C#反射机制实现开闭原则的简单工厂模式

using System;
public class MyClass
{
   [Obsolete("Don't use OldMethod, use NewMethod instead", true)]
   static void OldMethod()
   {
      Console.WriteLine("It is the old method");
   }
   static void NewMethod()
   {
      Console.WriteLine("It is the new method");
   }
   public static void Main()
   {
      OldMethod();
   }
}

using System;
[AttributeUsage(AttributeTargets.All)]
public class HelpAttribute : System.Attribute//这是一个自定义的特性
{
   public readonly string Url;
   public string Topic  // Topic 是一个命名（named）参数
   {
      get { return topic; } set { topic = value; }
   }
   public HelpAttribute(string url)  // url 是一个定位（positional）参数
   {
      this.Url = url;
   }
   private string topic;
}
[HelpAttribute("Information on the class MyClass")]//创建MyClass的特性
class MyClass
{
}
namespace AttributeAppl
{
   class Program
   {
      static void Main(string[] args)
      {
         System.Reflection.MemberInfo info = typeof(MyClass);//通过typeof反射机制获取MyClass的类型
         object[] attributes = info.GetCustomAttributes(true);//获取特性
         for (int i = 0; i < attributes.Length; i++)
         {
            System.Console.WriteLine(attributes[i]);//打印特性 “HelpAttribute”
         }
         Console.ReadKey();
      }
   }
}

public abstract class Product
{
  protected ControllerBase(string typeName)
  {
      TypeName = typeName;
  }
  public abstract void Show();
};
// 产品 A
public class ProductA : Product
{
  public class ProductA : base("产品A"){}
  public override void Show()
  {
      System.Console.WriteLine("Product A.");
  }
};
// 产品 B
public class ProductB Product
{
  public class ProductA : base("产品B"){}
  public override void Show()
  {
      System.Console.WriteLine("Porduct B.");
  }
};

// 工厂
public class Factory : InstanceBase<ControllerFactory>
{
  public Factory()
  {
    var types = Assembly.GetExecutingAssembly().GetExportedTypes().Where(p => typeof(Product).IsAssignableFrom(p) && !p.IsAbstract && p.IsClass).ToList();
    types.ForEach(p =>
    {
        var instance = (Product)Activator.CreateInstance(p);
        _productItems.Add(instance.TypeName, p);
    });
  }
  public Product CreateProduct(string typeName)
  {
    return (Product)Activator.CreateInstance(_productItems[typeName]);
  }
private Dictionary<string, Type> _productItems = new Dictionary<string, Type>();
};

class Program
 {
    static void Main(string[] args)
    {
       ControllerFactory.Instance().CreateProduct("产品A");
       ControllerFactory.Instance().CreateProduct("产品B");
    }
 }