[CLR via C#]18. Attribute

[assembly: MyAttr(1)]         // 应用于程序集
[module: MyAttr(2)]           // 应用于模块
 
[type: MyAttr(3)]             // 应用于类型
internal sealed class SomeType<[typevar: MyAttr(4)] T> { // 应用于泛型类型变量
 
   [field: MyAttr(5)]         // 应用于字段
   public Int32 SomeField = 0;
 
   [return: MyAttr(6)]        // 应用于返回值
   [method: MyAttr(7)]        // 应用于方法
   public Int32 SomeMethod(
      [param: MyAttr(8)]      // 应用于方法参数
      Int32 SomeParam) { return SomeParam; }
 
   [property: MyAttr(9)]      // 应用于属性
   public String SomeProp {
      [method: MyAttr(10)]    // 应用于get访问器方法
      get { return null; }
   }
 
   [event: MyAttr(11)]        // 应用于事件
   [field: MyAttr(12)]        // 应用于编译器生成的字段
   [method: MyAttr(13)]       // 应用于编译器生成的add&&remove方法
   public event EventHandler SomeEvent;
}
 
 
[AttributeUsage(AttributeTargets.All)]
public class MyAttr : Attribute {
   public MyAttr(Int32 x) { }
}
 

[DllImport("kernel32",CharSet = CharSet.Auto, SetLastError = true)]

public enum Color { Red }
 
[AttributeUsage(AttributeTargets.All)]
internal sealed class SomeAttribute : Attribute
{
    public SomeAttribute(String name, Object o, Type[] types)
    {
        // 'name'  引用了一个String类型
        // 'o'     引用了一个合法类型(如有必要，就进行装箱)
        // 'types' 引用一个一维0基Type数组
    }
}
 
[Some("Jeff", Color.Red, new Type[] { typeof(Math), typeof(Console) })]
internal sealed class SomeType
{
}

[Flags]
    internal enum Accounts
    {
        Savings = 0x0001,
        Checking = 0x0002,
        Brokerage = 0x0004
    }
 
 
    [AttributeUsage(AttributeTargets.Class)]
    internal sealed class AccountsAttribute : Attribute
    {
        private Accounts m_accounts;
 
        public AccountsAttribute(Accounts accounts)
        {
            m_accounts = accounts;
        }
 
        public override Boolean Match(Object obj)
        {
            // 如果基类实现了Match，而且基类
            // 不是Attribute,就取消对下面这行的注释
            //if (!base.Match(obj))
            //{
            //    return false;
            //}
 
            if (obj == null)
            {
                return false;
            }
 
 
            if (this.GetType() != obj.GetType())
            {
                return false;
            }
 
            AccountsAttribute other = (AccountsAttribute)obj;
 
            // 比较字段，判断它们是否有相同的值
            if ((other.m_accounts & m_accounts) != m_accounts)
            {
                return false;
            }
 
            return true;   // 对象匹配
        }
 
        public override Boolean Equals(Object obj)
        {
            //如果基类实现了Equals，而且基类不能Object
            //就取消对下面的注释
            //if (!base.Equals(obj))
            //{
            //    return false;
            //}
 
            if (obj == null)
            {
                return false;
            }
 
            if (this.GetType() != obj.GetType())
            {
                return false;
            }
 
            AccountsAttribute other = (AccountsAttribute)obj;
 
            // 比较字段，判断它们是否有相同的值
            if (other.m_accounts != m_accounts)
            {
                return false;
            }
 
            return true;   // Objects are equal
        }
 
        // 还需要重写GetHashCode，因为我们重写了Equals
        public override Int32 GetHashCode()
        {
            return (Int32)m_accounts;
        }
    }
 
 
    [Accounts(Accounts.Savings)]
    internal sealed class ChildAccount { }
 
    [Accounts(Accounts.Savings | Accounts.Checking | Accounts.Brokerage)]
    internal sealed class AdultAccount { }
 
    public class Program
    {
 
        public static void Main()
        {
 
            CanWriteCheck(new ChildAccount());
            CanWriteCheck(new AdultAccount());
 
            // 只是为了演示在一个没有应用AccountsAttribute的方法也能正确工作
            CanWriteCheck(new Program());
 
            Console.Read();
        }
 
 
        private static void CanWriteCheck(Object obj)
        {
            // 构造attribute类型的一个实例，并把它初始化我们要
            // 显示查找的内容
            Attribute checking = new AccountsAttribute(Accounts.Checking);
 
            // 构造应用于类型的attribute实例
            Attribute validAccounts = Attribute.GetCustomAttribute(
                obj.GetType(), typeof(AccountsAttribute), false);
 
            // 如果attribute应用于类型，而且attribute指定了
            // "Checking" 账户, 表面该类可以开支票
            if ((validAccounts != null) && checking.Match(validAccounts))
            {
                Console.WriteLine("{0} types can write checks.", obj.GetType());
            }
            else
            {
                Console.WriteLine("{0} types can NOT write checks.", obj.GetType());
            }
        }
    }