Les attributs C # peuvent-ils accéder à la classe cible?
Je souhaite accéder aux propriétés d'une classe à partir de la classe d'attribut en utilisant la réflexion. C'est possible?
Par exemple:
class MyAttribute : Attribute
{
private void AccessTargetClass()
{
// Do some operations
}
}
[MyAttribute]
class TargetClass
{
}
Pas directement, mais pour que l'attribut puisse faire quoi que ce soit, vous devez déjà avoir un descripteur sur le type/membre et appeler sa méthode GetCustomAttributes afin que vous puissiez simplement le transmettre à partir de là:
class MyAttribute : Attribute
{
public void DoSomethingWithDeclaringType(Type t)
{
}
}
L'utiliser comme ça:
Type classType = typeof(MyClass);
object[] attribs = classType.GetCustomAttributes(typeof(MyAttribute), true);
if(attribs.Length > 0)
{
((MyAttribute)attribs[0]).DoSomethingWithDeclaringType(classType);
}
Il existe des cas où vous ne pouvez pas compter sur le code d'appel pour transmettre une référence de type.
Voici une solution que j'ai concoctée pour résoudre ce problème. C'est un peu une approche de tir, mais ça marche ...
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);
}
}
}
Le principal objectif des attributs est l'annotation: vous déclarez? cette classe/méthode/propriété devrait être traitée de telle ou telle manière, mais le code qui analyse en fait tout cela devrait être ailleurs Donc, votre intention de faire quelque chose à l'intérieur de l'attribut brise ce modèle. Mon conseil: envisagez une autre solution.
Vous pouvez accéder au type d'appelant en utilisant CallerMemberNameAttribute à partir de .NET 4.5:
public CustomAttribute([CallerMemberName] string propertyName = null)
{
// ...
}
Et ensuite, utilisez une réflexion simple