C#使用反射获取对象变化的情况

　　记录日志时, 经常需要描述对象的状态发生了怎样的变化, 以前处理的非常简单粗暴:

　　a. 重写class的ToString()方法, 将重要的属性都输出来

　　b. 记录日志时:  谁谁谁  由  变更前实例.ToString()   变成   变更后实例.ToString()

　　但输出的日志总是太长了, 翻看日志时想找到差异也非常麻烦, 所以想输出为:  谁谁谁的哪个属性由  aaa 变成了 bbb

　　手写代码一个一个的比较字段然后输出这样的日志信息, 是不敢想象的事情. 本来想参考Dapper使用 System.Reflection.Emit 发射 来提高运行效率, 但实在没有功夫研究.Net Framework的中间语言, 所以准备用 Attribute特性 和 反射 来实现

/// <summary>
/// 要比较的字段或属性, 目前只支持C#基本类型, 比如 int, bool, string等, 你自己写的class或者struct 需要重写 ToString()、Equals(), 按理说如果重写了Equals(), 那也需要重写GetHashCode(), 但确实没有用到GetHashCode(), 所以可以忽略Warning不重写GetHashCode();
/// </summary>
[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false, Inherited = false)]
public class ComparePropertyFieldAttribute : Attribute
{
    /// <summary>
    /// 属性或字段的别名
    /// </summary>
    public string PropertyName { get; private set; }

    /// <summary>
    /// 要比较的字段或属性
    /// </summary>
    public ComparePropertyFieldAttribute()
    { }

    /// <summary>
    /// 要比较的字段或属性
    /// </summary>
    /// <param name="propertyName">属性或字段的别名</param>
    public ComparePropertyFieldAttribute(string propertyName)
    {
        PropertyName = propertyName;
    }

    // 缓存反射的结果,  Tuple<object, ComparePropertyAttribute> 中第一个参数之所以用object 是因为要保存 PropertyInfo 和 FieldInfo
    private static Dictionary<Type, Tuple<object, ComparePropertyFieldAttribute>[]> dict = new Dictionary<Type, Tuple<object, ComparePropertyFieldAttribute>[]>();

    /// <summary>
    /// 只对带有ComparePropertyAttribute的属性和字段进行比较
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="from"></param>
    /// <param name="to"></param>
    /// <param name="differenceMsg">不相同的字段或属性 的字符串说明</param>
    /// <returns>两者相同时, true; 两者不相同时, false</returns>
    public static bool CompareDifference<T>(T from, T to, out string differenceMsg)
    {
        var type = typeof(T);
        lock (dict)
        {
            if (!dict.ContainsKey(type))
            {
                var list = new List<Tuple<object, ComparePropertyFieldAttribute>>();
                // 获取带ComparePropertyAttribute的属性
                var properties = type.GetProperties();
                foreach (var property in properties)
                {
                    var comparePropertyAttribute = (ComparePropertyFieldAttribute)property.GetCustomAttributes(typeof(ComparePropertyFieldAttribute), false).FirstOrDefault();
                    if (comparePropertyAttribute != null)
                    {
                        list.Add(Tuple.Create<object, ComparePropertyFieldAttribute>(property, comparePropertyAttribute));
                    }
                }
                // 获取带ComparePropertyAttribute字段
                var fields = type.GetFields();
                foreach (var field in fields)
                {
                    var comparePropertyAttribute = (ComparePropertyFieldAttribute)field.GetCustomAttributes(typeof(ComparePropertyFieldAttribute), false).FirstOrDefault();
                    if (comparePropertyAttribute != null)
                    {
                        list.Add(Tuple.Create<object, ComparePropertyFieldAttribute>(field, comparePropertyAttribute));
                    }
                }

                dict.Add(type, list.ToArray());
            }
        }

        var sb = new StringBuilder(200); //估计200字节能覆盖大多数情况了吧
        var tupleArray = dict[type];
        foreach (var tuple in tupleArray)
        {
            object v1 = null, v2 = null;
            if (tuple.Item1 is System.Reflection.PropertyInfo)
            {
                if (from != null)
                {
                    v1 = ((System.Reflection.PropertyInfo)tuple.Item1).GetValue(from, null);
                }
                if (to != null)
                {
                    v2 = ((System.Reflection.PropertyInfo)tuple.Item1).GetValue(to, null);
                }
                if (!object.Equals(v1, v2))
                {
                    sb.AppendFormat("{0}从 {1} 变成 {2}; ", tuple.Item2.PropertyName ?? ((System.Reflection.PropertyInfo)tuple.Item1).Name, v1 ?? "null", v2 ?? "null");
                }
            }
            else if (tuple.Item1 is System.Reflection.FieldInfo)
            {
                if (from != null)
                {
                    v1 = ((System.Reflection.FieldInfo)tuple.Item1).GetValue(from);
                }
                if (to != null)
                {
                    v2 = ((System.Reflection.FieldInfo)tuple.Item1).GetValue(to);
                }
                if (!object.Equals(v1, v2))
                {
                    sb.AppendFormat("{0}从 {1} 变成 {2}; ", tuple.Item2.PropertyName ?? ((System.Reflection.FieldInfo)tuple.Item1).Name, v1 ?? "null", v2 ?? "null");
                }
            }
        }

        differenceMsg = sb.ToString();
        return differenceMsg == "";
    }
}

ComparePropertyFieldAttribute