Question

我们可以通过以下方式为引用的DLL中的类型设置自定义属性：

[AttributeUsage(AttributeTargets.Assembly, AllowMultiple = true)]
public sealed class AttributeForExternType : Attribute
{
    public ExternTypeAttribute(Type type)
    {
        ExternType = type;
    }
    public Type ExternType { get; }
}

// set in assembly info:
[assembly: ExternTypeAttribute(typeof(Extern.Type))]

在项目的枚举和字段中设置自定义属性：

[AttributeUsage(AttributeTargets.Enum)]
public sealed class InternEnumAttribute : Attribute { }

[AttributeUsage(AttributeTargets.Field)]
public sealed class InternFieldAttribute: Attribute { }

enum MyEnum
{
    FieldA,
    [InternFieldAttribute]
    FieldB
}

问题：是否可以在引用的枚举值上定义和使用自定义属性？

Answer 1

您无法将属性添加到预先存在的类型。如果可以修改源代码，则只能添加属性...

// set in assembly info:
[assembly: ExternTypeAttribute(typeof(System.Collections.Generic.List<int>))]
[assembly: ExternTypeAttribute(typeof(System.Attribute))]
[assembly: ExternTypeAttribute(typeof(System.Console))]
[assembly: ExternTypeAttribute(typeof(System.DateTime))]

[AttributeUsage(AttributeTargets.Assembly, AllowMultiple = true)]
public sealed class ExternTypeAttribute : Attribute
{
    public ExternTypeAttribute(Type type)
    {
        ExternType = type;
    }
    public Type ExternType { get; }
}

var attrs1 = Assembly.GetExecutingAssembly().GetCustomAttributes<ExternTypeAttribute>().ToArray();
var attrs2 = typeof(System.Collections.Generic.List<int>).GetCustomAttributes<ExternTypeAttribute>().ToArray();
var attrs3 = typeof(System.Attribute).GetCustomAttributes<ExternTypeAttribute>().ToArray();
var attrs4 = typeof(System.Console).GetCustomAttributes<ExternTypeAttribute>().ToArray();
var attrs5 = typeof(System.DateTime).GetCustomAttributes<ExternTypeAttribute>().ToArray();

Console.WriteLine("This assembly: {0}, attached to types: {1} {2} {3} {4}", attrs1.Length, attrs2.Length, attrs3.Length, attrs4.Length, attrs5.Length);

结果是：

此程序集：4，附加类型：0,0,0,0

请注意，有Type Descriptor子系统，但它是专用的，主要用于设计人员。

在引用的DLL

1 个答案: