Attribute.IsDefined doesn't see attributes applied with MetadataType class

问题

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?

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.

Thanks!

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
    {
    }
}

回答1:

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
    {
    }
}

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

回答2:

I had a similar situation. I ended up writing the following extension method for it. The idea is to hide the abstraction of looking in 2 places (main class and metadata class).

    static public Tattr GetSingleAttribute<Tattr>(this PropertyInfo pi, bool Inherit = true) where Tattr : Attribute
    {
        var attrs = pi.GetCustomAttributes(typeof(Tattr), Inherit);
        if (attrs.Length > 0)
            return (Tattr)attrs[0];
        var mt = pi.DeclaringType.GetSingleAttribute<MetadataTypeAttribute>();
        if (mt != null)
        {
            var pi2 = mt.MetadataClassType.GetProperty(pi.Name);
            if (pi2 != null)
                return pi2.GetSingleAttribute<Tattr>(Inherit);
        }
        return null;
    }

回答3:

My solution for generic use. Get the attribute the property that you are looking for. Return null if not found.

If found, it returns the attribute itself. So you can have access to the properties inside the attribute if you wihs.

Hopes this help.

public static Attribute GetAttribute<T>(this PropertyInfo PI, T t) where T: Type
{
    var Attrs = PI.DeclaringType.GetCustomAttributes(typeof(MetadataTypeAttribute), true);
    if (Attrs.Length < 1) return null;

    var metaAttr = Attrs[0] as MetadataTypeAttribute;
    var metaProp = metaAttr.MetadataClassType.GetProperty(PI.Name);
    if (metaProp == null) return null;

    Attrs = metaProp.GetCustomAttributes(t, true);
    if (Attrs.Length < 1) return null;
    return Attrs[0] as Attribute;
}

回答4:

Given the following classes:

public partial class Person
{
    public int PersonId { get; set; }
}

[MetadataType(typeof(PersonMetadata))]
public partial class Person
{
    public partial class PersonMetadata
    {
        [Key]
        public int PersonId { get; set; }
    }
}

I needed to get see if Key has been defined on a property for Person class. I then needed to get the value of the property. Using @AdamGrid answer, I modified the code like this to get it:

private static object GetPrimaryKeyValue(TEntity entity)
{
    MetadataTypeAttribute[] metadataTypes = typeof(TEntity).GetCustomAttributes(typeof(MetadataTypeAttribute), true).OfType<MetadataTypeAttribute>().ToArray();
    MetadataTypeAttribute metadata = metadataTypes.FirstOrDefault();
    if (metadata == null)
    {
        ThrowNotFound();
    }

    PropertyInfo[] properties = metadata.MetadataClassType.GetProperties();
    PropertyInfo primaryKeyProperty =
        properties.SingleOrDefault(x => Attribute.GetCustomAttribute(x, typeof(KeyAttribute)) as KeyAttribute != null);
    if (primaryKeyProperty == null)
    {
        ThrowNotFound();
    }

    object primaryKeyValue = typeof(TEntity).GetProperties().Single(x => x.Name == primaryKeyProperty.Name).GetValue(entity);

    return primaryKeyValue;
}

private static void ThrowNotFound()
{
    throw new InvalidOperationException
            ($"The type {typeof(TEntity)} does not have a property with attribute KeyAttribute to indicate the primary key. You must add that attribute to one property of the class.");
}

来源：https://stackoverflow.com/questions/1910532/attribute-isdefined-doesnt-see-attributes-applied-with-metadatatype-class