Attribute作为一种标记在我们的.net中随处可见，比如DatContract，DatMember，Serializable等等，各种用途的标记。是的我们的代码更加简洁，对于Attribute用好了，可以很好的简化我们的开发，比如实现就是一种基于Attribute的标记编译时注入。在随笔中有的实例。
在使用Attribute时候发现了些鲜为人知的特性：
1：利用GetCustomAttributes传入的Attribute返回得到包括派生类。
2：GetCustomAttributes每次返回的对象都是经过发射出来的没有缓存。
1：GetCustomAttributes传入的Attribute返回得到包括派生类：
这里将采用一个测试类来验证：
[AttributeUsage(AttributeTargets.Class)] public class TestImplementsAttribute : Attribute { public string Name { get; set; } } private static void TestMutilpeImplements() { var type = typeof(Program); var attrs = type.GetCustomAttributes( typeof(TestImplementsAttribute), false); Console.WriteLine( string.Format( " TestImplementsAttribute:({0}) ",attrs.Length)); foreach ( var item in attrs) { Console.WriteLine( " " + item.GetType().FullName); } attrs = type.GetCustomAttributes( typeof(SerializableAttribute), false); Console.WriteLine( string.Format( " SerializableAttribute:({0}) ", attrs.Length)); foreach ( var item in attrs) { Console.WriteLine( " " + item.GetType().FullName); } attrs = type.GetCustomAttributes( typeof(Attribute), false); Console.WriteLine( string.Format( " (base type)Attribute:({0}) ", attrs.Length)); foreach ( var item in attrs) { Console.WriteLine( " " + item.GetType().FullName); } }
输出为：
这里我们可以很清晰的看见当传入Attribute类型时候返回包含了SerializableAttribute和TestImplementsAttribute两个。
2：GetCustomAttributes每次返回的对象都是经过发射出来的没有缓存：
测试代码可以看出来,不是同一个地址引用：
private static void TestAttributeActiver() { var type = typeof(Program); var attr1 = type.GetCustomAttributes( typeof(TestImplementsAttribute), false)[ 0]; var attr2 = type.GetCustomAttributes( typeof(TestImplementsAttribute), false)[ 0]; Console.WriteLine(Object.ReferenceEquals(attr1, attr2)); }
输出值为false。
我们在看看
.下面是 reflector的反编译结果（Attribute.GetCustomAttributes）：
internal static unsafe object[] GetCustomAttributes(Module decoratedModule, int decoratedMetadataToken, int pcaCount, RuntimeType attributeFilterType, bool mustBeInheritable, IList derivedAttributes) { if (decoratedModule.Assembly.ReflectionOnly) { throw new InvalidOperationException(Environment.GetResourceString( " Arg_ReflectionOnlyCA ")); } MetadataImport metadataImport = decoratedModule.MetadataImport; CustomAttributeRecord[] customAttributeRecords = CustomAttributeData.GetCustomAttributeRecords(decoratedModule, decoratedMetadataToken); Type elementType = (((attributeFilterType == null) || attributeFilterType.IsValueType) || attributeFilterType.ContainsGenericParameters) ? typeof( object) : attributeFilterType; if ((attributeFilterType == null) && (customAttributeRecords.Length == 0)) { return (Array.CreateInstance(elementType, 0) as object[]); } object[] attributes = Array.CreateInstance(elementType, customAttributeRecords.Length) as object[]; int length = 0; SecurityContextFrame frame = new SecurityContextFrame(); frame.Push(decoratedModule.Assembly.InternalAssembly); Assembly lastAptcaOkAssembly = null; for ( int i = 0; i < customAttributeRecords.Length; i++) { bool flag2; bool flag3; object obj2 = null; CustomAttributeRecord caRecord = customAttributeRecords[i]; RuntimeMethodHandle ctor = new RuntimeMethodHandle(); RuntimeType attributeType = null; int namedArgs = 0; IntPtr signature = caRecord.blob.Signature; IntPtr blobEnd = (IntPtr) ((( void*) signature) + caRecord.blob.Length); if (FilterCustomAttributeRecord(caRecord, metadataImport, ref lastAptcaOkAssembly, decoratedModule, decoratedMetadataToken, attributeFilterType, mustBeInheritable, attributes, derivedAttributes, out attributeType, out ctor, out flag2, out flag3)) { if (!ctor.IsNullHandle()) { ctor.CheckLinktimeDemands(decoratedModule, decoratedMetadataToken); } RuntimeConstructorInfo.CheckCanCreateInstance(attributeType, flag3); if (flag2) { obj2 = CreateCaObject(decoratedModule, ctor, ref signature, blobEnd, out namedArgs); } else { obj2 = attributeType.TypeHandle.CreateCaInstance(ctor); if (Marshal.ReadInt16(signature) != 1) { throw new CustomAttributeFormatException(); } signature = (IntPtr) ((( void*) signature) + 2); namedArgs = Marshal.ReadInt16(signature); signature = (IntPtr) ((( void*) signature) + 2); } for ( int j = 0; j < namedArgs; j++) { string str; bool flag4; Type type3; object obj3; IntPtr ptr1 = caRecord.blob.Signature; GetPropertyOrFieldData(decoratedModule, ref signature, blobEnd, out str, out flag4, out type3, out obj3); try { if (flag4) { if ((type3 == null) && (obj3 != null)) { type3 = (obj3.GetType() == typeof(RuntimeType)) ? typeof(Type) : obj3.GetType(); } RuntimePropertyInfo property = null; if (type3 == null) { property = attributeType.GetProperty(str) as RuntimePropertyInfo; } else { property = attributeType.GetProperty(str, type3, Type.EmptyTypes) as RuntimePropertyInfo; } RuntimeMethodInfo setMethod = property.GetSetMethod( true) as RuntimeMethodInfo; if (setMethod.IsPublic) { setMethod.MethodHandle.CheckLinktimeDemands(decoratedModule, decoratedMetadataToken); setMethod.Invoke(obj2, BindingFlags.Default, null, new object[] { obj3 }, null, true); } } else { (attributeType.GetField(str) as RtFieldInfo).InternalSetValue(obj2, obj3, BindingFlags.Default, Type.DefaultBinder, null, false); } } catch (Exception exception) { throw new CustomAttributeFormatException( string.Format(CultureInfo.CurrentUICulture, Environment.GetResourceString(flag4 ? " RFLCT.InvalidPropFail " : " RFLCT.InvalidFieldFail "), new object[] { str }), exception); } } if (!signature.Equals(blobEnd)) { throw new CustomAttributeFormatException(); } attributes[length++] = obj2; } } frame.Pop(); if ((length == customAttributeRecords.Length) && (pcaCount == 0)) { return attributes; } if (length == 0) { Array.CreateInstance(elementType, 0); } object[] destinationArray = Array.CreateInstance(elementType, ( int) (length + pcaCount)) as object[]; Array.Copy(attributes, 0, destinationArray, 0, length); return destinationArray; }
在这里我们可以见数组的创建等等。
同时可以参见老赵前辈以前的关于Attribute反射的和。
不知道为什么在Attribute参数的检查是在我们的编译时期，参数必须是常量表达式，却在这里需要每次反射。
本篇随笔只是个人使用心得记录，请勿拍砖。