C#自定义Attribute值的获取与优化

　　C#自定义Attribute值的获取是开发中会经常用到的，一般我们的做法也就是用反射进行获取的，代码也不是很复杂。

1、首先有如下自定义的Attribute

     [AttributeUsage(AttributeTargets.All)]
     public sealed class NameAttribute : Attribute
     {
         private readonly string _name;

         public string Name
         {
             get { return _name; }
         }

         public NameAttribute(string name)
         {
             _name = name;
         }
     }

　　2、定义一个使用NameAttribute的类

[Description("Customer Information")]
[Name("customer_info")]
public class CustomerInfo
{
    [Name("name")]
    public string Name { get; set; }

    [Name("address")]
    public string Address;
}

　

　　3、获取CustomAttributes类上的"dept"也就很简单了

         private static string GetName()
         {
             var type = typeof(CustomAttributes);

             var attribute = type.GetCustomAttributes(typeof(NameAttribute), false).FirstOrDefault();

             if (attribute == null)
             {
                 return null;
             }

             return ((NameAttribute)attribute).Name;
         }

　　以上代码就可以简单的获取，类上的Attribute的值了，但是需求往往不是这么简单的，不仅要获取类头部Attribute上的值，还要获取字段Address头部Attribute上的值。有的同学可能就觉得这还不简单呀，直接上代码

         private static string GetAddress()
         {
             var type = typeof (CustomAttributes);

             var fieldInfo = type.GetField("Address");
             if (fieldInfo == null)
             {
                 return null;
             }

             var attribute = fieldInfo.GetCustomAttributes(typeof(NameAttribute), false).FirstOrDefault();

             if (attribute == null)
             {
                 return null;
             }

             return ((NameAttribute) attribute).Name;
         }

　　上面代码就是获取Address字段头部上的Attribute值了。虽然我们是获取到了我们想要的，但是我们发现这样做是不是太累了，如果又扩展一个自定义的Attribute，或者又在一个新的属性或字段上标上Attribute时，我们又要写一段代码来实现我想要的，这些严重代码违反了DRY的设计原则。我们知道获取Attribute是通过反射来取的，Attribute那个值又是不变的，这样就没必要每次都要进行反射来获取了。基于以上两点代码进行了如下的优化，优化后的代码如下：

 using System;
 using System.Collections.Concurrent;
 using System.Reflection;

 public static class CustomAttributeExtensions
 {
     /// <summary>
     /// Cache Data
     /// </summary>
     private static readonly ConcurrentDictionary<string, object> Cache = new ConcurrentDictionary<string, object>();

     /// <summary>
     /// 获取CustomAttribute Value
     /// </summary>
     /// <typeparam name="TAttribute">Attribute的子类型</typeparam>
     /// <typeparam name="TReturn">TReturn的子类型</typeparam>
     /// <param name="sourceType">头部标有CustomAttribute类的类型</param>
     /// <param name="attributeValueAction">取Attribute具体哪个属性值的匿名函数</param>
     /// <returns>返回Attribute的值，没有则返回null</returns>
     public static TReturn GetCustomAttributeValue<TAttribute, TReturn>(this Type sourceType, Func<TAttribute, TReturn> attributeValueAction)
         where TAttribute : Attribute
     {
         return _getAttributeValue(sourceType, attributeValueAction, null);
     }

     /// <summary>
     /// 获取CustomAttribute Value
     /// </summary>
     /// <typeparam name="TAttribute">Attribute的子类型</typeparam>
     /// <typeparam name="TReturn">TReturn的子类型</typeparam>
     /// <param name="sourceType">头部标有CustomAttribute类的类型</param>
     /// <param name="attributeValueAction">取Attribute具体哪个属性值的匿名函数</param>
     /// <param name="propertyName">field name或property name</param>
     /// <returns>返回Attribute的值，没有则返回null</returns>
     public static TReturn GetCustomAttributeValue<TAttribute, TReturn>(this Type sourceType, Func<TAttribute, TReturn> attributeValueAction, string propertyName)
         where TAttribute : Attribute
     {
         return _getAttributeValue(sourceType, attributeValueAction, propertyName);
     }

     #region private methods

     private static TReturn _getAttributeValue<TAttribute, TReturn>(Type sourceType, Func<TAttribute, TReturn> attributeFunc, string propertyName)
         where TAttribute : Attribute
     {
         var cacheKey = BuildKey<TAttribute>(sourceType, propertyName);
         var value = Cache.GetOrAdd(cacheKey, k => GetValue(sourceType, attributeFunc, propertyName));
         if (value is TReturn) return (TReturn)Cache[cacheKey];
         return default(TReturn);
     }

     private static string BuildKey<TAttribute>(Type type, string propertyName) where TAttribute : Attribute
     {
         var attributeName = typeof(TAttribute).FullName;
         if (string.IsNullOrEmpty(propertyName))
         {
             return type.FullName + "." + attributeName;
         }

         return type.FullName + "." + propertyName + "." + attributeName;
     }

     private static TReturn GetValue<TAttribute, TReturn>(this Type type, Func<TAttribute, TReturn> attributeValueAction, string name)
         where TAttribute : Attribute
     {
         TAttribute attribute = default(TAttribute);
         if (string.IsNullOrEmpty(name))
         {
             attribute = type.GetCustomAttribute<TAttribute>(false);
         }
         else
         {
             var propertyInfo = type.GetProperty(name);
             if (propertyInfo != null)
             {
                 attribute = propertyInfo.GetCustomAttribute<TAttribute>(false);
             }
             else
             {
                 var fieldInfo = type.GetField(name);
                 if (fieldInfo != null)
                 {
                     attribute = fieldInfo.GetCustomAttribute<TAttribute>(false);
                 }
             }
         }

         return attribute == null ? default(TReturn) : attributeValueAction(attribute);
     }

     #endregion
 }

　　优化后的代码：

　　把不同的代码用泛型T,Fun<TAttribute,TReturn>来处理来减少重复的代码；
　　把取过的Attribute值存到一个ConcurrentDictionary中，下次再来取时，如果有则直接取ConcurrentDictionary中的值，如果没有才通过反射来取相应的Attribute值，这样大大的提高效率；

　　调用方法也更加的简单了，代码如下：

            var customerInfoName = typeof(CustomerInfo).GetCustomAttributeValue<NameAttribute, string>(x => x.Name);
            var customerAddressName = typeof(CustomerInfo).GetCustomAttributeValue<NameAttribute, string>(x => x.Name, "Address");
            var customerInfoDesc = typeof(CustomerInfo).GetCustomAttributeValue<DescriptionAttribute, string>(x => x.Description);

            Console.WriteLine("CustomerInfo Name:" + customerInfoName);
            Console.WriteLine("customerInfo >Address Name:" + customerAddressName);
            Console.WriteLine("customerInfo Desc:" + customerInfoDesc);

　　运行结果：

　　如果你有什么好的或不好的意见欢迎拍砖!

　　谢谢大家的建议，己经更新（2019/11/21）

　　Code：Download