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