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：
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：
CustomAttributeDataAdapter
[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();
}
}
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。
ConditionalAttribute
[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");
}
}
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