自定义属性Attribute(三)

检测自定义属性的应用而不创建自定义属性的实例

　　 在调用GetCustomAttirbute及GetCustomAttirbutes方法时，都会调用属性类的构造器方法，同时也有可能调用property的get或set访问器。并且，首次访问一个类型会让CLR调用类型的类型构造器。而在构造器、访问器、类型构造器中，可能包含每次查找一个属性都会执行的代码，我们使用属性时并不知道这些代码，这样会允许未知的代码在AppDomain中运行，造成潜在的安全隐患。

         System.Reflection.CustomAttributeData类允许我们在查找自定义属性的同时禁止执行属性类的代码。其实现原理是 Assembly类的静态方法ReflectionOnlyLoad以一种特殊的方式加载程序集，期间会禁止CLR执行程序集中的任何代码，包括类型构造器。而CustomAttirbuteData类则对其加载的程序集进行分析。

         CustomAttributeData的GetCustomAttribute方法相当于一个Factory方法，它会返回IList<CustomAttirbuteData>集合。对于集合中的CustomAttributeData对象，可以访问其三个只读字段获取参数：

1. Constructor 返回一个ConstructorInfo对象，表示构造函数“要”如何调用。
2. ConstructorArguments  返回IList<CustomAttributeTypedArgument>集合，表示“要”传给构造函数的参数列表。
3. NamedArguments 返回IList<CustomAttributeNamedArgument>集合，返回“要”设置的字段。

这里的构造函数、参数列表以及字段的设置不会实际的调用构造器和set访问器，这样才会增加安全性。

代码示例：

[Serializable]
    [DefaultMember("ShowAttribute")]
    [DebuggerDisplay("Shi",Name="Jerry",Target=typeof(CustomAttributeDataAdapter))]
    public class CustomAttributeDataAdapter
    {
        public CustomAttributeDataAdapter()
        { }

        [Conditional("Debug")]
        [Conditional("Release")]
        public void DoSomething()
        { }

        [CLSCompliant(true)]
        [STAThread]
        public static void ShowAttribute()
        {
            ShowAttribute(typeof(CustomAttributeDataAdapter));

            MemberInfo[] members = typeof(CustomAttributeDataAdapter).FindMembers(
                MemberTypes.Constructor | MemberTypes.Method,
                BindingFlags.DeclaredOnly | BindingFlags.Instance | BindingFlags.Public | BindingFlags.Static,
                Type.FilterName, "*");
            foreach (MemberInfo member in members)
            {
                ShowAttribute(member);
            }
        }

        private static void ShowAttribute(MemberInfo attributeTarget)
        {
            IList<CustomAttributeData> attributes = CustomAttributeData.GetCustomAttributes(attributeTarget);

            Console.WriteLine("Attributes applied to {0}:{1}", attributeTarget.Name, (attributes.Count == 0 ? "None" : string.Empty));

            foreach (CustomAttributeData attribute in attributes)
            {
                Type t = attribute.Constructor.DeclaringType;
                Console.WriteLine(" {0}", t.ToString());
                Console.WriteLine(" Constructor called={0}", attribute.Constructor);

                IList<CustomAttributeTypedArgument> posArgs = attribute.ConstructorArguments;
                Console.WriteLine(" Positional arguments passed to constructor:{0}", (posArgs.Count == 0) ? "None" : string.Empty);
                foreach (CustomAttributeTypedArgument pa in posArgs)
                {
                    Console.WriteLine(" Type={0}, Value={1}", pa.ArgumentType, pa.Value);
                }

                IList<CustomAttributeNamedArgument> namedArgs = attribute.NamedArguments;
                Console.WriteLine(" Named arguments set after contruction:{0}", (namedArgs.Count == 0 ? "None" : string.Empty));
                foreach (CustomAttributeNamedArgument na in namedArgs)
                {
                    Console.WriteLine(" Name={0}, Type={1}, Value={2}", na.MemberInfo.Name, na.TypedValue.ArgumentType, na.TypedValue.Value);
                }
                Console.WriteLine();
            }
        }

 

条件属性类

自定义属性的目的及方便之处在于其反射或者晚期绑定中的应用，但过多的向目标定义属性会造成元数据不断扩大，并会影响程序的性能。因此.Net引出了条件属性类（ConditionalAttribute）来解决此问题。

ConditionalAttribute　仅对自定义属性有效，并且根据编译时 csc /define:condtionalParam 参数来判断是否将自定义属性生成到元数据。

 

[Conditional("Debug")]
    internal class DebugAttribute : Attribute
    { 
    
    }

    [Conditional("Release")]
    internal class ReleaseAttribute : Attribute
    { 
        
    }

    [Debug]
    internal class DebugApp
    {
        internal static void ShowAttribute()
        {
            //IList<CustomAttributeData> attributes = CustomAttributeData.GetCustomAttributes(typeof(DebugApp));

            //foreach (CustomAttributeData attribute in attributes)
            //{
            //    Console.WriteLine("{0} is applied to DebugApp", attribute.ToString());
            //}

            Console.WriteLine("Debug attribute is {0} applied to DebugApp", Attribute.IsDefined(typeof(DebugApp), typeof(DebugAttribute)) ? "" : "not");
        }
    }

    [Release]
    internal class ReleaseApp
    {
        internal static void ShowAttribute()
        {
            //IList<CustomAttributeData> attributes = CustomAttributeData.GetCustomAttributes(typeof(ReleaseApp));

            //foreach (CustomAttributeData attribute in attributes)
            //{
            //    Console.WriteLine("{0} is applied to ReleaseApp", attribute.ToString());
            //}
            Console.WriteLine("Release attribute is {0} applied to ReleaseApp", Attribute.IsDefined(typeof(ReleaseApp), typeof(ReleaseAttribute)) ? "" : "not");
        }
    }