main(string[] args)
{
    StoragePlatform platform = new StoragePlatform();
    platform.Attach(new Television());
    platform.Attach(new Computer());

    SizeVisitor sizeVisitor = new SizeVisitor();
    StateVisitor stateVisitor = new StateVisitor();

    platform.Operate(sizeVisitor);
    platform.Operate(stateVisitor);
}
//抽象訪問者角色 Visitor
public abstract class Visitor
{
    public abstract void PutTelevision(Television tv);

    public abstract void PutComputer(Computer comp);
}

//具體訪問者角色 ConcreteVisitor
public sealed class SizeVisitor : Visitor
{
    public override void PutTelevision(Television tv)
    {
        Console.WriteLine("按商品大小{0}排放", tv.Size);
    }

    public override void PutComputer(Computer comp)
    {
        Console.WriteLine("按商品大小{0}排放", comp.Size);
    }
}

//具體訪問者角色 ConcreteVisitor
public sealed class StateVisitor : Visitor
{
    public override void PutTelevision(Television tv)
    {
        Console.WriteLine("按商品新舊值{0}排放", tv.State);
    }

    public override void PutComputer(Computer comp)
    {
        Console.WriteLine("按商品新舊值{0}排放", comp.State);
    }
}

//抽象節點角色 Element
public abstract class Goods
{
    public abstract void Operate(Visitor visitor);

    private int nSize;
    public int Size
    {
        get { return nSize; }
        set { nSize = value; }
    }

    private int nState;
    public int State
    {
        get { return nState; }
        set { nState = value; }
    }
}

//具體節點角色 ConcreteElement
public sealed class Television : Goods
{
    public override void Operate(Visitor visitor)
    {
        visitor.PutTelevision(this);
    }
}

//具體節點角色 ConcreteElement
public sealed class Computer : Goods
{
    public override void Operate(Visitor visitor)
    {
        visitor.PutComputer(this);
    }
}

//結構對象角色
public sealed class StoragePlatform
{
    private IList<Goods> list = new List<Goods>();

    public void Attach(Goods element)
    {
        list.Add(element);
    }

    public void Detach(Goods element)
    {
        list.Remove(element);
    }

    public void Operate(Visitor visitor)
    {
        foreach (Goods g in list)
        {
            g.Operate(visitor);
        }
    }
}