Attribute.IsDefined 看不到与 MetadataType 类一起应用的属性 [英] Attribute.IsDefined doesn't see attributes applied with MetadataType class

问题描述

如果我通过 MetadataType 属性，通过 Attribute.IsDefined(). 有谁知道为什么，或者我做错了什么?

If I apply attributes to a partial class via the MetadataType attribute, those attributes are not found via Attribute.IsDefined(). Anyone know why, or what I'm doing wrong?

下面是我为此创建的一个测试项目，但我真的想将自定义属性应用于 LINQ to SQL 实体类 - 比如 这个问题的答案.

Below is a test project I created for this, but I'm really trying to apply custom attributes to a LINQ to SQL entity class - like this answer in this question.

谢谢！

using System;
using System.ComponentModel.DataAnnotations;
using System.Reflection;

namespace MetaDataTest
{
    class Program
    {
        static void Main(string[] args)
        {
            PropertyInfo[] properties = typeof(MyTestClass).GetProperties();

            foreach (PropertyInfo propertyInfo in properties)
            {
                Console.WriteLine(Attribute.IsDefined(propertyInfo, typeof(MyAttribute)));
                Console.WriteLine(propertyInfo.IsDefined(typeof(MyAttribute), true));
                Console.WriteLine(propertyInfo.GetCustomAttributes(true).Length);

                // Displays:
                // False
                // False
                // 0
            }

            Console.ReadLine();
        }
    }

    [MetadataType(typeof(MyMeta))]
    public partial class MyTestClass
    {
        public string MyField { get; set; }
    }

    public class MyMeta
    {
        [MyAttribute()]
        public string MyField { get; set; }
    }

    [AttributeUsage(AttributeTargets.All)]
    public class MyAttribute : System.Attribute
    {
    }
}

推荐答案

MetadataType 属性用于指定帮助指定数据对象的附加信息.要访问其他属性，您需要执行以下操作:

The MetadataType attribute is used to specify help specify the additional information on the data object. To access the additional attributes you would need to do something like the following:

using System;
using System.Linq;
using System.ComponentModel.DataAnnotations;
using System.Reflection;

namespace MetaDataTest
{
    class Program
    {
        static void Main(string[] args)
        {
            MetadataTypeAttribute[] metadataTypes = typeof(MyTestClass).GetCustomAttributes(typeof(MetadataTypeAttribute), true).OfType<MetadataTypeAttribute>().ToArray();
            MetadataTypeAttribute metadata = metadataTypes.FirstOrDefault();

            if (metadata != null)
            {
                PropertyInfo[] properties = metadata.MetadataClassType.GetProperties();

                foreach (PropertyInfo propertyInfo in properties)
                {
                    Console.WriteLine(Attribute.IsDefined(propertyInfo, typeof(MyAttribute)));
                    Console.WriteLine(propertyInfo.IsDefined(typeof(MyAttribute), true));
                    Console.WriteLine(propertyInfo.GetCustomAttributes(true).Length);
                    RequiredAttribute attrib = (RequiredAttribute)propertyInfo.GetCustomAttributes(typeof(RequiredAttribute), true)[0];
                    Console.WriteLine(attrib.ErrorMessage);
                }

                // Results:
                // True
                // True
                // 2
                // MyField is Required
            }

            Console.ReadLine();
        }
    }

    [MetadataType(typeof(MyMeta))]
    public partial class MyTestClass
    {
        public string MyField { get; set; }
    }

    public class MyMeta
    {
        [MyAttribute()]
        [Required(ErrorMessage="MyField is Required")]
        public string MyField { get; set; }
    }

    [AttributeUsage(AttributeTargets.All)]
    public class MyAttribute : System.Attribute
    {
    }
}

这还包括一个示例属性，用于展示如何提取添加的信息.

This also includes a sample attribute to show how to extract info that was added.

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