我想使用反射从属性类中访问类的属性。有可能吗？
例如：
class MyAttribute : Attribute
{
private void AccessTargetClass()
{
// Do some operations
}
}
[MyAttribute]
class TargetClass
{
}
【问题讨论】：
标签： c# reflection attributes
在某些情况下，您不能依赖调用代码来传递类型引用。
以下是我为解决此问题而设计的解决方案。这有点像霰弹枪的方法，但它确实有效......
using System;
using System.Reflection;
using System.Collections.Generic;
using System.Linq;
using System.Diagnostics;
namespace Ethica.Reflection
{
/// <summary>
/// Helps in discovery of the target of an Attribute
/// </summary>
/// <typeparam name="TAttribute">An Attribute derived type</typeparam>
/// <remarks>
/// The .NET framework does not provide navigation from attributes back to their targets, principally for the reason that
/// in typical usage scenarios for attributes, the attribute is discovered by a routine which already has a reference to a
/// member type.
///
/// There are, however, bona-fide cases where an attribute needs to detect it's target - an example is a localizable sub-class of the
/// DescriptionAttribute. In order for the DescriptionAttribute to return a localized string, it requires a resource key and, ideally,
/// a type reference as the base-key for the ResourceManager. A DescriptionAttribute could not provide this information without
/// a reference to it's target type.
///
/// Note to callers:
///
/// Your Attribute-derived class must implement Equals and GetHashCode, otherwise a run-time exception will occur, since this class
/// creates a dictionary of attributes in order to speed up target lookups.
/// </remarks>
public static class AttributeTargetHelper<TAttribute>
where TAttribute : Attribute
{
/// <summary>
/// Map of attributes and their respective targets
/// </summary>
private static Dictionary<TAttribute, object> targetMap;
/// <summary>
/// List of assemblies that should not be rescanned for types.
/// </summary>
private static List<string> skipAssemblies;
/// <summary>
/// Adds an attribute and it's target to the dictionary
/// </summary>
/// <param name="attribute"></param>
/// <param name="item"></param>
private static void Add(TAttribute attribute, object item)
{
targetMap.Add(attribute, item);
}
/// <summary>
/// Scans an assembly for all instances of the attribute.
/// </summary>
/// <param name="assembly"></param>
private static void ScanAssembly(Assembly assembly)
{
const BindingFlags memberInfoBinding = BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Static | BindingFlags.Instance;
if (!skipAssemblies.Contains(assembly.FullName))
{
skipAssemblies.Add(assembly.FullName);
Debug.WriteLine("Loading attribute targets for " + typeof(TAttribute).Name + " from assembly " + assembly.FullName);
foreach (TAttribute attr in assembly.GetCustomAttributes(typeof(TAttribute), false))
Add(attr, assembly);
foreach (Type type in assembly.GetTypes())
{
foreach (TAttribute attr in type.GetCustomAttributes(typeof(TAttribute), false))
Add(attr, type);
foreach (MemberInfo member in type.GetMembers(memberInfoBinding))
{
foreach (TAttribute attr in member.GetCustomAttributes(typeof(TAttribute), false))
Add(attr, member);
if (member.MemberType == MemberTypes.Method)
foreach (var parameter in ((MethodInfo)member).GetParameters())
foreach (TAttribute attr in parameter.GetCustomAttributes(typeof(TAttribute), false))
Add(attr, parameter);
}
}
}
foreach (var assemblyName in assembly.GetReferencedAssemblies())
{
if (!skipAssemblies.Contains(assemblyName.FullName))
ScanAssembly(Assembly.Load(assemblyName));
}
}
/// <summary>
/// Returns the target of an attribute.
/// </summary>
/// <param name="attribute">The attribute for which a target is sought</param>
/// <returns>The target of the attribute - either an Assembly, Type or MemberInfo instance.</returns>
public static object GetTarget(TAttribute attribute)
{
object result;
if (!targetMap.TryGetValue(attribute, out result))
{
// Since types can be loaded at any time, recheck that all assemblies are included...
// Walk up the stack in a last-ditch effort to find instances of the attribute.
StackTrace stackTrace = new StackTrace(); // get call stack
StackFrame[] stackFrames = stackTrace.GetFrames(); // get method calls (frames)
// write call stack method names
foreach (StackFrame stackFrame in stackFrames)
{
Console.WriteLine(stackFrame.GetMethod().Name); // write method name
ScanAssembly(stackFrame.GetMethod().GetType().Assembly);
}
if (!targetMap.TryGetValue(attribute, out result))
throw new InvalidProgramException("Cannot find assembly referencing attribute");
}
return result;
}
/// <summary>
/// Static constructor for type.
/// </summary>
static AttributeTargetHelper()
{
targetMap = new Dictionary<TAttribute, object>();
// Do not load any assemblies reference by the assembly which declares the attribute, since they cannot possibly use the attribute
skipAssemblies = new List<string>(typeof(TAttribute).Assembly.GetReferencedAssemblies().Select(c => c.FullName));
// Skip common system assemblies
skipAssemblies.Add("System.Core, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089");
skipAssemblies.Add("System.Security, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a");
skipAssemblies.Add("System.Xml, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089");
skipAssemblies.Add("System.Data.SqlXml, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089");
skipAssemblies.Add("System.Configuration, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a");
skipAssemblies.Add("System.Numerics, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089");
// Scan the entire application
ScanAssembly(Assembly.GetEntryAssembly());
}
}
/// <summary>
/// Extends attributes so that their targets can be discovered
/// </summary>
public static class AttributeTargetHelperExtension
{
/// <summary>
/// Gets the target of an attribute
/// </summary>
/// <typeparam name="TAttribute"></typeparam>
/// <param name="attribute">The attribute for which a target is sought</param>
/// <returns>The target of the attribute - either an Assembly, Type or MemberInfo instance.</returns>
public static object GetTarget<TAttribute>(this TAttribute attribute)
where TAttribute : Attribute
{
return AttributeTargetHelper<TAttribute>.GetTarget(attribute);
}
}
}
【讨论】：
不是直接的，但是为了让属性做任何事情，你必须有一个类型/成员的句柄并调用它的 GetCustomAttributes 方法，所以你可以简单地从那里传递它：
class MyAttribute : Attribute
{
public void DoSomethingWithDeclaringType(Type t)
{
}
}
像这样使用它：
Type classType = typeof(MyClass);
object[] attribs = classType.GetCustomAttributes(typeof(MyAttribute), true);
if(attribs.Length > 0)
{
((MyAttribute)attribs[0]).DoSomethingWithDeclaringType(classType);
}
【讨论】：
Attributes 主要目的是注解：你声明？该类/方法/属性应该以这种或那种方式处理，但实际分析所有这些东西的代码应该在其他地方。所以你打算在属性内部做一些事情正在打破这种模式。我的建议：考虑另一种解决方案。
【讨论】：
您可以使用 .NET 4.5 中的CallerMemberNameAttribute 访问调用者类型：
public CustomAttribute([CallerMemberName] string propertyName = null)
{
// ...
}
然后使用简单的反射
【讨论】：