Definition:  An attribute is a piece of additional declarative information that is specified for a declaration (MSDN).

So, we can look Attribute like a Decorator from the definition.

Example:

 [Obsolete("This method is substituted by xxx", true)]

public void Demo() { ... }

param1: 为什么该元素被荒弃，以及我们该使用什么元素来代替它.

param2: 告诉编译器把依然使用这被标识的元素视为一种错误，这就意味着编译器会因此而产生一个警告. 

Develop Customizd Attribute:

A class that derives from the abstract class System.Attribute, whether directly or indirectly, is an attribute class. The declaration of an attribute class defines a new kind of attribute that can be placed on a declaration (MSDN).

必选参数和命名参数

必选参数：在Contructor中进行初始化

命名参数：未在Contructor中初始化，但定义了Set方法的Property.

参数类型：simple data type, System.Type, object

public class MyAttribute : Attribute
{
protected string description;
protected string version;
public MyAttribute(string description)
{
this.description = description;
}
public string Description
{
get { return this.description;}
set { this.description = value;}
}
public string Version
{
get { return this.version;}
set { this.version = value;}
}
}

使用：

[MyAttribute("This is a class", Version = "1.0", Description = "This is a class 2")]
public class TestClass
{
...
}

此时

MyAttribute.Description  : This is a class 2  //先在构造时被设为This is a class, 后又被设为 ... 2

MyAttribute.Version : 2.0

约束自定义Attribute的使用

AttributeUsage 类:  描述了一个自定义attribute类能被怎样使用. attribute类本身用这个atrribute System.AttributeUsage来标记.

AttributeUsage的三个属性：

1. ValidOn: 指定约束的attribute可以应用在哪些语言元素上.　可选的值有：AttributeTargets. {Assembly, Moudle, Class, Struct, Enum, Constructor, Method, Property, Field, Event, Interface, Parameter, Delegate, All, ClassMembers = (Class | Struct | Enum | Constructor | Method | Property | Field | Event | Delegate | Interface)}.  default: AttributeTargets.All

2. AllowMultiple: 指示约束的attribute是否可在同一语言元素上使用多次. default: false

3. Inherited: 指示约束的attribute是否被派生类继承. default: false

示例：

1. Attribute Definition

using system;
[AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = false )]
public class MyAttribute : Attribute
{
public MyAttribute(string description){...}
...
}

2. Using

[MyAttribute("This is MyClass")]
[MyAttribute("This is MyClass")] //Error: because AllowMultiple = false
public class MyClass
{
[MyAttribute("This is a Method")] //Error: AttributeTarges.Class specified that we just can use this attribute on a class
public void Show() {...}
}

Attribute标记

功能: 约束将Attribute应用到哪一个语言元素

可用标识符: assembly, moudle, type, method, property, event, field, param, return

功能示例:

1. Attribute 放在哪儿才能让编译器确定该attribute是绑定至整个assembly?

2. 怎样才能让编译器确定我们是把attribute绑定至方法的返回类型上，而不是整个方法?

[assembly: MyAttribute(...)]

运行时查询Attributes (By Reflection)

1. 查询程序集的custom attribute

class TestApp
{
public static void Main()
{
Process p = Process.GetCurrentProcess();
string assemblyName = p.ProcessName + ".exe";
Assembly a = Assembly.LoadFrom(assemblyName);
foreach (Attribute attr in a.GetCustomAttributes(true))
{
if(typeof(MyAttribute).IsAssignableFrom(typeof(attr)))
{
Console.Write("Description of {0}: {1}", assemblyName, attr.Description);
}
}
}
}

2. 查询class, method, property的attributes

  a) class:  type.GetCustomAttributes(true) 得到所有绑定的attributes -> ...

  b) method　type.GetMethods() -> method.GetCustomAttributes(true) -> ...

  c) property  type.GetFields() -> field.GetCustomAttributes(true) -> ...

典型Attributes应用（待整理）

额外阅读推荐：

组件常用的设计期与解析期元数据Attribute