.NET Core 特性(Attribute)底层原理解析

    public enum Test
    {
        [EnumDescription("hhhhhh")]
        None = 0,
        [EnumDescription("xxxxxx")]
        Done =1
    }
	private static IEnumerable<string> GetEnumDescriptions(this Enum e)
	{
		IEnumerable<string> result = null;
        var type = e.GetType();
        var fieldInfo = type.GetField(e.ToString());
        var attr = fieldInfo?.GetCustomAttributes(typeof(EnumDescriptionAttribute), false);
        if (attr?.Length > 0)
        {
			result = attr.Cast<EnumDescriptionAttribute>().Select(x => x.Description);
        }
		return result ?? Enumerable.Empty<string>();
	}

        private static IEnumerable<string> GetEnumDescriptionsCache(this Enum e)
        {
            var key = $"{e.GetType().Name}_{e.ToString()}";
            if (_enumMap.ContainsKey(key))
            {
                return _enumMap[key];
            }
            else
            {
                var result = GetEnumDescriptions(e);
                _enumMap.TryAdd(key, result);
                return result;
            }
        }

    public class Person
    {
        [JsonConverter(typeof(DateTimeConverter))]
        public DateTime CreateTime { get; set; }
    }
	public class DateTimeConverter : JsonConverter<DateTime>
    {
        public override DateTime ReadJson(JsonReader reader, Type objectType, DateTime existingValue, bool hasExistingValue, JsonSerializer serializer)
        {
            if (reader.Value == null)
                return DateTime.MinValue;
            if (DateTime.TryParse(reader.Value.ToString(), out DateTime result))
                return result;
            return DateTime.MinValue;
        }
        public override void WriteJson(JsonWriter writer, DateTime value, JsonSerializer serializer)
        {
            writer.WriteValue(value.ToString("yyyy-MM-dd HH:mm:ss"));
        }
    }

        [RequiresUnreferencedCode(MiscellaneousUtils.TrimWarning)]
        [RequiresDynamicCode(MiscellaneousUtils.AotWarning)]
        public static JsonConverter? GetJsonConverter(object attributeProvider)
        {
			// 底层还是调用Reflection，为了性能，也缓存了对象元数据。
            JsonConverterAttribute? converterAttribute = GetCachedAttribute<JsonConverterAttribute>(attributeProvider);
            if (converterAttribute != null)
            {
                Func<object[]?, object> creator = CreatorCache.Instance.Get(converterAttribute.ConverterType);
                if (creator != null)
                {
                    return (JsonConverter)creator(converterAttribute.ConverterParameters);
                }
            }
            return null;
        }

    public class NativeMethods
    {
        [DllImport("xxxxx", EntryPoint = "add", CallingConvention = CallingConvention.Cdecl)]
        public extern static int ManagedAdd(int a, int b);
    }

    public class Person
    {
        public int id { get; set; } = 0;
        [MethodImpl(MethodImplOptions.Synchronized)]
        public void SyncMethod()
        {
            id++;
        }
    }